Subject: Re: multivariate statistics v10
To: Thom Brown <thom@linux.com>
References: <543AFA15.4080608@fuzzy.cz> <567AF104.80700@2ndquadrant.com>
 <569DBA67.9020403@2ndquadrant.com> <56D6FF1C.5060303@2ndquadrant.com>
 <CAA-aLv5cOaotb6-=mOEjVvm3Afh9638Ao+4UeczyZZkuQfHF8g@mail.gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <56D71921.9080509@2ndquadrant.com>
Date: Wed, 2 Mar 2016 17:47:29 +0100
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Hi,

On 03/02/2016 05:17 PM, Thom Brown wrote:
...
> Well, firstly, the patches all apply.
>
> But I have a question (which is coming really late, but I'll ask it
> anyway).  Is it intended that CREATE STATISTICS will only be for
> multivariate statistics?  Or do you think we could add support for
> expression statistics in future too?
>
> e.g.
>
> CREATE STATISTICS stats_comment_length ON comments (length(comment));

Hmmm, that's not a use case I had in mind while working on the patch, 
but it sounds interesting. I don't see why the syntax would not support 
this - I'd like to add support for expressions into the multivariate 
patch, but that will still require at least 2 columns to build 
multivariate statistics. But perhaps it'd be possible to relax the "at 
least 2 columns" requirement, and collect regular statistics somewhere.

So I don't see why the syntax could not work for that case too, but I'm 
not going to work on that.

>
>
> I also note that the docs contain this:
>
> CREATE STATISTICS [ IF NOT EXISTS ] statistics_name ON table_name ( [
>    { column_name } ] [, ...])
> [ WITH ( statistics_parameter [= value] [, ... ] )
>
> The open square bracket before WITH doesn't get closed.  Also, it
> indicates that columns are entirely options, so () would be valid, but
> that's not the case. Also, a space is missing after the first
> ellipsis.  So I think this should read:
>
> CREATE STATISTICS [ IF NOT EXISTS ] statistics_name ON table_name (
>    { column_name } [, ... ])
> [ WITH ( statistics_parameter [= value] [, ... ] ) ]

Yeah, will fix.


regards

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


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