Question

SELECT count(*) 
FROM contacts_lists 
     JOIN plain_contacts 
          ON contacts_lists.contact_id = plain_contacts.contact_id 
     JOIN contacts 
          ON contacts.id = plain_contacts.contact_id 
WHERE plain_contacts.has_email 
      AND NOT contacts.email_bad 
      AND NOT contacts.email_unsub 
      AND contacts_lists.list_id =67339

how can i optimize this query.. could you please explain…

Answer 1

Reformatting your query plan for clarity:

QUERY PLAN Aggregate (cost=126377.96..126377.97 rows=1 width=0)
  -> Hash Join (cost=6014.51..126225.38 rows=61033 width=0)
     Hash Cond: (contacts_lists.contact_id = plain_contacts.contact_id)
    -> Hash Join (cost=3067.30..121828.63 rows=61033 width=8)
       Hash Cond: (contacts_lists.contact_id = contacts.id)
      -> Index Scan using index_contacts_lists_on_list_id_and_contact_id
         on contacts_lists (cost=0.00..116909.97 rows=61033 width=4)
         Index Cond: (list_id = 66996)
         -> Hash (cost=1721.41..1721.41 rows=84551 width=4)
         -> Seq Scan on contacts (cost=0.00..1721.41 rows=84551 width=4)
            Filter: ((NOT email_bad) AND (NOT email_unsub))
            -> Hash (cost=2474.97..2474.97 rows=37779 width=4)
            -> Seq Scan on plain_contacts (cost=0.00..2474.97 rows=37779 width=4)
               Filter: has_email

Two partial indexes might eliminate seq scans depending on your data distribution:

-- if many contacts have bad emails or are unsubscribed:
CREATE INDEX contacts_valid_email_idx ON contacts (id)
WHERE (NOT email_bad AND NOT email_unsub);

-- if many contacts have no email:
CREATE INDEX plain_contacts_valid_email_idx ON plain_contacts (id)
WHERE (has_email);

You might be missing an index on a foreign key:

CREATE INDEX plain_contacts_contact_id_idx ON plain_contacts (contact_id);

Last but not least if you’ve never analyzed your data, you need to run:

VACUUM ANALYZE;

If it’s still slow once all that is done, there isn’t much you can do short of merging your plain_contacts and your contacts tables: getting the above query plan in spite of the above indexes means most/all of your subscribers are subscribed to that particular list — in which case the above query plan is the fastest you’ll get.