1256 lines
49 KiB
HTML
1256 lines
49 KiB
HTML
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PostgreSQL i<>in S<>k<EFBFBD>a Sorulan Sorular (SSS)
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Son g<>ncelleme : 19 May<61>s 2003 Pazartesi - 03:05:21
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us
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<mailto:pgman@candle.pha.pa.us>)
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<EFBFBD>eviren : Devrim G<>ND<4E>Z (devrim@gunduz.org <mailto:devrim@gunduz.org>)
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Nicolai Tufar (ntufar@yahoo.com <mailto:ntufar@yahoo.com>)
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Bu belgenin en g<>ncel hali,
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http://www.PostgreSQL.org/docs/faqs/FAQ_turkish.html ve
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http://www.gunduz.org/seminer/pg/FAQ_turkish adreslerinde g<>r<EFBFBD>lebilir.
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Platforma <20>zel sorular<61>n<EFBFBD>z,http://www.PostgreSQL.org/docs/index.html
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adresinde yan<61>tlan<61>r..
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------------------------------------------------------------------------
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Genel Sorular
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1.1 <#1.1>) PostgreSQL nedir? Nas<61>l okunur?
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1.2 <#1.2>) PostgreSQL' in haklar<61> nedir?
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1.3 <#1.3>) PostgreSQL, hangi Unix platformlar<61>nda <20>al<61>s<EFBFBD>r?
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1.4 <#1.4>) Hangi Unix olmayan uyarlamalar<61> bulunmaktad<61>r?
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1.5 <#1.5>) PostgreSQL'i nereden indirebilirim?
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1.6 <#1.6>) Deste<74>i nereden alabilirim?
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1.7 <#1.7>) En son s<>r<EFBFBD>m<EFBFBD> nedir?
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1.8 <#1.8>) Hangi belgelere ulasabilirim?
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1.9 <#1.9>) Bilinen hatalar ya da eksik <20>zelliklere nereden ulasabilirim?
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1.10 <#1.10>) Nas<61>l SQL <20><>renebilirim?
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1.11 <#1.11>) PostgreSQL 2000 y<>l<EFBFBD>na uyumlu mudur?
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1.12 <#1.12>) Geli<6C>tirme tak<61>m<EFBFBD>na nas<61>l kat<61>labilirim??
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1.13 <#1.13>) Bir hata raporunu nas<61>l g<>nderebilirim?
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1.14 <#1.14>) PostgreSQL, diger VTYS(DBMS lerle nas<61>l kar<61><72>la<6C>t<EFBFBD>r<EFBFBD>labilir?
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1.15 <#1.15>) PostgreSQL'e maddi a<><61>dan nas<61>l destek olabilirim?
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Kullan<61>c<EFBFBD>/istemci Sorular<61>
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2.1 <#2.1>) Are there ODBC drivers for PostgreSQL?
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2.2 <#2.2>) What tools are available for using PostgreSQL with Web pages?
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2.3 <#2.3>) Does PostgreSQL have a graphical user interface?
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2.4 <#2.4>) What languages are available to communicate with PostgreSQL?
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Administrative Questions
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3.1 <#3.1>) How do I install PostgreSQL somewhere other than
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//usr/local/pgsql/?
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3.2 <#3.2>) When I start /postmaster/, I get a /Bad System Call/ or core
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dumped message. Why?
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3.3 <#3.3>) When I try to start /postmaster/, I get /IpcMemoryCreate/
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errors. Why?
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3.4 <#3.4>) When I try to start /postmaster/, I get /IpcSemaphoreCreate/
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errors. Why?
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3.5 <#3.5>) How do I control connections from other hosts?
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3.6 <#3.6>) How do I tune the database engine for better performance?
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3.7 <#3.7>) What debugging features are available?
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3.8 <#3.8>) Why do I get /"Sorry, too many clients"/ when trying to connect?
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3.9 <#3.9>) What is in the /pgsql_tmp/ directory?
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3.10 <#3.10>) Why do I need to do a dump and restore to upgrade
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PostgreSQL releases?
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Operational Questions
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4.1 <#4.1>) What is the difference between binary cursors and normal
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cursors?
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4.2 <#4.2>) How do I SELECT only the first few rows of a query?
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4.3 <#4.3>) How do I get a list of tables or other things I can see in
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/psql/?
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4.4 <#4.4>) How do you remove a column from a table?
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4.5 <#4.5>) What is the maximum size for a row, a table, and a database?
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4.6 <#4.6>) How much database disk space is required to store data from
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a typical text file?
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4.7 <#4.7>) How do I find out what tables, indexes, databases, and users
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are defined?
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4.8 <#4.8>) My queries are slow or don't make use of the indexes. Why?
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4.9 <#4.9>) How do I see how the query optimizer is evaluating my query?
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4.10 <#4.10>) What is an R-tree index?
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4.11 <#4.11>) What is the Genetic Query Optimizer?
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4.12 <#4.12>) How do I perform regular expression searches and
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case-insensitive regular expression searches? How do I use an index for
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case-insensitive searches?
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4.13 <#4.13>) In a query, how do I detect if a field is NULL?
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4.14 <#4.14>) What is the difference between the various character types?
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4.15.1 <#4.15.1>) How do I create a serial/auto-incrementing field?
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4.15.2 <#4.15.2>) How do I get the value of a SERIAL insert?
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4.15.3 <#4.15.3>) Don't /currval()/ and /nextval()/ lead to a race
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condition with other users?
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4.15.4 <#4.15.4>) Why aren't my sequence numbers reused on transaction
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abort? Why are there gaps in the numbering of my sequence/SERIAL column?
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4.16 <#4.16>) What is an OID? What is a TID?
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4.17 <#4.17>) What is the meaning of some of the terms used in PostgreSQL?
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4.18 <#4.18>) Why do I get the error /"ERROR: Memory exhausted in
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AllocSetAlloc()"/?
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4.19 <#4.19>) How do I tell what PostgreSQL version I am running?
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4.20 <#4.20>) Why does my large-object operations get /"invalid large
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obj descriptor"/?
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4.21 <#4.21>) How do I create a column that will default to the current
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time?
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4.22 <#4.22>) Why are my subqueries using |IN| so slow?
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4.23 <#4.23>) How do I perform an outer join?
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4.24 <#4.24>) How do I perform queries using multiple databases?
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4.25 <#4.25>) How do I return multiple rows or columns from a function?
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4.26 <#4.26>) Why can't I reliably create/drop temporary tables in
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PL/PgSQL functions?
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4.27 <#4.27>) What replication options are available?
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4.28 <#4.28>) What encryption options are available?
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Extending PostgreSQL
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5.1 <#5.1>) I wrote a user-defined function. When I run it in /psql/,
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why does it dump core?
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5.2 <#5.2>) How can I contribute some nifty new types and functions to
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PostgreSQL?
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5.3 <#5.3>) How do I write a C function to return a tuple?
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5.4 <#5.4>) I have changed a source file. Why does the recompile not see
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the change?
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------------------------------------------------------------------------
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General Questions
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1.1) What is PostgreSQL? How is it pronounced?
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PostgreSQL is pronounced /Post-Gres-Q-L/.
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PostgreSQL is an enhancement of the POSTGRES database management system,
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a next-generation DBMS research prototype. While PostgreSQL retains the
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powerful data model and rich data types of POSTGRES, it replaces the
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PostQuel query language with an extended subset of SQL. PostgreSQL is
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free and the complete source is available.
