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Peter Geoghegan 92f375056c Fix nbtree maximum item size macro. 
Commit dd299df818, 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
2022-08-04 20:55:02 -07:00
config Unify DLSUFFIX on Darwin 2022-07-06 07:41:33 +02:00
contrib Remove dead pread and pwrite replacement code. 2022-08-05 09:49:21 +12:00
doc Remove configure probes for symlink/readlink, and dead code. 2022-08-05 09:22:56 +12:00
src Fix nbtree maximum item size macro. 2022-08-04 20:55:02 -07:00
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configure Simplify replacement code for preadv and pwritev. 2022-08-05 14:04:02 +12:00
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README 

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/.