Add a missing table key to improve performance of "Recently Completed Tasks" query

Summary:
Fixes T11490. Currently, this query can not use a key and the table size may be quite large.

Adjust the query so it can use a key for both selection and ordering, and add that key.

Test Plan: Ran `EXPLAIN` on the old query in production, then added the key and ran `EXPLAIN` on the new query. Saw key in use, and "rows" examined drop from 29,273 to 15.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11490

Differential Revision: https://secure.phabricator.com/D16423

src/infrastructure/daemon/workers/query/PhabricatorWorkerArchiveTaskQuery.php

@@ -104,6 +104,8 @@ final class PhabricatorWorkerArchiveTaskQuery
 
     if ($this->dateCreatedBefore) {
       return qsprintf($conn_r, 'ORDER BY dateCreated DESC, id DESC');
+    } else if ($this->dateModifiedSince) {
+      return qsprintf($conn_r, 'ORDER BY dateModified DESC, id DESC');
     } else {
       return qsprintf($conn_r, 'ORDER BY id DESC');
     }

src/infrastructure/daemon/workers/storage/PhabricatorWorkerArchiveTask.php

@@ -31,6 +31,9 @@ final class PhabricatorWorkerArchiveTask extends PhabricatorWorkerTask {
       'leaseOwner' => array(
         'columns' => array('leaseOwner', 'priority', 'id'),
       ),
+      'key_modified' => array(
+        'columns' => array('dateModified'),
+      ),
     ) + $parent[self::CONFIG_KEY_SCHEMA];
 
     return $config;