Re: Additional improvements to extended statistics

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Additional improvements to extended statistics
Date: 2020-03-09 00:06:21
Message-ID: 20200309000621.3jhvfxphagyf5bhg@development
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On Mon, Mar 09, 2020 at 01:01:57AM +0100, Tomas Vondra wrote:
>On Sun, Mar 08, 2020 at 07:17:10PM +0000, Dean Rasheed wrote:
>>On Fri, 6 Mar 2020 at 12:58, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>>>Here is a rebased version of this patch series. I've polished the first
>>>two parts a bit - estimation of OR clauses and (Var op Var) clauses.
>>I've been looking over the first patch (OR list support). It mostly
>>looks reasonable to me, except there's a problem with the way
>>statext_mcv_clauselist_selectivity() combines multiple stat_sel values
>>into the final result -- in the OR case, it needs to start with sel =
>>0, and then apply the OR formula to factor in each new estimate. I.e.,
>>this isn't right for an OR list:
>> /* Factor the estimate from this MCV to the oveall estimate. */
>> sel *= stat_sel;
>>(Oh and there's a typo in that comment: s/oveall/overall/).
>>For example, with the regression test data, this isn't estimated well:
>> SELECT * FROM mcv_lists_multi WHERE a = 0 OR b = 0 OR c = 0 OR d = 0;
>>Similarly, if no extended stats can be applied it needs to return 0
>>not 1, for example this query on the test data:
>> SELECT * FROM mcv_lists WHERE a = 1 OR a = 2 OR d IS NOT NULL;
>Ah, right. Thanks for noticing this. Attaches is an updated patch series
>with parts 0002 and 0003 adding tests demonstrating the issue and then
>fixing it (both shall be merged to 0001).

One day I won't forget to actually attach the files ...


Tomas Vondra
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment Content-Type Size
0001-Support-using-extended-stats-for-parts-of-O-20200309.patch text/plain 11.3 KB
0002-Fix-Add-regression-tests-for-OR-clauses-20200309.patch text/plain 4.8 KB
0003-Fix-Calculation-of-OR-selectivities-20200309.patch text/plain 1.4 KB
0004-Support-clauses-of-the-form-Var-op-Var-20200309.patch text/plain 16.1 KB

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