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PostgreSQL development is performed by a team of developers who all
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subscribe to the PostgreSQL development mailing list. The current
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coordinator is Marc G. Fournier (scrappy@PostgreSQL.org
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<mailto:scrappy@PostgreSQL.org>). (See section 1.6 <#1.6> on how to
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join). This team is now responsible for all development of PostgreSQL.
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The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many
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others have contributed to the porting, testing, debugging, and
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enhancement of the code. The original Postgres code, from which
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PostgreSQL is derived, was the effort of many graduate students,
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undergraduate students, and staff programmers working under the
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direction of Professor Michael Stonebraker at the University of
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California, Berkeley.
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The original name of the software at Berkeley was Postgres. When SQL
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functionality was added in 1995, its name was changed to Postgres95. The
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name was changed at the end of 1996 to PostgreSQL.
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1.2) What is the copyright on PostgreSQL?
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PostgreSQL is subject to the following COPYRIGHT:
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PostgreSQL Data Base Management System
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Portions copyright (c) 1996-2002, PostgreSQL Global Development Group
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Portions Copyright (c) 1994-6 Regents of the University of California
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Permission to use, copy, modify, and distribute this software and its
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documentation for any purpose, without fee, and without a written
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agreement is hereby granted, provided that the above copyright notice
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and this paragraph and the following two paragraphs appear in all copies.
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IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY
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FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES,
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INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS
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DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN ADVISED OF
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THE POSSIBILITY OF SUCH DAMAGE.
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THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES,
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INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
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AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER IS
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ON AN "AS IS" BASIS, AND THE UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS
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TO PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
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The above is the BSD license, the classic open-source license. It has no
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restrictions on how the source code may be used. We like it and have no
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intention of changing it.
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1.3) What Unix platforms does PostgreSQL run on?
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In general, a modern Unix-compatible platform should be able to run
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PostgreSQL. The platforms that had received explicit testing at the time
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of release are listed in the installation instructions.
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1.4) What non-Unix ports are available?
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*Client*
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It is possible to compile the /libpq/ C library, psql, and other
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interfaces and client applications to run on MS Windows platforms. In
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this case, the client is running on MS Windows, and communicates via
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TCP/IP to a server running on one of our supported Unix platforms. A
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file /win32.mak/ is included in the distribution for making a Win32
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/libpq/ library and /psql/. PostgreSQL also communicates with ODBC clients.
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*Server*
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The database server can run on Windows NT and Win2k using Cygwin, the
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Cygnus Unix/NT porting library. See /pgsql/doc/FAQ_MSWIN/ in the
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distribution or the MS Windows FAQ at
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http://www.PostgreSQL.org/docs/faqs/text/FAQ_MSWIN
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<http://www.PostgreSQL.org/docs/faqs/text/FAQ_MSWIN>.
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A native port to MS Win NT/2000/XP is currently being worked on. For
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more details on the current status of PostgreSQL on Windows see
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http://techdocs.postgresql.org/guides/Windows
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<http://techdocs.postgresql.org/guides/Windows>.
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There is also a Novell Netware 6 port at http://forge.novell.com.
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1.5) Where can I get PostgreSQL?
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The primary anonymous ftp site for PostgreSQL is
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ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main web site.
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1.6) Where can I get support?
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The main mailing list is: pgsql-general@PostgreSQL.org
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<mailto:pgsql-general@PostgreSQL.org>. It is available for discussion of
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matters pertaining to PostgreSQL. To subscribe, send mail with the
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following lines in the body (not the subject line):
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subscribe
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end
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to pgsql-general-request@PostgreSQL.org
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<mailto:pgsql-general-request@PostgreSQL.org>.
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There is also a digest list available. To subscribe to this list, send
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email to: pgsql-general-digest-request@PostgreSQL.org
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<mailto:pgsql-general-digest-request@PostgreSQL.org> with a body of:
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subscribe
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end
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Digests are sent out to members of this list whenever the main list has
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received around 30k of messages.
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The bugs mailing list is available. To subscribe to this list, send
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email to pgsql-bugs-request@PostgreSQL.org
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<mailto:pgsql-bugs-request@PostgreSQL.org> with a body of:
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subscribe
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end
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There is also a developers discussion mailing list available. To
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subscribe to this list, send email to
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pgsql-hackers-request@PostgreSQL.org
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<mailto:pgsql-hackers-request@PostgreSQL.org> with a body of:
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subscribe
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end
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Additional mailing lists and information about PostgreSQL can be found
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via the PostgreSQL WWW home page at:
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http://www.PostgreSQL.org
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There is also an IRC channel on EFNet and OpenProjects, channel
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/#PostgreSQL/. I use the Unix command |irc -c '#PostgreSQL' "$USER"
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irc.phoenix.net.|
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A list of commercial support companies is available at
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http://www.ca.PostgreSQL.org/users-lounge/commercial-support.html.
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1.7) What is the latest release?
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The latest release of PostgreSQL is version 7.3.3.
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We plan to have major releases every four months.
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1.8) What documentation is available?
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Several manuals, manual pages, and some small test examples are included
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in the distribution. See the //doc/ directory. You can also browse the
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manuals online at http://www.PostgreSQL.org/docs.
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There are two PostgreSQL books available online at
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http://www.PostgreSQL.org/docs/awbook.html and
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http://www.commandprompt.com/ppbook/. There is a list of PostgreSQL
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books available for purchase at http://www.ca.PostgreSQL.org/books/.
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There is also a collection of PostgreSQL technical articles at
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http://techdocs.PostgreSQL.org/.
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/psql/ has some nice \d commands to show information about types,
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operators, functions, aggregates, etc.
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Our web site contains even more documentation.
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1.9) How do I find out about known bugs or missing features?
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PostgreSQL supports an extended subset of SQL-92. See our TODO
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<http://developer.PostgreSQL.org/todo.php> list for known bugs, missing
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features, and future plans.
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1.10) How can I learn SQL?
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The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html
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teaches SQL. There is another PostgreSQL book at
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http://www.commandprompt.com/ppbook.
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<http://www.commandprompt.com/ppbook/> There is a nice tutorial at
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http://www.intermedia.net/support/sql/sqltut.shtm,
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<http://www.intermedia.net/support/sql/sqltut.shtm> at
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http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,
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<http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM>
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and at http://sqlcourse.com. <http://sqlcourse.com/>
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Another one is "Teach Yourself SQL in 21 Days, Second Edition" at
|
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http://members.tripod.com/er4ebus/sql/index.htm
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Many of our users like /The Practical SQL Handbook/, Bowman, Judith S.,
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et al., Addison-Wesley. Others like /The Complete Reference SQL/, Groff
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et al., McGraw-Hill.
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1.11) Is PostgreSQL Y2K compliant?
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Yes, we easily handle dates past the year 2000 AD, and before 2000 BC.
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1.12) How do I join the development team?
|
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First, download the latest source and read the PostgreSQL Developers
|
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documentation on our web site, or in the distribution. Second, subscribe
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to the /pgsql-hackers/ and /pgsql-patches/ mailing lists. Third, submit
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high quality patches to pgsql-patches.
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There are about a dozen people who have commit privileges to the
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PostgreSQL CVS archive. They each have submitted so many high-quality
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patches that it was impossible for the existing committers to keep up,
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and we had confidence that patches they committed were of high quality.
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1.13) How do I submit a bug report?
