From: Roan Kattouw <roan.kattouw@gmail.com>
Date: Mon, 11 Apr 2016 18:08:10 +0000 (-0700)
Subject: Force primary index for RevDelRevisionList query
X-Git-Tag: 1.31.0-rc.0~5667^2
X-Git-Url: http://git.cyclocoop.org///%22%40url%40//%22?a=commitdiff_plain;h=59f09caabc4c7e58b18d0a88dbb5a7d22d0ef7bd;p=lhc%2Fweb%2Fwiklou.git

Force primary index for RevDelRevisionList query

To work around an optimizer bug in MySQL where the
index on (rev_page, rev_id) is picked which mysteriously
leads to very slow execution times and timeouts.

In theory, the index being forced here is (rev_id),
which is fine for this query. In WMF production,
it can also be (rev_id, rev_user) (still fine), or
(rev_page, rev_id) (the index we're trying to avoid).
Mysteriously, the optimizer bug doesn't happen if
(rev_page, rev_id) is the primary key, so this still
behaves OK.

Bug: T104313
Change-Id: I15c68ba29309dca8dea274f19389d139a82784aa
---

diff --git a/includes/revisiondelete/RevDelRevisionList.php b/includes/revisiondelete/RevDelRevisionList.php
index 1d08dbe55a..f0b1907d83 100644
--- a/includes/revisiondelete/RevDelRevisionList.php
+++ b/includes/revisiondelete/RevDelRevisionList.php
@@ -66,7 +66,10 @@ class RevDelRevisionList extends RevDelList {
 				'rev_page' => $this->title->getArticleID(),
 				'rev_id' => $ids,
 			],
-			'options' => [ 'ORDER BY' => 'rev_id DESC' ],
+			'options' => [
+				'ORDER BY' => 'rev_id DESC',
+				'USE INDEX' => [ 'revision' => 'PRIMARY' ] // workaround for MySQL bug (T104313)
+			],
 			'join_conds' => [
 				'page' => Revision::pageJoinCond(),
 				'user' => Revision::userJoinCond(),