Performance impact with domain access: Is SELECT DISTINCT really necessary on every sql rewrite?

Hi agentrickard!

I have to do a performance tuning on a large drupal 7 site using the doman module.

While investigating, I compared the sql queries with a normal drupal 7 installation. I found out, that every view is rewritten into a SELECT DISTINT query.

Without Domain Access

SELECT node.title AS node_title, node.nid AS nid, node.language AS node_language, node.created AS node_created, 'test:default' AS view_name
FROM 
{node} node
INNER JOIN {node_access} na ON na.nid = node.nid
WHERE (( (node.status = '1') AND (node.type IN  ('story')) ))AND(( (na.gid = '0') AND (na.realm = 'all') )OR( (na.gid = '0') AND (na.realm = 'domain_site') )OR( (na.gid = '2') AND (na.realm = 'domain_id') ))AND (na.grant_view >= '1') 
ORDER BY node_created DESC
LIMIT 10 OFFSET 0

With Domain Access

SELECT DISTINCT node.title AS node_title, node.nid AS nid, node.language AS node_language, node.created AS node_created, 'test:default' AS view_name
FROM 
{node} node
INNER JOIN {node_access} na ON na.nid = node.nid
WHERE (( (node.status = '1') AND (node.type IN  ('story')) ))AND(( (na.gid = '0') AND (na.realm = 'all') )OR( (na.gid = '0') AND (na.realm = 'domain_site') )OR( (na.gid = '2') AND (na.realm = 'domain_id') ))AND (na.grant_view >= '1') 
ORDER BY node_created DESC
LIMIT 10 OFFSET 0

As far as I can see, the $query->distinct() is always set in domain_content/domain.module:

.....
  // Prevent duplicate records.
  $query->distinct();

  // Find all instances of the base table being joined -- could appear
  // more than once in the query, and could be aliased. Join each one to
  // the node_access table.

  $grants = node_access_grants($op, $account);
  if ($type == 'entity') {
....

As we all know, the SELECT DISTINCT is quite expensive, because every query have to be grouped to reduce duplicates. On my test system, I disabled the $query->distinct() line and had no impact in my environment (we did not see views with duplicates). I know this is not a solution, but to it seems, that the DISTINCT is not necessary in every use case.

I don't know well the architecture of domain access. Could you give me a litte help here? We feel a performance improvement disabling the "DISTINCT" in domain access. May I disable it and set the "DISTINCT" in views manually if necessary? Is there a solution for sites that would like to avoid the "DISTINCT" in every query because of performance reasons?