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OP - Can you please run your original query with a minor variation-

select dashboards.name, count(distinct time_on_site_logs.user_id) from time_on_site_logs join dashboards on time_on_site_logs.dashboard_id = dashboards.id group by time_on_site_logs.dashboard_id, dashboards.name order by count desc

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Good idea. Times are from my laptop, which is less beefy than the test rig we used:

  First query: 996s
  First query with your variation: 593s
  Second query: 67s
  Third query: 21s

Do you have a FK from dashboard_id to dashboards.id by chance? I assume no, as the query is presumably still forced to do an early existence test of the inner join before aggregating.

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