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Dean Rasheed e954a727f0 Improve the accuracy of floating point statistical aggregates. 
When computing statistical aggregates like variance, the common
schoolbook algorithm which computes the sum of the squares of the
values and subtracts the square of the mean can lead to a large loss
of precision when using floating point arithmetic, because the
difference between the two terms is often very small relative to the
terms themselves.

To avoid this, re-work these aggregates to use the Youngs-Cramer
algorithm, which is a proven, numerically stable algorithm that
directly aggregates the sum of the squares of the differences of the
values from the mean in a single pass over the data.

While at it, improve the test coverage to test the aggregate combine
functions used during parallel aggregation.

Per report and suggested algorithm from Erich Schubert.

Patch by me, reviewed by Madeleine Thompson.

Discussion: https://postgr.es/m/153313051300.1397.9594490737341194671@wrigleys.postgresql.org
 		2018-10-06 11:20:09 +01:00
config Try another way to detect the result type of strerror_r(). 2018-09-26 18:23:13 -04:00
contrib Allow btree comparison functions to return INT_MIN. 2018-10-05 16:01:29 -04:00
doc doc: update PG 11 release notes 2018-10-05 17:20:24 -04:00
src Improve the accuracy of floating point statistical aggregates. 2018-10-06 11:20:09 +01:00
.dir-locals.el Update documentation editor setup instructions 2018-07-13 21:23:41 +02:00
.gitattributes Remove contrib/tsearch2. 2017-02-13 11:06:11 -05:00
.gitignore Support for optimizing and emitting code in LLVM JIT provider. 2018-03-22 11:05:22 -07:00
COPYRIGHT Update copyright for 2018 2018-01-02 23:30:12 -05:00
GNUmakefile.in Remove unwanted "garbage cleanup" logic in Makefiles. 2018-08-08 14:32:29 -04:00
HISTORY Change documentation references to PG website to use https: not http: 2017-05-20 21:50:47 -04:00
Makefile Fix non-GNU makefiles for AIX make. 2017-11-30 00:57:22 -08:00
README Change documentation references to PG website to use https: not http: 2017-05-20 21:50:47 -04:00
README.git Change documentation references to PG website to use https: not http: 2017-05-20 21:50:47 -04:00
aclocal.m4 Add configure infrastructure (--with-llvm) to enable LLVM support. 2018-03-20 17:26:25 -07:00
configure Make assorted performance improvements in snprintf.c. 2018-10-03 10:18:15 -04:00
configure.in Make assorted performance improvements in snprintf.c. 2018-10-03 10:18:15 -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:

	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/.