Gregory Stark <stark@enterprisedb.com> writes:
> "Tom Lane" <tgl@sss.pgh.pa.us> writes:
>> Nested Loop (cost=5.39..198.81 rows=51 width=244)
>> -> HashAggregate (cost=1.06..1.11 rows=5 width=4)
>> -> Seq Scan on int4_tbl b (cost=0.00..1.05 rows=5 width=4)
>> -> Bitmap Heap Scan on tenk1 a (cost=4.33..39.41 rows=10 width=244)
>> Recheck Cond: (a.thousand = b.f1)
>> -> Bitmap Index Scan on tenk1_thous_tenthous (cost=0.00..4.33 rows=10 width=0)
>> Index Cond: (a.thousand = b.f1)
>> (7 rows)
> Sure, but that's still re-executing the bitmap index scan 51 times -- possibly
> having to fetch the same records off disk repeatedly.
It's not fetching any record repeatedly, because the HashAggregate
step eliminated duplicate keys on the other side.
regards, tom lane