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