<P>The most recent version of this document can be viewed at <Ahref="http://www.Postgresql.org/docs/faq-english.html">http://www.PostgreSQL.org/docs/faq-english.html</A>.</P>
<P>PostgreSQL is an enhancement of the POSTGRES database management system, a next-generation DBMS research prototype. While PostgreSQL retains the powerful data model and rich data types of POSTGRES, it replaces the PostQuel query language with an extended subset of SQL. PostgreSQL is free and the complete source is available.</P>
<P>PostgreSQL development is performed by a team of Internet developers who all subscribe to the PostgreSQL development mailing list. The current coordinator is Marc G. Fournier (<Ahref="mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See below on how to join). This team is now responsible for all development of PostgreSQL.</P>
<P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many others have contributed to the porting, testing, debugging, and enhancement of the code. The original Postgres code, from which PostgreSQL is derived, was the effort of many graduate students, undergraduate students, and staff programmers working under the direction of Professor Michael Stonebraker at the University of California, Berkeley.</P>
<P>The original name of the software at Berkeley was Postgres. When SQL functionality was added in 1995, its name was changed to Postgres95. The name was changed at the end of 1996 to PostgreSQL.</P>
<P>Permission to use, copy, modify, and distribute this software and its documentation for any purpose, without fee, and without a written agreement is hereby granted, provided that the above copyright notice and this paragraph and the following two paragraphs appear in all copies.</P>
<P>IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.</P>
<P>THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
<P>In general, a modern Unix-compatible platform should be able to run PostgreSQL. The platforms that had received explicit testing at the time of release are listed in the installation instructions.</P>
<P>It is possible to compile the <I>libpq</I> C library, psql, and other interfaces and binaries to run on MS Windows platforms. In this case, the client is running on MS Windows, and communicates via TCP/IP to a server running on one of our supported Unix platforms. A file <I>win31.mak</I> is included in the distribution for making a Win32 <I>libpq</I> library and psql.</P>
<P>The database server can run on Windows NT and later using Cygwin, the Cygnus Unix/NT porting library. See <I>pgsql/doc/FAQ_MSWIN</I> in the distribution. The database server does not run on MS Windows 9X because Cygwin does not support the required features on those platforms. We have no plans to do a native port to any Microsoft platform.</P>
<P>The primary anonymous ftp site for PostgreSQL is <Ahref="ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>. For mirror sites, see our main Web site.</P>
<P>The main mailing list is: <Ahref="mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>. It is available for discussion of matters pertaining to PostgreSQL. To subscribe, send mail with the following lines in the body (not the subject line)</P>
<P>There is also a digest list available. To subscribe to this list, send email to: <Ahref="mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A> with a body of:</P>
<P>The bugs mailing list is available. To subscribe to this list, send email to <Ahref="mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A> with a body of:</P>
There is also a developers discussion mailing list available. To subscribe to this list, send email to <Ahref="mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A> with a body of:
<P>There is also an IRC channel on EFNet, channel <I>#PostgreSQL.</I> I use the unix command <CODE>irc -c '#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
<P>A list of commercial PostgreSQL support is available at <Ahref="http://www.postgresql.org/users-lounge/commercial-support.html">http://www.postgresql.org/users-lounge/commercial-support.html</A>.</P>
<P>Several manuals, manual pages, and some small test examples are included in the distribution. See the <I>/doc</I> directory. You can also browse the manual online at <Ahref="http://www.PostgreSQL.org/docs/postgres">http://www.PostgreSQL.org/docs/postgres</A>.</P>
<P>PostgreSQL supports an extended subset of SQL-92. See our <Ahref="http://www.PostgreSQL.org/docs/todo.html">TODO</A> list for known bugs, missing features, and future plans.</P>
<P>The PostgreSQL book at <Ahref="http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A> teaches SQL. There is a nice tutorial at <Ahref="http://w3.one.net/~jhoffman/sqltut.htm">http://w3.one.net/~jhoffman/sqltut.htm</A> and at <Ahref="http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM.</A></P>
<P>Another one is "Teach Yourself SQL in 21 Days, Second Edition" at <Ahref="http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
