Sort

I'm currently using v7.0.2 of PostgreSQL.

I have a query that performs a group by on three columns.  The EXPLAIN of
the query is as follows:

Aggregate  (cost=4116.05..4125.47 rows=94 width=24)
  ->  Group  (cost=4116.05..4123.12 rows=942 width=24)
        ->  Sort  (cost=4116.05..4116.05 rows=942 width=24)
              ->  Nested Loop  (cost=0.00..4069.52 rows=942 width=24)
                    ->  Seq Scan on click  (cost=0.00..15.42 rows=942
width=8)
                    ->  Index Scan using impression_pkey on impression
(cost=0.00..4.29 rows=1 width=16)

I need to speed up this query.  I have already created an index on the three
columns query_idx(columna, columnb,columnc).  Is there some other index that
I could add which would speed up the query.  There is a high frequency in
the table of each group.  Each group probably makes up 10% of the table.
Does this force a sequence scan when sorting and grouping?  Basically just
looking for suggestions.


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Nathan Barnett