2008/8/19 Simon Riggs <simon@2ndquadrant.com>:
>
> 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
yes, Agg get this value directly, but it wrong
postgres=# explain select * from (select sum(a) from twocol group by b
offset 0) c; QUERY PLAN
----------------------------------------------------------------------------Subquery Scan c (cost=42.10..46.60
rows=200width=8) -> Limit (cost=42.10..44.60 rows=200 width=8) -> HashAggregate (cost=42.10..44.60 rows=200
width=8) -> Seq Scan on twocol (cost=0.00..31.40 rows=2140 width=8)
(4 rows)
limit, subquery scan has wrong width now.
regards
Pavel
>
> --
> Simon Riggs www.2ndQuadrant.com
> PostgreSQL Training, Services and Support
>
>