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|
|
|||
|
Please visit the PostgreSQL BugTool page at
|
|||
|
http://www.PostgreSQL.org/bugs/bugs.php, which gives guidelines and
|
|||
|
directions on how to submit a bug report.
|
|||
|
|
|||
|
Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if there
|
|||
|
is a more recent PostgreSQL version or patches.
|
|||
|
|
|||
|
|
|||
|
1.14) How does PostgreSQL compare to other DBMSs?
|
|||
|
|
|||
|
There are several ways of measuring software: features, performance,
|
|||
|
reliability, support, and price.
|
|||
|
|
|||
|
*Features*
|
|||
|
PostgreSQL has most features present in large commercial DBMSs, like
|
|||
|
transactions, subselects, triggers, views, foreign key referential
|
|||
|
integrity, and sophisticated locking. We have some features they do
|
|||
|
not have, like user-defined types, inheritance, rules, and
|
|||
|
multi-version concurrency control to reduce lock contention.
|
|||
|
|
|||
|
*Performance*
|
|||
|
PostgreSQL has performance similar to other commercial and open
|
|||
|
source databases. it is faster for some things, slower for others.
|
|||
|
In comparison to MySQL or leaner database systems, we are slower on
|
|||
|
inserts/updates because of transaction overhead. Of course, MySQL
|
|||
|
does not have any of the features mentioned in the /Features/
|
|||
|
section above. We are built for reliability and features, though we
|
|||
|
continue to improve performance in every release. There is an
|
|||
|
interesting Web page comparing PostgreSQL to MySQL at
|
|||
|
http://openacs.org/philosophy/why-not-mysql.html
|
|||
|
<http://openacs.org/philosophy/why-not-mysql.html>
|
|||
|
|
|||
|
*Reliability*
|
|||
|
We realize that a DBMS must be reliable, or it is worthless. We
|
|||
|
strive to release well-tested, stable code that has a minimum of
|
|||
|
bugs. Each release has at least one month of beta testing, and our
|
|||
|
release history shows that we can provide stable, solid releases
|
|||
|
that are ready for production use. We believe we compare favorably
|
|||
|
to other database software in this area.
|
|||
|
|
|||
|
*Support*
|
|||
|
Our mailing lists provide contact with a large group of developers
|
|||
|
and users to help resolve any problems encountered. While we cannot
|
|||
|
guarantee a fix, commercial DBMSs do not always supply a fix either.
|
|||
|
Direct access to developers, the user community, manuals, and the
|
|||
|
source code often make PostgreSQL support superior to other DBMSs.
|
|||
|
There is commercial per-incident support available for those who
|
|||
|
need it. (See FAQ section 1.6 <#1.6>.)
|
|||
|
|
|||
|
*Price*
|
|||
|
We are free for all use, both commercial and non-commercial. You can
|
|||
|
add our code to your product with no limitations, except those
|
|||
|
outlined in our BSD-style license stated above.
|
|||
|
|
|||
|
|
|||
|
1.15) How can I financially assist PostgreSQL?
|
|||
|
|
|||
|
PostgreSQL has had a first-class infrastructure since we started in
|
|||
|
1996. This is all thanks to Marc Fournier, who has created and managed
|
|||
|
this infrastructure over the years.
|
|||
|
|
|||
|
Quality infrastructure is very important to an open-source project. It
|
|||
|
prevents disruptions that can greatly delay forward movement of the project.
|
|||
|
|
|||
|
Of course, this infrastructure is not cheap. There are a variety of
|
|||
|
monthly and one-time expenses that are required to keep it going. If you
|
|||
|
or your company has money it can donate to help fund this effort, please
|
|||
|
go to http://store.pgsql.com/shopping/ and make a donation.
|
|||
|
|
|||
|
Although the web page mentions PostgreSQL, Inc, the "contributions" item
|
|||
|
is solely to support the PostgreSQL project and does not fund any
|
|||
|
specific company. If you prefer, you can also send a check to the
|
|||
|
contact address.
|
|||
|
|
|||
|
------------------------------------------------------------------------
|
|||
|
|
|||
|
|
|||
|
User Client Questions
|
|||
|
|
|||
|
|
|||
|
2.1) Are there ODBC drivers for PostgreSQL?
|
|||
|
|
|||
|
There are two ODBC drivers available, PsqlODBC and OpenLink ODBC.
|
|||
|
|
|||
|
You can download PsqlODBC from
|
|||
|
http://gborg.postgresql.org/project/psqlodbc/projdisplay.php
|
|||
|
<http://gborg.postgresql.org/project/psqlodbc/projdisplay.php>.
|
|||
|
|
|||
|
OpenLink ODBC can be gotten from http://www.openlinksw.com
|
|||
|
<http://www.openlinksw.com/>. It works with their standard ODBC client
|
|||
|
software so you'll have PostgreSQL ODBC available on every client
|
|||
|
platform they support (Win, Mac, Unix, VMS).
|
|||
|
|
|||
|
They will probably be selling this product to people who need
|
|||
|
commercial-quality support, but a freeware version will always be
|
|||
|
available. Please send questions to postgres95@openlink.co.uk
|
|||
|
<mailto:postgres95@openlink.co.uk>.
|
|||
|
|
|||
|
|
|||
|
2.2) What tools are available for using PostgreSQL with Web pages?
|
|||
|
|
|||
|
A nice introduction to Database-backed Web pages can be seen at:
|
|||
|
http://www.webreview.com
|
|||
|
|
|||
|
For Web integration, PHP is an excellent interface. It is at
|
|||
|
http://www.php.net.
|
|||
|
|
|||
|
For complex cases, many use the Perl interface and CGI.pm or mod_perl.
|
|||
|
|
|||
|
|
|||
|
2.3) Does PostgreSQL have a graphical user interface?
|
|||
|
|
|||
|
Yes, there are several graphical interfaces to PostgreSQL available.
|
|||
|
These include PgAccess http://www.pgaccess.org
|
|||
|
<http://www.pgaccess.org>), PgAdmin II (http://www.pgadmin.org,
|
|||
|
Win32-only), RHDB Admin (http://sources.redhat.com/rhdb/ ) and Rekall (
|
|||
|
http://www.thekompany.com/products/rekall/
|
|||
|
<http://www.thekompany.com/products/rekall/>, proprietary). There is
|
|||
|
also PHPPgAdmin ( http://phppgadmin.sourceforge.net/
|
|||
|
<http://phppgadmin.sourceforge.net/>), a web-based interface to PostgreSQL.
|
|||
|
|
|||
|
See http://techdocs.postgresql.org/guides/GUITools for a more detailed list.
|
|||
|
|
|||
|
|
|||
|
2.4) What languages are able to communicate with PostgreSQL?
|
|||
|
|
|||
|
Most popular programming languages contain an interface to PostgreSQL.
|
|||
|
Check your programming language's list of extension modules.
|
|||
|
|
|||
|
The following interfaces are included in the PostgreSQL distribution:
|
|||
|
|
|||
|
* C (libpq)
|
|||
|
* Embedded C (ecpg)
|
|||
|
* Java (jdbc)
|
|||
|
* Python (PyGreSQL)
|
|||
|
* TCL (libpgtcl)
|
|||
|
|
|||
|
Additional interfaces are available at http://gborg.postgresql.org in
|
|||
|
the /Drivers/Interfaces/ section.
|
|||
|
|
|||
|
------------------------------------------------------------------------
|
|||
|
|
|||
|
|
|||
|
Administrative Questions
|
|||
|
|
|||
|
|
|||
|
3.1) How do I install PostgreSQL somewhere other than
|
|||
|
//usr/local/pgsql/?