<P>Many of our users like <I>The Practical SQL Handbook</I>, Bowman, Judith S., et al., Addison-Wesley. Others like <I>The Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
<P>First, download the latest source and read the PostgreSQL Developers documentation on our Web site, or in the distribution. Second, subscribe to the <I>pgsql-hackers</I> and <I>pgsql-patches</I> mailing lists. Third, submit high-quality patches to pgsql-patches.</P>
<P>There are about a dozen people who have commit privileges to the PostgreSQL CVS archive. They each have submitted so many high-quality patches that it was impossible for the existing committers to keep up, and we had confidence that patches they committed were of high quality.</P>
<P>Also check out our ftp site <Ahref="ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to see if there is a more recent PostgreSQL version or patches.</P>
<DD>PostgreSQL has most features present in large commercial DBMS's, like transactions, subselects, triggers, views, foreign key referential integrity, and sophisticated locking. We have some features they don't have, like user-defined types, inheritance, rules, and multi-version concurrency control to reduce lock contention. We don't have outer joins, but are working on them.<BR>
<DD>PostgreSQL runs in two modes. Normal <I>fsync</I> mode flushes every completed transaction to disk, guaranteeing that if the OS crashes or loses power in the next few seconds, all your data is safely stored on disk. In this mode, we are slower than most commercial databases, partly because few of them do such conservative flushing to disk in their default modes. In <I>no-fsync</I> mode, we are usually faster than commercial databases, though in this mode, an OS crash could cause data corruption. We are working to provide an intermediate mode that suffers less performance overhead than full fsync mode, and will allow data integrity within 30 seconds of an OS crash.<BR>
In comparison to MySQL or leaner database systems, we are slower on inserts/updates because we have transaction overhead. Of course, MySQL doesn't have any of the features mentioned in the <I>Features</I> section above. We are built for flexibility and features, though we continue to improve performance through profiling and source code analysis. There is an interesting Web page comparing PostgreSQL to MySQL at <Ahref="http://openacs.org/why-not-mysql.html">http://openacs.org/why-not-mysql.html</A><BR>
We handle each user connection by creating a Unix process. Backend processes share data buffers and locking information. With multiple CPU's, multiple backends can easily run on different CPU's.<BR>
<DD>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.<BR>
<DD>Our mailing list provides a large group of developers and users to help resolve any problems encountered. While we can not guarantee a fix, commercial DBMS's don't always supply a fix either. Direct access to developers, the user community, manuals, and the source code often make PostgreSQL support superior to other DBMS's. There is commercial per-incident support available for those who need it. (See support FAQ item.)<BR>
<DD>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.<BR>
<P>PsqlODBC is included in the distribution. More information about it can be gotten from <Ahref="ftp://ftp.PostgreSQL.org/pub/odbc/">ftp://ftp.PostgreSQL.org/pub/odbc/</A>.</P>
<P>OpenLink ODBC can be gotten from <Ahref="http://www.openlinksw.com/">http://www.openlinksw.com</A>. 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).</P>
<P>They will probably be selling this product to people who need commercial-quality support, but a freeware version will always be available. Questions to <Ahref="mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
<P>A WWW gateway based on WDB using Perl can be downloaded from <Ahref="http://www.eol.ists.ca/~dunlop/wdb-p95">http://www.eol.ists.ca/~dunlop/wdb-p95</A></P>
<P>We have a nice graphical user interface called <I>pgaccess,</I> which is shipped as part of the distribution. <I>Pgaccess</I> also has a report generator. The Web page is <Ahref="http://www.flex.ro/pgaccess">http://www.flex.ro/pgaccess</A></P>
<H4><Aname="3.2">3.2</A>) When I start the <I>postmaster</I>, I get a <I>Bad System Call</I> or core dumped message. Why?</H4>
<P>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.</P>
<H4><Aname="3.3">3.3</A>) When I try to start the <I>postmaster,</I> I get <I>IpcMemoryCreate</I> errors. Why?</H4>
<P>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 the <I>postmaster.</I> For most systems, with default numbers of buffers and processes, you need a minimum of ~1MB.</P>
<H4><Aname="3.4">3.4</A>) When I try to start the <I>postmaster,</I> I get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
