Multi-value merge queries

I see that this may not be supported currently. However, I know MySQL supports doing this in a single query. The base type MergeQuery already implements a degenerate case with multiple queries, surely adding another such case that loops over the rows would not be so bad? MySQL and other drivers that support single query multiple-record merging could then extend this.

To show the problem:

  // Should work with multiple-record merge support:
  $query = db_merge('topic');
  $query->key(array('post_id')); // (needs a better API)
  
  foreach ($topics as $topic) {
    $query->values(array(
      'post_id'  => $topic->post_id,
      'forum_id' => $forum_id,
      'author'   => $topic->author,
      'lastpost' => $topic->lastpost,
      'length'   => $topic->length,
      'title'    => $topic->title,
    ));
  }
  $query->execute();
  
  // This requires a separate query for each record, as it is now.
  foreach ($topics as $topic) {
    db_merge('topic')
      ->key(array('post_id' => $topic->post_id))
      ->fields(array(
      'post_id'  => $topic->post_id,
      'forum_id' => $forum_id,
      'author'   => $topic->author,
      'lastpost' => $topic->lastpost,
      'length'   => $topic->length,
      'title'    => $topic->title,
    ))
      ->execute();
  }

Might be MySQL 5.0 specific. Not sure if we require that version already.

http://dev.mysql.com/doc/refman/5.0/en/insert-on-duplicate.html

The ON DUPLICATE KEY UPDATE clause can contain multiple column assignments, separated by commas.

You can use the VALUES(col_name) function in the UPDATE clause to refer to column values from the INSERT portion of the INSERT ... UPDATE statement. In other words, VALUES(col_name) in the UPDATE clause refers to the value of col_name that would be inserted, had no duplicate-key conflict occurred. This function is especially useful in multiple-row inserts. The VALUES() function is meaningful only in INSERT ... UPDATE statements and returns NULL otherwise. Example:

INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6)
ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);

That statement is identical to the following two statements:

INSERT INTO table (a,b,c) VALUES (1,2,3)
ON DUPLICATE KEY UPDATE c=3;
INSERT INTO table (a,b,c) VALUES (4,5,6)
ON DUPLICATE KEY UPDATE c=9;