From: Brad Jorsch <bjorsch@wikimedia.org>
Date: Thu, 11 Dec 2014 15:56:36 +0000 (-0500)
Subject: API: Avoid MySQL filesort with list=allpages&apfilterlanglinks=withlanglinks
X-Git-Tag: 1.31.0-rc.0~12913
X-Git-Url: https://git.cyclocoop.org/%28%28?a=commitdiff_plain;h=c394cdc04dd1ee4058a90c09630f0dee7f70026f;p=lhc%2Fweb%2Fwiklou.git

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 a85c9c9361..e243593b1b 100644
--- 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;
 		}