Issue #937284 by chx, chrisdolby, deviantintegral, Berdir, tim.plunkett | hefox: DEADLOCK errors on MergeQuery INSERT due to InnoDB gap locking when condition in SELECT ... FOR UPDATE results in 0 rows.

CHANGELOG.txt

@@ -1,6 +1,8 @@
 
 Drupal 7.25, xxxx-xx-xx (development version)
 -----------------------
+- Fixed a bug in the database API that caused frequent deadlock errors when
+  running merge queries on some servers.
 - Performance improvement: Prevent block rehashing from writing blocks to the
   database on every cache clear and cron run when the blocks have not changed.
   This fix results in an extra 'saved' key which is added and set to TRUE for

includes/database/query.inc

@@ -1606,15 +1606,11 @@ class MergeQuery extends Query implements QueryConditionInterface {
   }
 
   public function execute() {
-    // Wrap multiple queries in a transaction, if the database supports it.
-    $transaction = $this->connection->startTransaction();
-    try {
     if (!count($this->condition)) {
       throw new InvalidMergeQueryException(t('Invalid merge query: no conditions'));
     }
     $select = $this->connection->select($this->conditionTable)
-        ->condition($this->condition)
-        ->forUpdate();
+      ->condition($this->condition);
     $select->addExpression('1');
     if (!$select->execute()->fetchField()) {
       try {
@@ -1623,7 +1619,7 @@ class MergeQuery extends Query implements QueryConditionInterface {
           $insert->useDefaults($this->defaultFields);
         }
         $insert->execute();
-          return MergeQuery::STATUS_INSERT;
+        return self::STATUS_INSERT;
       }
       catch (Exception $e) {
         // The insert query failed, maybe it's because a racing insert query
@@ -1645,17 +1641,9 @@ class MergeQuery extends Query implements QueryConditionInterface {
         }
       }
       $update->execute();
-        return MergeQuery::STATUS_UPDATE;
+      return self::STATUS_UPDATE;
      }
   }
-    catch (Exception $e) {
-      // Something really wrong happened here, bubble up the exception to the
-      // caller.
-      $transaction->rollback();
-      throw $e;
-    }
-    // Transaction commits here where $transaction looses scope.
-  }
 }
 
 /**