Date: Tue, 7 May 2019 17:09:31 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Thomas Munro <thomas.munro@gmail.com>,
	Melanie Plageman <melanieplageman@gmail.com>,
	hackers <pgsql-hackers@postgresql.org>
Subject: Re: accounting for memory used for BufFile during hash joins
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On Tue, May 07, 2019 at 10:42:36AM -0400, Tom Lane wrote:
>Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>> On Mon, May 06, 2019 at 11:18:28PM -0400, Tom Lane wrote:
>>> Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>>>> Do we actually check how many duplicates are there during planning?
>
>>> Certainly that's part of the planner's cost estimates ... but it's
>>> only as good as the planner's statistical knowledge.
>
>> I'm looking at the code, and the only place where I see code dealing with
>> MCVs (probably the best place for info about duplicate values) is
>> estimate_hash_bucketsize in final_cost_hashjoin.
>
>What I'm thinking of is this bit in final_cost_hashjoin:
>
>    /*
>     * If the bucket holding the inner MCV would exceed work_mem, we don't
>     * want to hash unless there is really no other alternative, so apply
>     * disable_cost.  (The executor normally copes with excessive memory usage
>     * by splitting batches, but obviously it cannot separate equal values
>     * that way, so it will be unable to drive the batch size below work_mem
>     * when this is true.)
>     */
>    if (relation_byte_size(clamp_row_est(inner_path_rows * innermcvfreq),
>                           inner_path->pathtarget->width) >
>        (work_mem * 1024L))
>        startup_cost += disable_cost;
>
>It's certainly likely that that logic needs improvement in view of this
>discussion --- I was just pushing back on the claim that we weren't
>considering the issue at all.
>

Ah, this code is new in 11, and I was looking at code from 10 for some
reason. I don't think we can do much better than this, except perhaps
falling back to (1/ndistinct) when there's no MCV available.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services