|
|||
|
|
|||
|
Specify the /--prefix/ option when running /configure/.
|
|||
|
|
|||
|
|
|||
|
3.2) When I start /postmaster/, I get a /Bad System Call/ or
|
|||
|
core dumped message. Why?
|
|||
|
|
|||
|
It could be a variety of problems, but first check to see that you have
|
|||
|
System V extensions installed in your kernel. PostgreSQL requires kernel
|
|||
|
support for shared memory and semaphores.
|
|||
|
|
|||
|
|
|||
|
3.3) When I try to start /postmaster/, I get /IpcMemoryCreate/
|
|||
|
errors. Why?
|
|||
|
|
|||
|
You either do not have shared memory configured properly in your kernel
|
|||
|
or you need to enlarge the shared memory available in the kernel. The
|
|||
|
exact amount you need depends on your architecture and how many buffers
|
|||
|
and backend processes you configure for /postmaster/. For most systems,
|
|||
|
with default numbers of buffers and processes, you need a minimum of ~1
|
|||
|
MB. See the PostgreSQL Administrator's Guide
|
|||
|
<http://www.PostgreSQL.org/docs/view.php?version=current&idoc=1&file=kernel-resources.html>
|
|||
|
for more detailed information about shared memory and semaphores.
|
|||
|
|
|||
|
|
|||
|
3.4) When I try to start /postmaster/, I get
|
|||
|
/IpcSemaphoreCreate/ errors. Why?
|
|||
|
|
|||
|
If the error message is /IpcSemaphoreCreate: semget failed (No space
|
|||
|
left on device)/ then your kernel is not configured with enough
|
|||
|
semaphores. Postgres needs one semaphore per potential backend process.
|
|||
|
A temporary solution is to start /postmaster/ with a smaller limit on
|
|||
|
the number of backend processes. Use /-N/ with a parameter less than the
|
|||
|
default of 32. A more permanent solution is to increase your kernel's
|
|||
|
SEMMNS and SEMMNI parameters.
|
|||
|
|
|||
|
Inoperative semaphores can also cause crashes during heavy database access.
|
|||
|
|
|||
|
If the error message is something else, you might not have semaphore
|
|||
|
support configured in your kernel at all. See the PostgreSQL
|
|||
|
Administrator's Guide for more detailed information about shared memory
|
|||
|
and semaphores.
|
|||
|
|
|||
|
|
|||
|
3.5) How do I control connections from other hosts?
|
|||
|
|
|||
|
By default, PostgreSQL only allows connections from the local machine
|
|||
|
using Unix domain sockets. Other machines will not be able to connect
|
|||
|
unless you add the /-i/ flag to /postmaster/, *and* enable host-based
|
|||
|
authentication by modifying the file /$PGDATA/pg_hba.conf/ accordingly.
|
|||
|
This will allow TCP/IP connections.
|
|||
|
|
|||
|
|
|||
|
3.6) How do I tune the database engine for better performance?
|
|||
|
|
|||
|
Certainly, indexes can speed up queries. The EXPLAIN command allows you
|
|||
|
to see how PostgreSQL is interpreting your query, and which indexes are
|
|||
|
being used.
|
|||
|
|
|||
|
If you are doing many INSERTs, consider doing them in a large batch
|
|||
|
using the COPY command. This is much faster than individual INSERTS.
|
|||
|
Second, statements not in a BEGIN WORK/COMMIT transaction block are
|
|||
|
considered to be in their own transaction. Consider performing several
|
|||
|
statements in a single transaction block. This reduces the transaction
|
|||
|
overhead. Also, consider dropping and recreating indexes when making
|
|||
|
large data changes.
|
|||
|
|
|||
|
There are several tuning options. You can disable /fsync()/ by starting
|
|||
|
/postmaster/ with a /-o -F/ option. This will prevent /fsync()/s from
|
|||
|
flushing to disk after every transaction.
|
|||
|
|
|||
|
You can also use the /postmaster/ /-B/ option to increase the number of
|
|||
|
shared memory buffers used by the backend processes. If you make this
|
|||
|
parameter too high, the /postmaster/ may not start because you have
|
|||
|
exceeded your kernel's limit on shared memory space. Each buffer is 8K
|
|||
|
and the default is 64 buffers.
|
|||
|
|
|||
|
You can also use the backend /-S/ option to increase the maximum amount
|
|||
|
of memory used by the backend process for temporary sorts. The /-S/
|
|||
|
value is measured in kilobytes, and the default is 512 (i.e. 512K).
|
|||
|
|
|||
|
You can also use the CLUSTER command to group data in tables to match an
|
|||
|
index. See the CLUSTER manual page for more details.
|
|||
|
|
|||
|
|
|||
|
3.7) What debugging features are available?
|
|||
|
|
|||
|
PostgreSQL has several features that report status information that can
|
|||
|
be valuable for debugging purposes.
|
|||
|
|
|||
|
First, by running /configure/ with the --enable-cassert option, many
|
|||
|
/assert()/s monitor the progress of the backend and halt the program
|
|||
|
when something unexpected occurs.
|
|||
|
|
|||
|
Both /postmaster/ and /postgres/ have several debug options available.
|
|||
|
First, whenever you start /postmaster/, make sure you send the standard
|
|||
|
output and error to a log file, like:
|
|||
|
|
|||
|
cd /usr/local/pgsql
|
|||
|
./bin/postmaster >server.log 2>&1 &
|
|||
|
|
|||
|
This will put a server.log file in the top-level PostgreSQL directory.
|
|||
|
This file contains useful information about problems or errors
|
|||
|
encountered by the server. /Postmaster/ has a /-d/ option that allows
|
|||
|
even more detailed information to be reported. The /-d/ option takes a
|
|||
|
number that specifies the debug level. Be warned that high debug level
|
|||
|
values generate large log files.
|
|||
|
|
|||
|
If /postmaster/ is not running, you can actually run the /postgres/
|
|||
|
backend from the command line, and type your SQL statement directly.
|
|||
|
This is recommended *only* for debugging purposes. Note that a newline
|
|||
|
terminates the query, not a semicolon. If you have compiled with
|
|||
|
debugging symbols, you can use a debugger to see what is happening.
|
|||
|
Because the backend was not started from /postmaster/, it is not running
|
|||
|
in an identical environment and locking/backend interaction problems may
|
|||
|
not be duplicated.
|
|||
|
|
|||
|
If /postmaster/ is running, start /psql/ in one window, then find the
|
|||
|
PID of the /postgres/ process used by /psql/. Use a debugger to attach
|
|||
|
to the /postgres/ PID. You can set breakpoints in the debugger and issue
|
|||
|
queries from /psql/. If you are debugging /postgres/ startup, you can
|
|||
|
set PGOPTIONS="-W n", then start /psql/. This will cause startup to
|
|||
|
delay for /n/ seconds so you can attach to the process with the
|
|||
|
debugger, set any breakpoints, and continue through the startup sequence.
|
|||
|
|
|||
|
The /postgres/ program has /-s, -A/, and /-t/ options that can be very
|
|||
|
useful for debugging and performance measurements.
|
|||
|
|
|||
|
You can also compile with profiling to see what functions are taking
|
|||
|
execution time. The backend profile files will be deposited in the
|
|||
|
/pgsql/data/base/dbname/ directory. The client profile file will be put
|
|||
|
in the client's current directory. Linux requires a compile with
|
|||
|
/-DLINUX_PROFILE/ for proper profiling.
