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Andres Freund d4c62a6b62 Make simplehash.h grow hashtable in additional cases. 
Increase the size when either the distance between actual and optimal
slot grows too large, or when too many subsequent entries would have
to be moved.

This addresses reports that the simplehash performed, sometimes
considerably, worse than dynahash.

The reason turned out to be that insertions into the hashtable where,
due to the use of parallel query, in effect done from another
hashtable, in hash-value order.  If the target hashtable, due to
mis-estimation, was sized a lot smaller than the source table(s) that
lead to very imbalanced tables; a lot of entries in many close-by
buckets from the source tables were inserted into a single, wider,
bucket on the target table.  As the growth factor was solely computed
based on the fillfactor, the performance of the table decreased
further and further.

b81b5a96f4 was an attempt to address this problem for hash
aggregates (but not for bitmap scans), but it turns out that the
current method of mixing hash values often actually leaves neighboring
hash-values close to each other, just in different value range.  It
might be worth revisiting that independently of the performance issues
addressed in this patch..

To address that problem resize tables in two additional cases: Firstly
when the optimal position for an entry would be far from the actual
position, secondly when many entries would have to be moved to make
space for the new entry (while satisfying the robin hood property).

Due to the additional resizing threshold it seems possible, and
testing confirms that so far, that a higher fillfactor doesn't hurt
performance and saves a bit of memory.  It seems better to increase it
now, before a release containing any of this code, rather than wonder
in some later release.

The various boundaries aren't determined in a particularly scientific
manner, they might need some fine-tuning.

In all my tests the new code now, even with parallelism, performs at
least as good as the old code, in several scenarios significantly
better.

Reported-By: Dilip Kumar, Robert Haas, Kuntal Ghosh
Discussion:
    https://postgr.es/m/CAFiTN-vagvuAydKG9VnWcoK=ADAhxmOa4ZTrmNsViBBooTnriQ@mail.gmail.com
    https://postgr.es/m/CAGz5QC+=fNTYgzMLTBUNeKt6uaWZFXJbkB5+7oWm-n9DwVxcLA@mail.gmail.com
2017-03-06 14:13:06 -08:00
config Remove some configure header-file checks that we weren't really using. 2017-02-25 18:10:09 -05:00
contrib Move atooid() definition to a central place 2017-03-01 11:55:28 -05:00
doc Combine several DROP variants into generic DropStmt 2017-03-06 13:31:47 -05:00
src Make simplehash.h grow hashtable in additional cases. 2017-03-06 14:13:06 -08:00
.dir-locals.el emacs: Set indent-tabs-mode in perl-mode 2015-04-12 23:53:23 -04:00
.gitattributes Remove contrib/tsearch2. 2017-02-13 11:06:11 -05:00
.gitignore Allow .so minor version numbers above 9 in .gitignore. 2016-08-15 17:35:35 -04:00
aclocal.m4 Replace our hacked version of ax_pthread.m4 with latest upstream version. 2015-07-08 20:36:06 +03:00
configure Remove some configure header-file checks that we weren't really using. 2017-02-25 18:10:09 -05:00
configure.in Remove some configure header-file checks that we weren't really using. 2017-02-25 18:10:09 -05:00
COPYRIGHT Update copyright for 2017 2017-01-03 12:37:53 -05:00
GNUmakefile.in Have "make coverage" recurse into contrib as well 2016-09-05 18:44:36 -03:00
HISTORY Improve text of stub HISTORY file. 2014-02-12 18:16:17 -05:00
Makefile Allow make check in PL directories 2011-02-15 06:52:12 +02:00
README Don't generate plain-text HISTORY and src/test/regress/README anymore. 2014-02-10 20:48:04 -05:00
README.git Don't generate plain-text HISTORY and src/test/regress/README anymore. 2014-02-10 20:48:04 -05:00

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.  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/.