| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Felix(dot)徐 <ygnhzeus(at)gmail(dot)com> |
| Cc: | Amit Langote <amitlangote09(at)gmail(dot)com>, Atri Sharma <atri(dot)jiit(at)gmail(dot)com>, pgsql-general <pgsql-general(at)postgresql(dot)org> |
| Subject: | Re: How to specify/mock the statistic data of tables in PostgreSQL |
| Date: | 2014-01-13 15:21:10 |
| Message-ID: | 9051.1389626470@sss.pgh.pa.us |
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| Lists: | pgsql-general |
=?GB2312?B?RmVsaXgu0Ow=?= <ygnhzeus(at)gmail(dot)com> writes:
> //line 194 : In a "most common values" slot, staop is the OID of the "="
> operator used to decide whether values are the same or not.
> //line 206 : A "histogram" slot describes the distribution of scalar data.
> staop is the OID of the "<" operator that describes the sort ordering.
> I don't understand the function of staop here, how is it used in optimizer,
In principle a data type could have more than one sort ordering, and if
we were to collect stats according to multiple orderings, staop would be
needed to identify which ordering a particular set of statistics was
created with. That flexibility isn't being used right now, at least not
by any built-in code. There are types with more than one ordering (more
than one btree opclass), but ANALYZE only collects stats for the default
btree opclass.
regards, tom lane
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