Slow query with sub-select

[- - <loh.law@xxxxxxxxxxx>]

The following query seems to take ages despite the EXPLAIN stating that an index is used.

Also, the condition (WHERE t.mid = q.mid) should be a one-to-one mapping, should it not? In this case the mapping is to 3641527 rows.

Table q has no indexes and not referenced by other tables.  

Table t has an index on column mid.

Does anyone know why the query is slow?

SELECT COUNT(*) FROM q

      WHERE NOT EXISTS (SELECT 1

                          FROM t AS t

                         WHERE t.mid = q.mid);

                                              QUERY PLAN                                                

< font class="Apple-style-span" face="Tahoma" size="2">---------------------------------------------------------------------------------------------------------

 Aggregate  (cost=10021304028.93..10021304028.94 rows=1 width=0)

   ->  Hash Anti Join  (cost=10007145480.54..10021028896.24 rows=110053073 width=0)

         Hash Cond: ((q.mid)::text = (t.mid)::text)

         ->  Seq Scan on q (cost=10000000000.00..10007993328.46 rows=220106146 width=38)

         ->  Hash  (cost=7083958.46..7083958.46 rows=364 1527 width=10)

               ->  Index Scan using t_pkey on t  (cost=0.00..7083958.46 rows=3641527 width=10)

(6 rows)