Re: Re: Suspicious call of initial_cost_hashjoin()

Hi Antonin,

On 12/22/17 6:13 AM, Thomas Munro wrote:
> On Fri, Dec 22, 2017 at 10:45 PM, Antonin Houska <ah(at)cybertec(dot)at> wrote:
>> try_partial_hashjoin_path() passes constant true to for the parallel_hash
>> argument of initial_cost_hashjoin(). Shouldn't it instead pass the
>> parallel_hash argument that it receives?
>
> Thanks. Yeah. When initial_cost_hashjoin() calls
> get_parallel_divisor() on a non-partial inner path I think it would
> return 1.0, so no damage was done there, but when
> ExecChooseHashTableSize() receives try_combined_work_mem == true it
> might underestimate the number of batches required for a partial hash
> join without parallel hash, because it would incorrectly assume that a
> single batch join could use the combined work_mem budget. This was
> quite well hidden because ExecHashTableCreate() calls
> ExecChooseHashTableSize() again (rather than reusing the results from
> planning time), so the bad nbatch estimate doesn't show up anywhere.

Does this look right to you? If so, can you sign up as a reviewer and
mark it Ready for Committer?

Thanks,
--
-David
david(at)pgmasters(dot)net