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Commit dd299df8189, which made heap TID a tiebreaker nbtree index column, introduced new rules on page space management to make suffix truncation safe for v4+ indexes. New pivot tuples (generated by suffix truncation during leaf page splits) sometimes require dedicated extra space at the end of a new leaf page high key/pivot to store a heap TID using a special representation (a representation only used in pivot tuples). The definition of "1/3 of a page" was reduced by a single MAXALIGN() quantum for v4 indexes to make sure that the final enlarged pivot tuple always fit, even with a split point whose firstright tuple happened to already be at the "1/3 of a page" limit (limit for non-pivot tuples). Internal pages (which only contain pivot tuples) stuck with the original "1/3 of a page" definition. This scheme made it impossible for any page split to fail to free enough space for its newitem, which is never okay. The macro that determines whether non-pivot tuples exceed their "1/3 of a leaf page" restriction was structured as if space was needed for all three tuples during a leaf page split (the new pivot plus two very large adjoining non-pivots that are separated by the split). This was subtly wrong, in that it accidentally relied on implementation details that could (at least in theory) change in the future. To fix, make the macro subtract a single MAXALIGN() quantum, once. The macro evaluates to exactly the same value as before in practice. But it no longer depends on the current layout of nbtree's special area struct. No backpatch, since this isn't a live bug. Author: Peter Geoghegan <pg@bowt.ie> Reported-By: Robert Haas <robertmhaas@gmail.com> Diagnosed-By: Robert Haas <robertmhaas@gmail.com> Discussion: https://postgr.es/m/CA+Tgmoa7UBxivM7f6Ocx_qbq4=ky3uXc+WZNOBcVX+kvJvWOEA@mail.gmail.com
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/.