|
|||
|
|
|||
|
|
|||
|
3.8) Why do I get /"Sorry, too many clients"/ when trying to
|
|||
|
connect?
|
|||
|
|
|||
|
You need to increase /postmaster/'s limit on how many concurrent backend
|
|||
|
processes it can start.
|
|||
|
|
|||
|
The default limit is 32 processes. You can increase it by restarting
|
|||
|
/postmaster/ with a suitable /-N/ value or modifying /postgresql.conf/.
|
|||
|
|
|||
|
Note that if you make /-N/ larger than 32, you must also increase /-B/
|
|||
|
beyond its default of 64; /-B/ must be at least twice /-N/, and probably
|
|||
|
should be more than that for best performance. For large numbers of
|
|||
|
backend processes, you are also likely to find that you need to increase
|
|||
|
various Unix kernel configuration parameters. Things to check include
|
|||
|
the maximum size of shared memory blocks, SHMMAX; the maximum number of
|
|||
|
semaphores, SEMMNS and SEMMNI; the maximum number of processes, NPROC;
|
|||
|
the maximum number of processes per user, MAXUPRC; and the maximum
|
|||
|
number of open files, NFILE and NINODE. The reason that PostgreSQL has a
|
|||
|
limit on the number of allowed backend processes is so your system won't
|
|||
|
run out of resources.
|
|||
|
|
|||
|
|
|||
|
3.9) What is in the /pgsql_tmp/ directory?
|
|||
|
|
|||
|
This directory contains temporary files generated by the query executor.
|
|||
|
For example, if a sort needs to be done to satisfy an ORDER BY and the
|
|||
|
sort requires more space than the backend's /-S/ parameter allows, then
|
|||
|
temporary files are created here to hold the extra data.
|
|||
|
|
|||
|
The temporary files are usually deleted automatically, but might remain
|
|||
|
if a backend crashes during a sort. A stop and restart of the
|
|||
|
/postmaster/ will remove files from those directories.
|
|||
|
|
|||
|
|
|||
|
3.10) Why do I need to do a dump and restore to upgrade between
|
|||
|
major PostgreSQL releases?
|
|||
|
|
|||
|
The PostgreSQL team makes only small changes between minor releases, so
|
|||
|
upgrading from 7.2 to 7.2.1 does not require a dump and restore.
|
|||
|
However, major releases (e.g. from 7.2 to 7.3) often change the internal
|
|||
|
format of system tables and data files. These changes are often complex,
|
|||
|
so we don't maintain backward compatability for data files. A dump
|
|||
|
outputs data in a generic format that can then be loaded in using the
|
|||
|
new internal format.
|
|||
|
|
|||
|
In releases where the on-disk format does not change, the /pg_upgrade/
|
|||
|
script can be used to upgrade without a dump/restore. The release notes
|
|||
|
mention whether /pg_upgrade/ is available for the release.
|
|||
|
|
|||
|
------------------------------------------------------------------------
|
|||
|
|
|||
|
|
|||
|
Operational Questions
|
|||
|
|
|||
|
|
|||
|
4.1) What is the difference between binary cursors and normal
|
|||
|
cursors?
|
|||
|
|
|||
|
See the DECLARE manual page for a description.
|
|||
|
|
|||
|
|
|||
|
4.2) How do I SELECT only the first few rows of a query?
|
|||
|
|
|||
|
See the FETCH manual page, or use SELECT ... LIMIT....
|
|||
|
|
|||
|
The entire query may have to be evaluated, even if you only want the
|
|||
|
first few rows. Consider using a query that has an ORDER BY. If there is
|
|||
|
an index that matches the ORDER BY, PostgreSQL may be able to evaluate
|
|||
|
only the first few records requested, or the entire query may have to be
|
|||
|
evaluated until the desired rows have been generated.
|
|||
|
|
|||
|
|
|||
|
4.3) How do I get a list of tables or other things I can see in
|
|||
|
/psql/?
|
|||
|
|
|||
|
You can read the source code for /psql/ in file
|
|||
|
/pgsql/src/bin/psql/describe.c/. It contains SQL commands that generate
|
|||
|
the output for psql's backslash commands. You can also start /psql/ with
|
|||
|
the /-E/ option so it will print out the queries it uses to execute the
|
|||
|
commands you give.
|
|||
|
|
|||
|
|
|||
|
4.4) How do you remove a column from a table?
|
|||
|
|
|||
|
This functionality was added in release 7.3 with ALTER TABLE DROP
|
|||
|
COLUMN. In earlier versions, you can do this:
|
|||
|
|
|||
|
BEGIN;
|
|||
|
LOCK TABLE old_table;
|
|||
|
SELECT ... -- select all columns but the one you want to remove
|
|||
|
INTO TABLE new_table
|
|||
|
FROM old_table;
|
|||
|
DROP TABLE old_table;
|
|||
|
ALTER TABLE new_table RENAME TO old_table;
|
|||
|
COMMIT;
|
|||
|
|
|||
|
|
|||
|
4.5) What is the maximum size for a row, a table, and a database?
|
|||
|
|
|||
|
These are the limits:
|
|||
|
|
|||
|
Maximum size for a database? unlimited (4 TB databases exist)
|
|||
|
Maximum size for a table? 16 TB
|
|||
|
Maximum size for a row? 1.6TB
|
|||
|
Maximum size for a field? 1 GB
|
|||
|
Maximum number of rows in a table? unlimited
|
|||
|
Maximum number of columns in a table? 250-1600 depending on column types
|
|||
|
Maximum number of indexes on a table? unlimited
|
|||
|
|
|||
|
Of course, these are not actually unlimited, but limited to available
|
|||
|
disk space and memory/swap space. Performance may suffer when these
|
|||
|
values get unusually large.
|
|||
|
|
|||
|
The maximum table size of 16 TB does not require large file support from
|
|||
|
the operating system. Large tables are stored as multiple 1 GB files so
|
|||
|
file system size limits are not important.
|
|||
|
|
|||
|
The maximum table size and maximum number of columns can be increased if
|
|||
|
the default block size is increased to 32k.
|
|||
|
|
|||
|
|
|||
|
4.6) How much database disk space is required to store data from
|
|||
|
a typical text file?
|
|||
|
|
|||
|
A PostgreSQL database may require up to five times the disk space to
|
|||
|
store data from a text file.
|
|||
|
|
|||
|
As an example, consider a file of 100,000 lines with an integer and text
|
|||
|
description on each line. Suppose the text string avergages twenty bytes
|
|||
|
in length. The flat file would be 2.8 MB. The size of the PostgreSQL
|
|||
|
database file containing this data can be estimated as 6.4 MB:
|
|||
|
|
|||
|
36 bytes: each row header (approximate)
|
|||
|
24 bytes: one int field and one text field
|
|||
|
+ 4 bytes: pointer on page to tuple
|
|||
|
----------------------------------------
|
|||
|
64 bytes per row
|
|||
|
|
|||
|
The data page size in PostgreSQL is 8192 bytes (8 KB), so:
|
|||
|
|
|||
|
8192 bytes per page
|
|||
|
------------------- = 128 rows per database page (rounded down)
|
|||
|
64 bytes per row
|
|||
|
|
|||
|
100000 data rows
|
|||
|
-------------------- = 782 database pages (rounded up)
|
|||
|
128 rows per page
|
|||
|
|
|||
|
782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
|
|||
|
|
|||
|
Indexes do not require as much overhead, but do contain the data that is
|
|||
|
being indexed, so they can be large also.
