Make CategoryMembershipChangeJob query more readable

Previously, the INNER JOIN had "rc_timestamp >= rev_timestamp" which
complicates query planning. Even with "equals" it still was techinally
ambiguous. Instead, just use EXISTS and an exact equality operator.

Bug: T180793
Change-Id: I1e9ae7c2ce0f95484e09e867550283d816d151f0

diff --git a/includes/jobqueue/jobs/CategoryMembershipChangeJob.php b/includes/jobqueue/jobs/CategoryMembershipChangeJob.php
index 55c1367..4251a25 100644 (file)
--- a/includes/jobqueue/jobs/CategoryMembershipChangeJob.php
+++ b/includes/jobqueue/jobs/CategoryMembershipChangeJob.php
@@ -25,6 +25,8 @@ use Wikimedia\Rdbms\LBFactory;
 /**
  * Job to add recent change entries mentioning category membership changes
  *
+ * This allows users to easily scan categories for recent page membership changes
+ *
  * Parameters include:
  *   - pageId : page ID
  *   - revTimestamp : timestamp of the triggering revision
@@ -81,28 +83,28 @@ class CategoryMembershipChangeJob extends Job {
                // between COMMIT and actual enqueueing of the CategoryMembershipChangeJob job.
                $cutoffUnix -= self::ENQUEUE_FUDGE_SEC;
 
-               // Get the newest revision that has a SRC_CATEGORIZE row...
+               // Get the newest page revision that has a SRC_CATEGORIZE row.
+               // Assume that category changes before it were already handled.
                $row = $dbr->selectRow(
-                       [ 'revision', 'recentchanges' ],
+                       'revision',
                        [ 'rev_timestamp', 'rev_id' ],
                        [
                                'rev_page' => $page->getId(),
-                               'rev_timestamp >= ' . $dbr->addQuotes( $dbr->timestamp( $cutoffUnix ) )
-                       ],
-                       __METHOD__,
-                       [ 'ORDER BY' => 'rev_timestamp DESC, rev_id DESC' ],
-                       [
-                               'recentchanges' => [
-                                       'INNER JOIN',
+                               'rev_timestamp >= ' . $dbr->addQuotes( $dbr->timestamp( $cutoffUnix ) ),
+                               'EXISTS (' . $dbr->selectSQLText(
+                                       'recentchanges',
+                                       '1',
                                        [
                                                'rc_this_oldid = rev_id',
                                                'rc_source' => RecentChange::SRC_CATEGORIZE,
                                                // Allow rc_cur_id or rc_timestamp index usage
                                                'rc_cur_id = rev_page',
-                                               'rc_timestamp >= rev_timestamp'
+                                               'rc_timestamp = rev_timestamp'
                                        ]
-                               ]
-                       ]
+                               ) . ')'
+                       ],
+                       __METHOD__,
+                       [ 'ORDER BY' => 'rev_timestamp DESC, rev_id DESC' ]
                );
                // Only consider revisions newer than any such revision
                if ( $row ) {