Re: possible minor EXPLAIN bug?

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
>
>