From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>
Cc: hackers <pgsql-hackers@postgresql.org>
Subject: Re: accounting for memory used for BufFile during hash joins
Date: Tue, 07 May 2019 10:42:36 -0400
Message-ID: <27778.1557240156@sss.pgh.pa.us> (raw)
In-Reply-To: <20190507131742.nixczzyyyuuoig7m@development>
References: <20190504003414.bulcbnge3rhwhcsh@development>
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<CA+hUKG+ppih9DtZwGJh4KMZXdtD=c9+fYoS1CSeBQBs9JzZtbA@mail.gmail.com>
<20190507031511.lvp6bqn2e5dpsnga@development>
<27902.1557199108@sss.pgh.pa.us>
<20190507131742.nixczzyyyuuoig7m@development>
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.
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, tomas.vondra@2ndquadrant.com, thomas.munro@gmail.com, melanieplageman@gmail.com
Subject: Re: accounting for memory used for BufFile during hash joins
In-Reply-To: <27778.1557240156@sss.pgh.pa.us>
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