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Tom Lane 31f38f28b0 Redesign the planner's handling of index-descent cost estimation. 
Historically we've used a couple of very ad-hoc fudge factors to try to
get the right results when indexes of different sizes would satisfy a
query with the same number of index leaf tuples being visited.  In
commit 21a39de580 I tweaked one of these
fudge factors, with results that proved disastrous for larger indexes.
Commit bf01e34b55 fudged it some more,
but still with not a lot of principle behind it.

What seems like a better way to address these issues is to explicitly model
index-descent costs, since that's what's really at stake when considering
diferent indexes with similar leaf-page-level costs.  We tried that once
long ago, and found that charging random_page_cost per page descended
through was way too much, because upper btree levels tend to stay in cache
in real-world workloads.  However, there's still CPU costs to think about,
and the previous fudge factors can be seen as a crude attempt to account
for those costs.  So this patch replaces those fudge factors with explicit
charges for the number of tuple comparisons needed to descend the index
tree, plus a small charge per page touched in the descent.  The cost
multipliers are chosen so that the resulting charges are in the vicinity of
the historical (pre-9.2) fudge factors for indexes of up to about a million
tuples, while not ballooning unreasonably beyond that, as the old fudge
factor did (even more so in 9.2).

To make this work accurately for btree indexes, add some code that allows
extraction of the known root-page height from a btree.  There's no
equivalent number readily available for other index types, but we can use
the log of the number of index pages as an approximate substitute.

This seems like too much of a behavioral change to risk back-patching,
but it should improve matters going forward.  In 9.2 I'll just revert
the fudge-factor change.
2013-01-11 12:56:58 -05:00
config Detect Windows perl linkage parameters in configure script. 2013-01-09 17:49:23 -05:00
contrib Allow parallel copy/link in pg_upgrade 2013-01-09 08:57:47 -05:00
doc Allow parallel copy/link in pg_upgrade 2013-01-09 08:57:47 -05:00
src Redesign the planner's handling of index-descent cost estimation. 2013-01-11 12:56:58 -05:00
.gitignore Add gitignore for mingw/cygwin build outputs 2011-06-09 18:11:47 +02:00
aclocal.m4 Remove cvs keywords from all files. 2010-09-20 22:08:53 +02:00
configure Last-gasp attempt to save libperl.so configure probe. 2013-01-10 22:16:22 -05:00
configure.in Last-gasp attempt to save libperl.so configure probe. 2013-01-10 22:16:22 -05:00
COPYRIGHT Update copyrights for 2013 2013-01-01 17:15:01 -05:00
GNUmakefile.in Make init-po and update-po recursive make targets 2012-06-29 14:01:54 +03:00
Makefile Allow make check in PL directories 2011-02-15 06:52:12 +02:00
README Remove useless whitespace at end of lines 2010-11-23 22:34:55 +02:00
README.git Trivial typo fix. 2010-09-21 14:16:00 -04:00

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:

	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.  Changes between all PostgreSQL releases are recorded in the
file HISTORY.  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/.