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Attribute missing values might be needed past the lifetime of the tuple descriptors from which they are extracted. To avoid possibly using pointers for by-reference values which might thus be left dangling, we cache a datumCopy'd version of the datum in the TopMemoryContext. Since we first search for the value this only needs to be done once per session for any such value. Original complaint from Tom Lane, idea for mitigation by Andrew Dunstan, tweaked by Tom Lane. Backpatch to version 11 where missing values were introduced. Discussion: https://postgr.es/m/1306569.1687978174@sss.pgh.pa.us
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. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ 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. 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 https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.