I have a table t with a column c, which is an int and has a btree index on it.
Why does the following query not utilize this index?
explain select c from t group by c;
The result I get is:
HashAggregate (cost=1005817.55..1005817.71 rows=16 width=4) -> Seq Scan on t (cost=0.00..946059.84 rows=23903084 width=4)
My understanding of indexes is limited, but I thought such queries were the purpose of indexes.