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* Merge xmin/xmax/cmin/cmax back into three header fields Before subtransactions, there used to be only three fields needed to store these four values. This was possible because only the current transaction looks at the cmin/cmax values. If the current transaction created and expired the row the fields stored where xmin (same as xmax), cmin, cmax, and if the transaction was expiring a row from a another transaction, the fields stored were xmin (cmin was not needed), xmax, and cmax. Such a system worked because a transaction could only see committed rows from another transaction. However, subtransactions can see rows from outer transactions, and once the subtransaction completes, the outer transaction continues, requiring the storage of all four fields. With subtransactions, an outer transaction can create a row, a subtransaction expire it, and when the subtransaction completes, the outer transaction still has to have proper visibility of the row, for example, for cursors. One possible solution is to create a phantom cid which represents a cmin/cmax pair and is stored in local memory.
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. The JDBC, ODBC, C++, Python, and Tcl interfaces have been moved to the PostgreSQL Projects Web Site at http://gborg.postgresql.org for separate maintenance. A Perl DBI/DBD driver is available from CPAN. See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Changes between all PostgreSQL releases are recorded in the file HISTORY. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at ftp://ftp.postgresql.org/pub/. For more information look at our web site located at http://www.postgresql.org/.