API: Avoid MySQL filesort with list=allpages&apfilterlanglinks=withlanglinks

I'm not sure whether r44584 didn't go far enough or if MySQL's behavior
has changed since 2008, but MySQL is now filesorting when a
constant-in-WHERE field is included in GROUP BY.

If all our supported databases used the 1999 SQL standard rules for
GROUP BY[1] this would be an easy fix. But PostgreSQL before 9.1 uses
the older 1992 rules.[2] And then there's Oracle and MSSQL, which aren't
listed as supported[1] but are still in the code. Simplest thing to do
is probably to check if we're on MySQL, Sqlite, or Postgres >= 9.1 and
use the 1999 rules, and otherwise use the older rules.

 [1]: Basically "any non-aggregate field in the SELECT must be
      functionally dependent on the grouped-by fields", meaning if you
      include the primary key you're good.
 [2]: Basically "any non-aggregate field in the SELECT must be in the
      GROUP BY".
 [3]: https://www.mediawiki.org/wiki/Manual:Installation_requirements#Database_server

Bug: T78276
Change-Id: I80b515bb06d194b146897155b318a3d1c908e8b6

diff --git a/includes/api/ApiQueryAllPages.php b/includes/api/ApiQueryAllPages.php
index a85c9c9..e243593 100644 (file)
--- a/includes/api/ApiQueryAllPages.php
+++ b/includes/api/ApiQueryAllPages.php
@@ -168,9 +168,23 @@ class ApiQueryAllPages extends ApiQueryGeneratorBase {
                        $this->addTables( 'langlinks' );
                        $this->addWhere( 'page_id=ll_from' );
                        $this->addOption( 'STRAIGHT_JOIN' );
-                       // We have to GROUP BY all selected fields to stop
-                       // PostgreSQL from whining
-                       $this->addOption( 'GROUP BY', $selectFields );
+
+                       // MySQL filesorts if we use a GROUP BY that works with the rules
+                       // in the 1992 SQL standard (it doesn't like having the
+                       // constant-in-WHERE page_namespace column in there). Using the
+                       // 1999 rules works fine, but that breaks other DBs. Sigh.
+                       /// @todo Once we drop support for 1992-rule DBs, we can simplify this.
+                       $dbType = $db->getType();
+                       if ( $dbType === 'mysql' || $dbType === 'sqlite' ||
+                               $dbType === 'postgres' && $db->getServerVersion() >= 9.1
+                       ) {
+                               // 1999 rules, or screw-the-rules
+                               $this->addOption( 'GROUP BY', array( 'page_title', 'page_id' ) );
+                       } else {
+                               // 1992 rules
+                               $this->addOption( 'GROUP BY', $selectFields );
+                       }
+
                        $forceNameTitleIndex = false;
                }