Michael Lewis <mlewis 'at' entrata.com> writes: > By the way, I expect the time is cut in half while heap fetches stays similar > because the index is now in OS cache on the > second run and didn't need to be fetched from disk. Definitely need to check > on vacuuming as Justin says. If you have a fairly > active system, you would need to run this query many times in order to push > other stuff out of shared_buffers and get this > query to perform more like it does on dev. > > Do you have the option to re-write the query or is this generated by an ORM? > You are forcing the looping as I read this query. > If you aggregate before you join, then the system should be able to do a > single scan of the index, aggregate, then join those > relatively few rows to the multicards table records. > > SELECT transaction_uid, COALESCE( sub.count, 0 ) AS count FROM multicards > LEFT JOIN (SELECT multicard_uid, COUNT(*) AS count > FROM tickets GROUP BY multicard_uid ) AS sub ON sub.multicard_uid = > multicards.uid;
Thanks for this hint! I always hit this fact that I never write good queries using explicit joins :/ Execution time (before vacuuming the table as adviced by Justin) down 38x to 44509ms using this query :) Real query was an UPDATE of the multicards table to set the count value. I rewrote this using your approach but I think I lack what coalesce did in your query, this would update only the rows where count >= 1 obviously: UPDATE multicards SET defacements = count FROM ( SELECT multicard_uid, COUNT(*) AS count FROM tickets GROUP BY multicard_uid ) AS sub WHERE uid = multicard_uid; Any hinted solution to do that in one pass? I could do a first pass setting defacements = 0, but that would produce more garbage :/ Thanks! -- Guillaume Cottenceau