<P>If the error message is <I>IpcSemaphoreCreate: semget failed (No space left on device)</I> then your kernel is not configured with enough semaphores. Postgres needs one semaphore per potential backend process. A temporary solution is to start the <I>postmaster</I> with a smaller limit on the number of backend processes. Use <I>-N</I> with a parameter less than the default of 32. A more permanent solution is to increase your kernel's <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
<P>If the error message is something else, you might not have semaphore support configured in your kernel at all.</P>
<H4><Aname="3.5">3.5</A>) How do I prevent other hosts from accessing my PostgreSQL database?</H4>
<P>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>-i</I> flag to the <I>postmaster,</I><B>and</B> enable host-based authentication by modifying the file <I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP connections.</P>
<H4><Aname="3.6">3.6</A>) Why can't I connect to my database from another machine?</H4>
<P>The default configuration allows only unix domain socket connections from the local machine. To enable TCP/IP connections, make sure the <I>postmaster</I> has been started with the <I>-i</I> option, and add an appropriate host entry to the file <I>pgsql/data/pg_hba.conf</I>.</P>
<H4><Aname="3.7">3.7</A>) All my servers crash under concurrent table access. Why?</H4>
<P>This problem can be caused by a kernel that is not configured to support semaphores.</P>
<H4><Aname="3.8">3.8</A>) How do I tune the database engine for better performance?</H4>
<P>Certainly, indices can speed up queries. The <SMALL>EXPLAIN</SMALL> command allows you to see how PostgreSQL is interpreting your query, and which indices are being used.</P>
<P>If you are doing a lot of <SMALL>INSERTs</SMALL>, consider doing them in a large batch using the <SMALL>COPY</SMALL> command. This is much faster than individual <SMALL>INSERTS.</SMALL> Second, statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> 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 indices when making large data changes.</P>
<P>There are several tuning options. You can disable <I>fsync()</I> by starting the <I>postmaster</I> with a <I>-o -F</I> option. This will prevent <I>fsync()'s</I> from flushing to disk after every transaction.</P>
<P>You can also use the <I>postmaster</I><I>-B</I> option to increase the number of shared memory buffers used by the backend processes. If you make this parameter too high, the <I>postmaster</I> may not start because you've exceeded your kernel's limit on shared memory space. Each buffer is 8K and the default is 64 buffers.</P>
<P>You can also use the backend <I>-S</I> option to increase the maximum amount of memory used by the backend process for temporary sorts. The <I>-S</I> value is measured in kilobytes, and the default is 512 (ie, 512K).</P>
<P>You can also use the <SMALL>CLUSTER</SMALL> command to group data in tables to match an index. See the <SMALL>CLUSTER</SMALL> manual page for more details.</P>
<H4><Aname="3.9">3.9</A>) What debugging features are available?</H4>
<P>PostgreSQL has several features that report status information that can be valuable for debugging purposes.</P>
<P>First, by running <I>configure</I> with the --enable-cassert option, many <I>assert()'s</I> monitor the progress of the backend and halt the program when something unexpected occurs.</P>
<P>Both <I>postmaster</I> and <I>postgres</I> have several debug options available. First, whenever you start the <I>postmaster,</I> make sure you send the standard output and error to a log file, like:</P>
<P>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. <I>Postmaster</I> has a <I>-d</I> option that allows even more detailed information to be reported. The <I>-d</I> option takes a number that specifies the debug level. Be warned that high debug level values generate large log files.</P>
<P>If the <I>postmaster</I> is not running, you can actually run the <I>postgres</I> backend from the command line, and type your SQL statement directly. This is recommended <B>only</B> 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 the <I>postmaster,</I> it is not running in an identical environment and locking/backend interaction problems may not be duplicated.</P>
<P>If the <I>postmaster</I> is running, start <I>psql</I> in one window, then find the <SMALL>PID</SMALL> of the <I>postgres</I> process used by <I>psql.</I> Use a debugger to attach to the <I>postgres</I><SMALL>PID.</SMALL> You can set breakpoints in the debugger and issue queries from <I>psql.</I> If you are debugging <I>postgres</I> startup, you can set PGOPTIONS="-W n", then start <I>psql.</I> This will cause startup to delay for <I>n</I> seconds so you can attach with the debugger and trace through the startup sequence.</P>
<P>The <I>postgres</I> program has <I>-s, -A,</I> and <I>-t</I> options that can be very useful for debugging and performance measurements.</P>
<P>You can also compile with profiling to see what functions are taking execution time. The backend profile files will be deposited in the <I>pgsql/data/base/dbname</I> directory. The client profile file will be put in the client's current directory.</P>
