For information about the MediaWiki database layout, such as a
description of the tables and their contents, please see:
- http://www.mediawiki.org/wiki/Manual:Database_layout
- http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/maintenance/tables.sql?view=markup
+ https://www.mediawiki.org/wiki/Manual:Database_layout
+ https://gerrit.wikimedia.org/r/gitweb?p=mediawiki/core.git;a=blob_plain;f=maintenance/tables.sql;hb=HEAD
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at the first query, and commits it before the output is sent. Locks will
be held from the time when the query is done until the commit. So you
can reduce lock time by doing as much processing as possible before you
-do your write queries. Update operations which do not require database
-access can be delayed until after the commit by adding an object to
-$wgPostCommitUpdateList.
+do your write queries.
Often this approach is not good enough, and it becomes necessary to
enclose small groups of queries in their own transaction. Use the
following syntax:
$dbw = wfGetDB( DB_MASTER );
-$dbw->begin();
+$dbw->begin( __METHOD__ );
/* Do queries */
-$dbw->commit();
+$dbw->commit( __METHOD__ );
Use of locking reads (e.g. the FOR UPDATE clause) is not advised. They
are poorly implemented in InnoDB and will cause regular deadlock errors.
* PostgreSQL
* SQLite
* Oracle
-* IBM DB2
* MSSQL
More information can be found about each of these databases (known issues,
level of support, extra configuration) in the "databases" subdirectory in
this folder.
+
+------------------------------------------------------------------------
+ Use of GROUP BY
+------------------------------------------------------------------------
+
+MySQL supports GROUP BY without checking anything in the SELECT clause.
+Other DBMSs (especially Postgres) are stricter and require that all the
+non-aggregate items in the SELECT clause appear in the GROUP BY. For
+this reason, it is highly discouraged to use SELECT * with GROUP BY
+queries.
+