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Peter Geoghegan 1542e16f2c Consider outliers in split interval calculation. 
Commit 0d861bbb, which introduced deduplication to nbtree, added some
logic to take large posting list tuples into account when choosing a
split point.  We subtract firstright posting list overhead from the
projected new high key size when calculating leftfree/rightfree values
for an affected candidate split point.  Posting list tuples aren't
special to nbtsplitloc.c, but taking them into account like this makes a
huge difference in practice.  Posting list tuples are frequently tuple
size outliers.

However, commit 0d861bbb missed a closely related issue: split interval
itself is calculated based on the assumption that tuples on the page
being split are roughly equisized.  That assumption was acceptable back
when commit fab25024 taught the logic for choosing a split point about
suffix truncation, but it's pretty questionable now that very large
tuple sizes are common.  This oversight led to unbalanced page splits in
low cardinality multi-column indexes when deduplication was used: page
splits that don't give sufficient weight to how unbalanced the split is
when the interval happens to include some large posting list tuples (and
when most other tuples on the page are not so large).

Nail this down by calculating an initial split interval in a way that's
attuned to the actual cost that we want to keep under control (not a
fuzzy proxy for the cost): apply a leftfree + rightfree evenness test to
each candidate split point that actually gets included in the split
interval (for the default strategy).  This replaces logic that used a
percentage of all legal split points for the page as the basis of the
initial split interval.

Discussion: https://postgr.es/m/CAH2-WznJt5aT2uUB2Bs+JBLdwe0XTX67+xeLFcaNvCKxO=QBVQ@mail.gmail.com
 		2020-04-21 09:59:24 -07:00
config Enable Unix-domain sockets support on Windows 2020-03-28 15:01:01 +01:00
contrib Fix cache reference leak in contrib/sepgsql. 2020-04-16 14:45:54 -04:00
doc Mention pg_promote() as a method to trigger promotion in documentation. 2020-04-21 14:05:43 +09:00
src Consider outliers in split interval calculation. 2020-04-21 09:59:24 -07:00
.dir-locals.el Make Emacs perl-mode indent more like perltidy. 2019-01-13 11:32:31 -08:00
.editorconfig Add .editorconfig 2019-12-18 09:13:13 +01:00
.gitattributes gitattributes: Add new file 2019-11-12 08:13:55 +01:00
.gitignore Support for optimizing and emitting code in LLVM JIT provider. 2018-03-22 11:05:22 -07:00
COPYRIGHT Update copyrights for 2020 2020-01-01 12:21:45 -05:00
GNUmakefile.in Add support for automatically updating Unicode derived files 2020-01-09 10:08:14 +01:00
HISTORY Canonicalize some URLs 2020-02-10 20:47:50 +01:00
Makefile Don't unset MAKEFLAGS in non-GNU Makefile. 2019-06-25 09:36:21 +12:00
README Canonicalize some URLs 2020-02-10 20:47:50 +01:00
README.git Canonicalize some URLs 2020-02-10 20:47:50 +01:00
aclocal.m4 Fix configure's AC_CHECK_DECLS tests to work correctly with clang. 2018-11-19 12:01:47 -05:00
configure Enable Unix-domain sockets support on Windows 2020-03-28 15:01:01 +01:00
configure.in Define EXEC_BACKEND in pg_config_manual.h 2020-03-25 14:31:14 +01:00

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/.