Re: [HACKERS] Partition-wise aggregation/grouping

On Thu, Dec 14, 2017 at 4:01 PM, Ashutosh Bapat
<ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:
> +
> +-- Test when input relation for grouping is dummy
> +EXPLAIN (COSTS OFF)
> +SELECT c, sum(a) FROM pagg_tab WHERE 1 = 2 GROUP BY c;
> + QUERY PLAN
> +--------------------------------
> + HashAggregate
> + Group Key: pagg_tab.c
> + -> Result
> + One-Time Filter: false
> +(4 rows)
>
> Not part of your patch, I am wondering if we can further optimize this plan by
> converting HashAggregate to Result (One-time Filter: false) and the aggregate
> target. Just an idea.
>
This comment is also wrong. The finalization step of aggregates needs
to be executed irrespective of whether or not the underlying scan
produces any rows. It may, for example, add a constant value to the
transition result. We may apply this optimization only when none of
aggregations have finalization functions, but it may not be worth the
code.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company