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This allows squeezing out the unused space in full-page writes. And more importantly, it can be a useful debugging aid. In hindsight we should've done this back when GIN was added - we wouldn't need the 'maxoff' field in the page opaque struct if we had used pd_lower and pd_upper like on normal pages. But as long as there can be pages in the index that have been binary-upgraded from pre-9.4 versions, we can't rely on that, and have to continue using 'maxoff'. Most of the code churn comes from renaming some macros, now that they're used on internal pages, too. This change is completely backwards-compatible, no effect on pg_upgrade.
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: http://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 http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.