|
|||
|
|
|||
|
NULLs are stored in bitmaps, so they use very little space.
|
|||
|
|
|||
|
|
|||
|
4.7) How do I find out what tables, indexes, databases, and
|
|||
|
users are defined?
|
|||
|
|
|||
|
/psql/ has a variety of backslash commands to show such information. Use
|
|||
|
\? to see them. There are also system tables beginning with /pg_/ that
|
|||
|
describe these too. Also, /psql -l/ will list all databases.
|
|||
|
|
|||
|
Also try the file /pgsql/src/tutorial/syscat.source/. It illustrates
|
|||
|
many of the SELECTs needed to get information from the database system
|
|||
|
tables.
|
|||
|
|
|||
|
|
|||
|
4.8) My queries are slow or don't make use of the indexes. Why?
|
|||
|
|
|||
|
Indexes are not automatically used by every query. Indexes are only used
|
|||
|
if the table is larger than a minimum size, and the query selects only a
|
|||
|
small percentage of the rows in the table. This is because the random
|
|||
|
disk access caused by an index scan can be slower than a straight read
|
|||
|
through the table, or sequential scan.
|
|||
|
|
|||
|
To determine if an index should be used, PostgreSQL must have statistics
|
|||
|
about the table. These statistics are collected using VACUUM ANALYZE, or
|
|||
|
simply ANALYZE. Using statistics, the optimizer knows how many rows are
|
|||
|
in the table, and can better determine if indexes should be used.
|
|||
|
Statistics are also valuable in determining optimal join order and join
|
|||
|
methods. Statistics collection should be performed periodically as the
|
|||
|
contents of the table change.
|
|||
|
|
|||
|
Indexes are normally not used for ORDER BY or to perform joins. A
|
|||
|
sequential scan followed by an explicit sort is usually faster than an
|
|||
|
index scan of a large table.
|
|||
|
|
|||
|
However, LIMIT combined with ORDER BY often will use an index because
|
|||
|
only a small portion of the table is returned. In fact, though MAX() and
|
|||
|
MIN() don't use indexes, it is possible to retrieve such values using an
|
|||
|
index with ORDER BY and LIMIT:
|
|||
|
|
|||
|
SELECT col
|
|||
|
FROM tab
|
|||
|
ORDER BY col [ DESC ]
|
|||
|
LIMIT 1;
|
|||
|
|
|||
|
If you believe the optimizer is incorrect in choosing a sequential scan,
|
|||
|
use |SET enable_seqscan TO 'off'| and run tests to see if an index scan
|
|||
|
is indeed faster.
|
|||
|
|
|||
|
When using wild-card operators such as LIKE or /~/, indexes can only be
|
|||
|
used in certain circumstances:
|
|||
|
|
|||
|
* The beginning of the search string must be anchored to the start
|
|||
|
of the string, i.e.
|
|||
|
o LIKE patterns must not start with /%/.
|
|||
|
o /~/ (regular expression) patterns must start with /^/.
|
|||
|
* The search string can not start with a character class, e.g. [a-e].
|
|||
|
* Case-insensitive searches such as ILIKE and /~*/ do not utilise
|
|||
|
indexes. Instead, use functional indexes, which are described in
|
|||
|
section 4.12 <#4.12>.
|
|||
|
* The default /C/ locale must be used during /initdb/.
|
|||
|
|
|||
|
|
|||
|
4.9) How do I see how the query optimizer is evaluating my query?
|
|||
|
|
|||
|
See the EXPLAIN manual page.
|
|||
|
|
|||
|
|
|||
|
4.10) What is an R-tree index?
|
|||
|
|
|||
|
An R-tree index is used for indexing spatial data. A hash index can't
|
|||
|
handle range searches. A B-tree index only handles range searches in a
|
|||
|
single dimension. R-trees can handle multi-dimensional data. For
|
|||
|
example, if an R-tree index can be built on an attribute of type
|
|||
|
/point/, the system can more efficiently answer queries such as "select
|
|||
|
all points within a bounding rectangle."
|
|||
|
|
|||
|
The canonical paper that describes the original R-tree design is:
|
|||
|
|
|||
|
Guttman, A. "R-trees: A Dynamic Index Structure for Spatial Searching."
|
|||
|
Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt of Data, 45-57.
|
|||
|
|
|||
|
You can also find this paper in Stonebraker's "Readings in Database
|
|||
|
Systems".
|
|||
|
|
|||
|
Built-in R-trees can handle polygons and boxes. In theory, R-trees can
|
|||
|
be extended to handle higher number of dimensions. In practice,
|
|||
|
extending R-trees requires a bit of work and we don't currently have any
|
|||
|
documentation on how to do it.
|
|||
|
|
|||
|
|
|||
|
4.11) What is the Genetic Query Optimizer?
|
|||
|
|
|||
|
The GEQO module speeds query optimization when joining many tables by
|
|||
|
means of a Genetic Algorithm (GA). It allows the handling of large join
|
|||
|
queries through nonexhaustive search.
|
|||
|
|
|||
|
|
|||
|
4.12) How do I perform regular expression searches and
|
|||
|
case-insensitive regular expression searches? How do I use an
|
|||
|
index for case-insensitive searches?
|
|||
|
|
|||
|
The /~/ operator does regular expression matching, and /~*/ does
|
|||
|
case-insensitive regular expression matching. The case-insensitive
|
|||
|
variant of LIKE is called ILIKE.
|
|||
|
|
|||
|
Case-insensitive equality comparisons are normally expressed as:
|
|||
|
|
|||
|
SELECT *
|
|||
|
FROM tab
|
|||
|
WHERE lower(col) = 'abc';
|
|||
|
|
|||
|
This will not use an standard index. However, if you create a functional
|
|||
|
index, it will be used:
|
|||
|
|
|||
|
CREATE INDEX tabindex ON tab (lower(col));
|
|||
|
|
|||
|
|
|||
|
4.13) In a query, how do I detect if a field is NULL?
|
|||
|
|
|||
|
You test the column with IS NULL and IS NOT NULL.
|
|||
|
|
|||
|
|
|||
|
4.14) What is the difference between the various character types?
|
|||
|
|
|||
|
Type Internal Name Notes
|
|||
|
--------------------------------------------------
|
|||
|
VARCHAR(n) varchar size specifies maximum length, no padding
|
|||
|
CHAR(n) bpchar blank padded to the specified fixed length
|
|||
|
TEXT text no specific upper limit on length
|
|||
|
BYTEA bytea variable-length byte array (null-byte safe)
|
|||
|
"char" char one character
|
|||
|
|
|||
|
You will see the internal name when examining system catalogs and in
|
|||
|
some error messages.
|
|||
|
|
|||
|
The first four types above are "varlena" types (i.e., the first four
|
|||
|
bytes on disk are the length, followed by the data). Thus the actual
|
|||
|
space used is slightly greater than the declared size. However, these
|
|||
|
data types are also subject to compression or being stored out-of-line
|
|||
|
by TOAST, so the space on disk might also be less than expected.
|
|||
|
|
|||
|
VARCHAR(n) is best when storing variable-length strings and it limits
|
|||
|
how long a string can be. TEXT is for strings of unlimited length, with
|
|||
|
a maximum of one gigabyte.
|
|||
|
|
|||
|
CHAR(n) is for storing strings that are all the same length. CHAR(n)
|
|||
|
pads with blanks to the specified length, while VARCHAR(n) only stores
|
|||
|
the characters supplied. BYTEA is for storing binary data, particularly
|
|||
|
values that include NULL bytes. All the types described here have
|
|||
|
similar performance characteristics.
