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Tom Lane fc8adddd9a Improve ANALYZE's handling of concurrent-update scenarios. 
This patch changes the rule for whether or not a tuple seen by ANALYZE
should be included in its sample.

When we last touched this logic, in commit 51e1445f1, we weren't
thinking very hard about tuples being UPDATEd by a long-running
concurrent transaction.  In such a case, we might see the pre-image as
either LIVE or DELETE_IN_PROGRESS depending on timing; and we might see
the post-image not at all, or as INSERT_IN_PROGRESS.  Since the existing
code will not sample either DELETE_IN_PROGRESS or INSERT_IN_PROGRESS
tuples, this leads to concurrently-updated rows being omitted from the
sample entirely.  That's not very helpful, and it's especially the wrong
thing if the concurrent transaction ends up rolling back.

The right thing seems to be to sample DELETE_IN_PROGRESS rows just as if
they were live.  This makes the "sample it" and "count it" decisions the
same, which seems good for consistency.  It's clearly the right thing
if the concurrent transaction ends up rolling back; in effect, we are
sampling as though IN_PROGRESS transactions haven't happened yet.
Also, this combination of choices ensures maximum robustness against
the different combinations of whether and in which state we might see the
pre- and post-images of an update.

It's slightly annoying that we end up recording immediately-out-of-date
stats in the case where the transaction does commit, but on the other
hand the stats are fine for columns that didn't change in the update.
And the alternative of sampling INSERT_IN_PROGRESS rows instead seems
like a bad idea, because then the sampling would be inconsistent with
the way rows are counted for the stats report.

Per report from Mark Chambers; thanks to Jeff Janes for diagnosing
what was happening.  Back-patch to all supported versions.

Discussion: https://postgr.es/m/CAFh58O_Myr6G3tcH3gcGrF-=OExB08PJdWZcSBcEcovaiPsrHA@mail.gmail.com
2019-01-03 17:00:08 -05:00
config Postpone LLVM-related uses of AC_CHECK_DECLS. 2018-11-19 12:43:05 -05:00
contrib Fix generation of padding message before encrypting Elgamal in pgcrypto 2019-01-01 10:39:29 +09:00
doc Update copyright for 2019 2019-01-02 12:44:25 -05:00
src Improve ANALYZE's handling of concurrent-update scenarios. 2019-01-03 17:00:08 -05:00
.dir-locals.el Update documentation editor setup instructions 2018-07-13 21:28:18 +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
aclocal.m4 Fix configure's AC_CHECK_DECLS tests to work correctly with clang. 2018-11-19 12:01:47 -05:00
configure Postpone LLVM-related uses of AC_CHECK_DECLS. 2018-11-19 12:43:05 -05:00
configure.in Postpone LLVM-related uses of AC_CHECK_DECLS. 2018-11-19 12:43:05 -05:00
COPYRIGHT Update copyright for 2019 2019-01-02 12:44:25 -05:00
GNUmakefile.in Process EXTRA_INSTALL serially, during the first temp-install. 2018-12-31 13:55:01 -08:00
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

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/.