From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrei Lepikhov <lepihov@gmail.com>
cc: Thomas Munro <thomas.munro@gmail.com>,
 Craig Milhiser <craig@milhiser.com>,
        pgsql-bugs@lists.postgresql.org
Subject: Re: Reference to - BUG #18349: ERROR: invalid DSA memory alloc
 request size 1811939328, CONTEXT: parallel worker
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Comments: In-reply-to Andrei Lepikhov <lepihov@gmail.com>
	message dated "Mon, 14 Oct 2024 12:45:09 +0700"
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Date: Mon, 14 Oct 2024 02:26:09 -0400
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Andrei Lepikhov <lepihov@gmail.com> writes:
> My explanation (correct if I'm wrong):
> OUTER JOINs allow NULLs to be in a hash table. At the same time, a hash 
> value for NULL is 0, and it goes to the batch==0.
> If batch number 0 gets overfilled, the 
> ExecParallelHashIncreaseNumBatches routine attempts to increase the 
> number of batches - but nothing happens. The initial batch is still too 
> big, and the number of batches doubles up to the limit.

Interesting point.  If memory serves (I'm too tired to actually look)
the planner considers the statistical most-common-value when
estimating whether an unsplittable hash bucket is likely to be too
big.  It does *not* think about null values ... but it ought to.

However, this does not explain why PHJ would be more subject to
the problem than non-parallel HJ.

			regards, tom lane