|
|||
|
|
|||
|
|
|||
|
4.15.1) How do I create a serial/auto-incrementing field?
|
|||
|
|
|||
|
PostgreSQL supports a SERIAL data type. It auto-creates a sequence and
|
|||
|
index on the column. For example, this:
|
|||
|
|
|||
|
CREATE TABLE person (
|
|||
|
id SERIAL,
|
|||
|
name TEXT
|
|||
|
);
|
|||
|
|
|||
|
is automatically translated into this:
|
|||
|
|
|||
|
CREATE SEQUENCE person_id_seq;
|
|||
|
CREATE TABLE person (
|
|||
|
id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
|
|||
|
name TEXT
|
|||
|
);
|
|||
|
CREATE UNIQUE INDEX person_id_key ON person ( id );
|
|||
|
|
|||
|
See the /create_sequence/ manual page for more information about
|
|||
|
sequences. You can also use each row's /OID/ field as a unique value.
|
|||
|
However, if you need to dump and reload the database, you need to use
|
|||
|
/pg_dump/'s /-o/ option or COPY WITH OIDS option to preserve the OIDs.
|
|||
|
|
|||
|
|
|||
|
4.15.2) How do I get the value of a SERIAL insert?
|
|||
|
|
|||
|
One approach is to retrieve the next SERIAL value from the sequence
|
|||
|
object with the /nextval()/ function /before/ inserting and then insert
|
|||
|
it explicitly. Using the example table in 4.15.1 <#4.15.1>, an example
|
|||
|
in a pseudo-language would look like this:
|
|||
|
|
|||
|
new_id = execute("SELECT nextval('person_id_seq')");
|
|||
|
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
|
|||
|
|
|||
|
You would then also have the new value stored in |new_id| for use in
|
|||
|
other queries (e.g., as a foreign key to the |person| table). Note that
|
|||
|
the name of the automatically created SEQUENCE object will be named
|
|||
|
</table/>_</serialcolumn/>_/seq/, where /table/ and /serialcolumn/ are
|
|||
|
the names of your table and your SERIAL column, respectively.
|
|||
|
|
|||
|
Alternatively, you could retrieve the assigned SERIAL value with the
|
|||
|
/currval()/ function /after/ it was inserted by default, e.g.,
|
|||
|
|
|||
|
execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
|
|||
|
new_id = execute("SELECT currval('person_id_seq')");
|
|||
|
|
|||
|
Finally, you could use the OID <#4.16> returned from the INSERT
|
|||
|
statement to look up the default value, though this is probably the
|
|||
|
least portable approach. In Perl, using DBI with Edmund Mergl's DBD::Pg
|
|||
|
module, the oid value is made available via /$sth->{pg_oid_status}/
|
|||
|
after /$sth->execute()/.
|
|||
|
|
|||
|
|
|||
|
4.15.3) Don't /currval()/ and /nextval()/ lead to a race
|
|||
|
condition with other users?
|
|||
|
|
|||
|
No. /currval()/ returns the current value assigned by your backend, not
|
|||
|
by all users.
|
|||
|
|
|||
|
|
|||
|
4.15.4) Why aren't my sequence numbers reused on transaction
|
|||
|
abort? Why are there gaps in the numbering of my sequence/SERIAL
|
|||
|
column?
|
|||
|
|
|||
|
To improve concurrency, sequence values are given out to running
|
|||
|
transactions as needed and are not locked until the transaction
|
|||
|
completes. This causes gaps in numbering from aborted transactions.
|
|||
|
|
|||
|
|
|||
|
4.16) What is an OID? What is a TID?
|
|||
|
|
|||
|
OIDs are PostgreSQL's answer to unique row ids. Every row that is
|
|||
|
created in PostgreSQL gets a unique OID. All OIDs generated during
|
|||
|
/initdb/ are less than 16384 (from /include/access/transam.h/). All
|
|||
|
user-created OIDs are equal to or greater than this. By default, all
|
|||
|
these OIDs are unique not only within a table or database, but unique
|
|||
|
within the entire PostgreSQL installation.
|
|||
|
|
|||
|
PostgreSQL uses OIDs in its internal system tables to link rows between
|
|||
|
tables. These OIDs can be used to identify specific user rows and used
|
|||
|
in joins. It is recommended you use column type OID to store OID values.
|
|||
|
You can create an index on the OID field for faster access.
|
|||
|
|
|||
|
OIDs are assigned to all new rows from a central area that is used by
|
|||
|
all databases. If you want to change the OID to something else, or if
|
|||
|
you want to make a copy of the table, with the original OIDs, there is
|
|||
|
no reason you can't do it:
|
|||
|
|
|||
|
CREATE TABLE new_table(old_oid oid, mycol int);
|
|||
|
SELECT old_oid, mycol INTO new FROM old;
|
|||
|
COPY new TO '/tmp/pgtable';
|
|||
|
DELETE FROM new;
|
|||
|
COPY new WITH OIDS FROM '/tmp/pgtable';
|
|||
|
|
|||
|
OIDs are stored as 4-byte integers, and will overflow at 4 billion. No
|
|||
|
one has reported this ever happening, and we plan to have the limit
|
|||
|
removed before anyone does.
|
|||
|
|
|||
|
TIDs are used to identify specific physical rows with block and offset
|
|||
|
values. TIDs change after rows are modified or reloaded. They are used
|
|||
|
by index entries to point to physical rows.
|
|||
|
|
|||
|
|
|||
|
4.17) What is the meaning of some of the terms used in PostgreSQL?
|
|||
|
|
|||
|
Some of the source code and older documentation use terms that have more
|
|||
|
common usage. Here are some:
|
|||
|
|
|||
|
* table, relation, class
|
|||
|
* row, record, tuple
|
|||
|
* column, field, attribute
|
|||
|
* retrieve, select
|
|||
|
* replace, update
|
|||
|
* append, insert
|
|||
|
* OID, serial value
|
|||
|
* portal, cursor
|
|||
|
* range variable, table name, table alias
|
|||
|
|
|||
|
A list of general database terms can be found at:
|
|||
|
http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html
|
|||
|
|
|||
|
|
|||
|
4.18) Why do I get the error /"ERROR: Memory exhausted in
|
|||
|
AllocSetAlloc()"/?
|
|||
|
|
|||
|
You probably have run out of virtual memory on your system, or your
|
|||
|
kernel has a low limit for certain resources. Try this before starting
|
|||
|
/postmaster/:
|
|||
|
|
|||
|
ulimit -d 262144
|
|||
|
limit datasize 256m
|
|||
|
|
|||
|
Depending on your shell, only one of these may succeed, but it will set
|
|||
|
your process data segment limit much higher and perhaps allow the query
|
|||
|
to complete. This command applies to the current process, and all
|
|||
|
subprocesses created after the command is run. If you are having a
|
|||
|
problem with the SQL client because the backend is returning too much
|
|||
|
data, try it before starting the client.
|
|||
|
|
|||
|
|
|||
|
4.19) How do I tell what PostgreSQL version I am running?
|
|||
|
|
|||
|
From /psql/, type |SELECT version();|
|
|||
|
|
|||
|
|
|||
|
4.20) Why does my large-object operations get /"invalid large
|
|||
|
obj descriptor"/?
