Re: [HACKERS] PATCH: multivariate histograms and MCV lists

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Cc: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Mark Dilger <hornschnorter(at)gmail(dot)com>, Adrien Nayrat <adrien(dot)nayrat(at)dalibo(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] PATCH: multivariate histograms and MCV lists
Date: 2018-04-01 16:07:55
Message-ID: abfb7b66-af22-ac32-a74d-3fcf9bdb5c5c@2ndquadrant.com
Views: Raw Message | Whole Thread | Download mbox
Thread:
Lists: pgsql-hackers

Hi,

The attached patch version modifies how the non-MCV selectivity is
computed, along the lines explained in the previous message.

The comments in statext_clauselist_selectivity() explain it in far more
detail, but we this:

1) Compute selectivity using the MCV (s1).

2) To compute the non-MCV selectivity (s2) we do this:

2a) See how many top-level equalities are there (and compute ndistinct
estimate for those attributes).

2b) If there is an equality on each column, we know there can only be a
single matching item. If we found it in the MCV (i.e. s1 > 0) we're
done, and 's1' is the answer.

2c) If only some columns have equalities, we estimate the selectivity
for equalities as

s2 = ((1 - mcv_total_sel) / ndistinct)

If there are no remaining conditions, we're done.

2d) To estimate the non-equality clauses (on non-MCV part only), we
either repeat the whole process by calling clauselist_selectivity() or
approximating s1 to the non-MCV part. This needs a bit of care to
prevent infinite loops.

Of course, with 0002 this changes slightly, because we may try using a
histogram to estimate the non-MCV part. But that's just an extra step
right before (2a).

regards

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

Attachment Content-Type Size
0001-multivariate-MCV-lists-20180401.patch.gz application/gzip 38.4 KB
0002-multivariate-histograms-20180401.patch.gz application/gzip 43.0 KB

In response to

Responses

Browse pgsql-hackers by date

  From Date Subject
Next Message legrand legrand 2018-04-01 16:33:25 Re: Planning counters in pg_stat_statements
Previous Message Magnus Hagander 2018-04-01 16:07:31 Re: [PATCH] pg_hba.conf : new auth option : clientcert=verify-full