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