|
|||
|
|
|||
|
You need to put |BEGIN WORK| and |COMMIT| around any use of a large
|
|||
|
object handle, that is, surrounding |lo_open| ... |lo_close.|
|
|||
|
|
|||
|
Currently PostgreSQL enforces the rule by closing large object handles
|
|||
|
at transaction commit. So the first attempt to do anything with the
|
|||
|
handle will draw /invalid large obj descriptor/. So code that used to
|
|||
|
work (at least most of the time) will now generate that error message if
|
|||
|
you fail to use a transaction.
|
|||
|
|
|||
|
If you are using a client interface like ODBC you may need to set
|
|||
|
|auto-commit off.|
|
|||
|
|
|||
|
|
|||
|
4.21) How do I create a column that will default to the current
|
|||
|
time?
|
|||
|
|
|||
|
Use /CURRENT_TIMESTAMP/:
|
|||
|
|
|||
|
|CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
|
|||
|
|
|
|||
|
|
|||
|
|
|||
|
4.22) Why are my subqueries using |IN| so slow?
|
|||
|
|
|||
|
Currently, we join subqueries to outer queries by sequentially scanning
|
|||
|
the result of the subquery for each row of the outer query. If the
|
|||
|
subquery returns only a few rows and the outer query returns many rows,
|
|||
|
|IN| is fastest. To speed up other queries, replace |IN| with |EXISTS|:
|
|||
|
|
|||
|
SELECT *
|
|||
|
FROM tab
|
|||
|
WHERE col IN (SELECT subcol FROM subtab);
|
|||
|
|
|||
|
to:
|
|||
|
|
|||
|
SELECT *
|
|||
|
FROM tab
|
|||
|
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
|
|||
|
|
|||
|
For this to be fast, |subcol| should be an indexed column. This
|
|||
|
preformance problem will be fixed in 7.4.
|
|||
|
|
|||
|
|
|||
|
4.23) How do I perform an outer join?
|
|||
|
|
|||
|
PostgreSQL supports outer joins using the SQL standard syntax. Here are
|
|||
|
two examples:
|
|||
|
|
|||
|
SELECT *
|
|||
|
FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
|
|||
|
|
|||
|
or
|
|||
|
|
|||
|
SELECT *
|
|||
|
FROM t1 LEFT OUTER JOIN t2 USING (col);
|
|||
|
|
|||
|
These identical queries join t1.col to t2.col, and also return any
|
|||
|
unjoined rows in t1 (those with no match in t2). A RIGHT join would add
|
|||
|
unjoined rows of t2. A FULL join would return the matched rows plus all
|
|||
|
unjoined rows from t1 and t2. The word OUTER is optional and is assumed
|
|||
|
in LEFT, RIGHT, and FULL joins. Ordinary joins are called INNER joins.
|
|||
|
|
|||
|
In previous releases, outer joins can be simulated using UNION and NOT
|
|||
|
IN. For example, when joining /tab1/ and /tab2/, the following query
|
|||
|
does an /outer/ join of the two tables:
|
|||
|
|
|||
|
SELECT tab1.col1, tab2.col2
|
|||
|
FROM tab1, tab2
|
|||
|
WHERE tab1.col1 = tab2.col1
|
|||
|
UNION ALL
|
|||
|
SELECT tab1.col1, NULL
|
|||
|
FROM tab1
|
|||
|
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
|
|||
|
ORDER BY col1
|
|||
|
|
|||
|
|
|||
|
4.24) How do I perform queries using multiple databases?
|
|||
|
|
|||
|
There is no way to query a database other than the current one. Because
|
|||
|
PostgreSQL loads database-specific system catalogs, it is uncertain how
|
|||
|
a cross-database query should even behave.
|
|||
|
|
|||
|
/contrib/dblink/ allows cross-database queries using function calls. Of
|
|||
|
course, a client can make simultaneous connections to different
|
|||
|
databases and merge the results on the client side.
|
|||
|
|
|||
|
|
|||
|
4.25) How do I return multiple rows or columns from a function?
|
|||
|
|
|||
|
In 7.3, you can easily return multiple rows or columns from a function,
|
|||
|
http://techdocs.postgresql.org/guides/SetReturningFunctions
|
|||
|
<http://techdocs.postgresql.org/guides/SetReturningFunctions>.
|
|||
|
|
|||
|
|
|||
|
4.26) Why can't I reliably create/drop temporary tables in
|
|||
|
PL/PgSQL functions?
|
|||
|
|
|||
|
PL/PgSQL caches function contents, and an unfortunate side effect is
|
|||
|
that if a PL/PgSQL function accesses a temporary table, and that table
|
|||
|
is later dropped and recreated, and the function called again, the
|
|||
|
function will fail because the cached function contents still point to
|
|||
|
the old temporary table. The solution is to use EXECUTE for temporary
|
|||
|
table access in PL/PgSQL. This will cause the query to be reparsed every
|
|||
|
time.
|
|||
|
|
|||
|
|
|||
|
4.27) What replication options are available?
|
|||
|
|
|||
|
There are several master/slave replication options available. These
|
|||
|
allow only the master to make database changes and the slave can only do
|
|||
|
database reads. The bottom of
|
|||
|
http://gborg.PostgreSQL.org/genpage?replication_research
|
|||
|
<http://gborg.PostgreSQL.org/genpage?replication_research> lists them. A
|
|||
|
multi-master replication solution is being worked on at
|
|||
|
http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php.
|
|||
|
|
|||
|
|
|||
|
4.28) What encryption options are available?
|
|||
|
|
|||
|
* /contrib/pgcrypto/ contains many encryption functions for use in
|
|||
|
SQL queries.
|
|||
|
* The only way to encrypt transmission from the client to the server
|
|||
|
is by using /hostssl/ in /pg_hba.conf/.
|
|||
|
* Database user passwords are automatically encrypted when stored in
|
|||
|
version 7.3. In previous versions, you must enable the option
|
|||
|
/PASSWORD_ENCRYPTION/ in /postgresql.conf/.
|
|||
|
* The server can run using an encrypted file system.
|
|||
|
|
|||
|
------------------------------------------------------------------------
|
|||
|
|
|||
|
|
|||
|
Extending PostgreSQL
|
|||
|
|
|||
|
|
|||
|
5.1) I wrote a user-defined function. When I run it in /psql/,
|
|||
|
why does it dump core?
|
|||
|
|
|||
|
The problem could be a number of things. Try testing your user-defined
|
|||
|
function in a stand-alone test program first.
|
|||
|
|
|||
|
|
|||
|
5.2) How can I contribute some nifty new types and functions to
|
|||
|
PostgreSQL?
|
|||
|
|
|||
|
Send your extensions to the /pgsql-hackers/ mailing list, and they will
|
|||
|
eventually end up in the /contrib// subdirectory.
|
|||
|
|
|||
|
|
|||
|
5.3) How do I write a C function to return a tuple?
|
|||
|
|
|||
|
In versions of PostgreSQL beginning with 7.3, table-returning functions
|
|||
|
are fully supported in C, PL/PgSQL, and SQL. See the Programmer's Guide
|
|||
|
for more information. An example of a table-returning function defined
|
|||
|
in C can be found in /contrib/tablefunc/.
|
|||
|
|
|||
|
|
|||
|
5.4) I have changed a source file. Why does the recompile not
|
|||
|
see the change?
|
|||
|
|
|||
|
The /Makefiles/ do not have the proper dependencies for include files.
|
|||
|
You have to do a /make clean/ and then another /make/. If you are using
|
|||
|
GCC you can use the /--enable-depend/ option of /configure/ to have the
|
|||
|
compiler compute the dependencies automatically.
|
|||
|
|