<H4><Aname="3.10">3.10</A>) I get 'Sorry, too many clients' when trying to connect. Why?</H4>
<P>You need to increase the <I>postmaster's</I> limit on how many concurrent backend processes it can start.</P>
<P>In PostgreSQL 6.5 and up, the default limit is 32 processes. You can increase it by restarting the <I>postmaster</I> with a suitable <I>-N</I> value. With the default configuration you can set <I>-N</I> as large as 1024. If you need more, increase <SMALL>MAXBACKENDS</SMALL> in <I>include/config.h</I> and rebuild. You can set the default value of <I>-N</I> at configuration time, if you like, using <I>configure's</I><I>--with-maxbackends</I> switch.</P>
<P>Note that if you make <I>-N</I> larger than 32, you must also increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at least twice <I>-N,</I> 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, <SMALL>SHMMAX;</SMALL> the maximum number of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the maximum number of processes, <SMALL>NPROC;</SMALL> the maximum number of processes per user, <SMALL>MAXUPRC;</SMALL> and the maximum number of open files, <SMALL>NFILE</SMALL> and <SMALL>NINODE.</SMALL> The reason that PostgreSQL has a limit on the number of allowed backend processes is so your system won't run out of resources.</P>
<P>In PostgreSQL versions prior to 6.5, the maximum number of backends was 64, and changing it required a rebuild after altering the MaxBackendId constant in <I>include/storage/sinvaladt.h.</I></P>
<H4><Aname="3.11">3.11</A>) What are the <I>pg_sorttempNNN.NN</I> files in my database directory?</H4>
<P>They are temporary files generated by the query executor. For example, if a sort needs to be done to satisfy an <SMALL>ORDER BY,</SMALL> and the sort requires more space than the backend's <I>-S</I> parameter allows, then temporary files are created to hold the extra data.</P>
<P>The temporary files should be deleted automatically, but might not if a backend crashes during a sort. If you have no backends running at the time, it is safe to delete the pg_tempNNN.NN files.</P>
<P>Check your locale configuration. PostgreSQL uses the locale setting of the user that ran the <I>postmaster</I> process. There are postgres and psql SET commands to control the date format. Set those accordingly for your operating environment.</P>
<P>The entire query may have to be evaluated, even if you only want the first few rows. Consider a query that has an <SMALL>ORDER BY.</SMALL> If there is an index that matches the <SMALL>ORDER BY</SMALL>, 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.</P>
<P>You can read the source code for <I>psql</I> in file <I>pgsql/src/bin/psql/describe.c.</I> It contains SQL commands that generate the output for psql's backslash commands. You can also start <I>psql</I> with the <I>-E</I> option so it will print out the queries it uses to execute the commands you give.</P>
<P>Consider a file of 300,000 lines with two integers on each line. The flat file is 2.4MB. The size of the PostgreSQL database file containing this data can be estimated at 14MB:</P>
<P>Also try the file <I>pgsql/src/tutorial/syscat.source.</I> It illustrates many of the <SMALL>SELECT</SMALL>s needed to get information from the database system tables.</P>
<P>PostgreSQL does not automatically maintain statistics. V<SMALL>ACUUM</SMALL> must be run to update the statistics. After statistics are updated, the optimizer knows how many rows in the table, and can better decide if it should use indices. Note that the optimizer does not use indices in cases when the table is small because a sequential scan would be faster.</P>
<P>For column-specific optimization statistics, use <SMALL>VACUUM ANALYZE.</SMALL><SMALL>VACUUM ANALYZE</SMALL> is important for complex multijoin queries, so the optimizer can estimate the number of rows returned from each table, and choose the proper join order. The backend does not keep track of column statistics on its own, so <SMALL>VACUUM ANALYZE</SMALL> must be run to collect them periodically.</P>
<P>Indexes are usually not used for <SMALL>ORDER BY</SMALL> operations: a sequential scan followed by an explicit sort is faster than an indexscan of all tuples of a large table, because it takes fewer disk accesses.</P>
<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or <I>~,</I> indices can only be used if the beginning of the search is anchored to the start of the string. So, to use indices, <SMALL>LIKE</SMALL> searches should not begin with <I>%,</I> and <I>~</I>(regular expression searches) should start with <I>^.</I></P>
<P>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-tree's can handle multi-dimensional data. For example, if an R-tree index can be built on an attribute of type <I>point,</I> the system can more efficiently answer queries such as "select all points within a bounding rectangle."</P>
