Re: Number of parallel workers chosen by the optimizer for parallel append

On Wed, 2020-11-25 at 17:36 +0100, Laurenz Albe wrote:
> I have a partitioned table, each partition has "parallel_workers = 10" set.
>
> SET max_parallel_workers_per_gather = 8;
>
> SET enable_partitionwise_aggregate = on;
>
> EXPLAIN (COSTS OFF)
> SELECT applicant_name, count(ipc_4)
> FROM laurenz.z_flat
> GROUP BY applicant_name;
>
> QUERY PLAN
> --------------------------------------------------
> Gather
> Workers Planned: 4
> -> Parallel Append
> -> HashAggregate
> Group Key: z_flat_3.applicant_name
> -> Seq Scan on xyz_4 z_flat_3
> -> HashAggregate
> Group Key: z_flat.applicant_name
> -> Seq Scan on xyz_1 z_flat
> [8 more such partition scans]
> (33 rows)
>
> How does the optimizer decide to use 4 parallel workers?
>
> No matter what I try, I cannot influence that number.

I figured it out.

This is automatically calculated from the number of partitions, and the
number of parallel workers is

ld(#partitions) + 1

where "ld" is the logarithm of base 2 (function "fls" in the source).

It might be nice to make this configurable, but since we don't have
storage parameters on partitioned tables, I wonder how.

Yours,
Laurenz Albe
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