On 6 December 2012 17:21, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
>> On 5 December 2012 23:37, David Rowley <dgrowleyml@gmail.com> wrote:
>>> Though this plan might not be quite as optimal as it could be as it performs
>>> the grouping after the join.
>
>> PostgreSQL always calculates aggregation as the last step.
>
>> It's a well known optimisation to push-down GROUP BY clauses to the
>> lowest level, but we don't do that, yet.
>
>> You're right that it can make a massive difference to many queries.
>
> In the case being presented here, it's not apparent to me that there's
> any advantage to be had at all. You still need to aggregate over the
> rows joining to each uniquely-keyed row. So how exactly are you going
> to "push down the GROUP BY", and where does the savings come from?
David presents SQL that shows how that is possible.
In terms of operators, after push down we aggregate 1 million rows and
then join 450. Which seems cheaper than join 1 million rows and
aggregate 1 million. So we're passing nearly 1 million fewer rows into
the join.
-- Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services