<P>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.</P>
<P>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.</P>
<P>The <I>~</I> operator does regular expression matching, and <I>~*</I> does case-insensitive regular expression matching. There is no case-insensitive variant of the LIKE operator, but you can get the effect of case-insensitive <SMALL>LIKE</SMALL> with this:</P>
<P>The last 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.</P>
See the <I>create_sequence</I> manual page for more information about sequences. You can also use each row's <I>OID</I> field as a unique value. However, if you need to dump and reload the database, you need to use <I>pg_dump's -o</I> option or <SMALL>COPY WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
<H4><Aname="4.16.2">4.16.2</A>) How do I get the value of a <SMALL>SERIAL</SMALL> insert?</H4>
<P>One approach is to to retrieve the next SERIAL value from the sequence object with the <I>nextval()</I> function <I>before</I> inserting and then insert it explicitly. Using the example table in <Ahref="#4.16.1">4.16.1</A>, that might look like this:</P>
You would then also have the new value stored in <CODE>$newSerialID</CODE> for use in other queries (e.g., as a foreign key to the <CODE>person</CODE> table). Note that the name of the automatically created SEQUENCE object will be named <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where <I>table</I> and <I>serialcolumn</I> are the names of your table and your SERIAL column, respectively.
<P>Alternatively, you could retrieve the assigned SERIAL value with the <I>currval</I>() function <I>after</I> it was inserted by default, e.g.,</P>
Finally, you could use the <Ahref="#4.17"><SMALL>OID</SMALL></A> returned from the INSERT statement to look up the default value, though this is probably the least portable approach. In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid value is made available via <I>$sth->{pg_oid_status} after $sth->execute().</I>
<H4><Aname="4.17">4.17</A>) What is an <SMALL>OID</SMALL>? What is a <SMALL>TID</SMALL>?</H4>
<P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids. Every row that is created in PostgreSQL gets a unique <SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during <I>initdb</I> are less than 16384 (from <I>backend/access/transam.h</I>). All user-created <SMALL>OID</SMALL>s are equal to or greater than this. By default, all these <SMALL>OID</SMALL>s are unique not only within a table or database, but unique within the entire PostgreSQL installation.</P>
<P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system tables to link rows between tables. These <SMALL>OID</SMALL>s can be used to identify specific user rows and used in joins. It is recommended you use column type <SMALL>OID</SMALL> to store <SMALL>OID</SMALL> values. You can create an index on the <SMALL>OID</SMALL> field for faster access.</P>
<P>O<SMALL>id</SMALL>s are assigned to all new rows from a central area that is used by all databases. If you want to change the <SMALL>OID</SMALL> to something else, or if you want to make a copy of the table, with the original <SMALL>OID</SMALL>'s, there is no reason you can't do it:</P>
<P>O<SMALL>ID</SMALL>s 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.</P>
<P>T<SMALL>ID</SMALL>s 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.</P>
<P>A list of general database terms can be found at: <Ahref="http://www.comptechnews.com/~reaster/dbdesign.html">http://www.comptechnews.com/~reaster/dbdesign.html</A></P>
<P>It is possible you have run out of virtual memory on your system, or your kernel has a low limit for certain resources. Try this before starting the <I>postmaster:</I></P>
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.
<H4><Aname="4.20">4.20</A>) How do I tell what PostgreSQL version I am running?<BR>
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE> around any use of a large object handle, that is, surrounding <CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
<P>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 <I>invalid large obj descriptor.</I> 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.</P>
<P>Currently, we join subqueries to outer queries by sequentially scanning the result of the subquery for each row of the outer query. A workaround is to replace <CODE>IN</CODE> with <CODE>EXISTS</CODE>:</P>
<P>PostgreSQL 7.1 and later supports outer joins. In previous releases, outer joins can be simulated using <SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when joining <I>tab1</I> and <I>tab2,</I> the following query does an <I>outer</I> join of the two tables:</P>
<P>The <I>Makefiles</I> do not have the proper dependencies for include files. You have to do a <I>make clean</I> and then another <I>make</I>. If you are using GCC you can use the <I>--enable-depend</I> option of <I>configure</I> to have the compiler compute the dependencies automatically.</P>
