Re: group by query plan on already clustered index

Hello

2013/7/25 Sandeep Gupta <gupta(dot)sandeep(at)gmail(dot)com>:
> On a table T with two fields, f1 and f2, the sql command
>
> select count(f2), f1
> from T
> group by f1
>
> result is seq scan followed by a sort on f1 (see the query plan below):
>
> GroupAggregate (cost=21566127.88..22326004.09 rows=987621 width=8)
> -> Sort (cost=21566127.88..21816127.88 rows=100000000 width=8)
> Sort Key: pid
> -> Seq Scan on tc (cost=0.00..1442478.00 rows=100000000 width=8)
>
>
> However, the table is already has clustered index on f1. My question is why
> doesn't
> postgres simply scan the table and compute the group? Why it needs to sort
> on f1 again?
> How can I force postgres to use the clustered index for group by?
>

PostgreSQL doesn't support index organized tables. Statement CLUSTER
doesn't ensure order of heap for ever. After any INSERT, UPDATE you
can lost a order.

So every time, you have to check order.

PostgreSQL 9.2 support a index only scan - but it is not used in your plan.

Regards

Pavel Stehule

>
> Thanks.
> Sandeep
>
>