On Tue, 2008-08-19 at 09:45 +0200, Pavel Stehule wrote:
> postgres=# explain select sum(a) from twocol group by b;
> QUERY PLAN
> ----------------------------------------------------------------
> HashAggregate (cost=42.10..44.60 rows=200 width=8) --< wrong should be 4
> -> Seq Scan on twocol (cost=0.00..31.40 rows=2140 width=8)
> (2 rows)
Although column b is not displayed it is kept in the HashAgg node to
allow your request to GROUP BY B. I'm happy that it tells me the width
of 8 so I can work out space used by hash, but perhaps it should say 12
(or even 16) to include hash value also, so we include the full cost per
row in the hash table.
If you do explain select sum(a) from twocol
you will see the width is only 4
-- Simon Riggs www.2ndQuadrant.comPostgreSQL Training, Services and Support