From: | Jaime Casanova <jcasanov(at)systemguards(dot)com(dot)ec> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: ALTER TABLE ... ALTER COLUMN ... SET DISTINCT |
Date: | 2009-05-04 03:14:19 |
Message-ID: | 3073cc9b0905032014u47118344t5b3acc3489404a06@mail.gmail.com |
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Lists: | pgsql-hackers |
On Sun, May 3, 2009 at 3:13 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Mon, Apr 6, 2009 at 11:15 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> So based on this comment and Stephen's remarks, I'm going to assume
>> that I'm succumbing to a fit of unjustified paranoia and re-implement
>> as you suggest.
>
> OK, new version of patch, this time with the weird scaling removed and
> the datatype changed to float4.
>
In this paragraph i think you mean: "ALTER TABLE ... ALTER COLUMN ...
SET DISTINCT"?
<para>
+ The analyzer also estimates the number of distinct values that appear
+ in each column. Because only a subset of pages are scanned, this method
+ can sometimes be quite inaccurate, especially for large tables. If this
+ inaccuracy leads to bad query plans, the analyzer can be forced to use
+ a more correct value with <command>ALTER TABLE ... ALTER COLUMN ... SET
+ STATISTICS</command> (see <xref linkend="sql-altertable"
+ endterm="sql-altertable-title">).
+ </para>
--
Atentamente,
Jaime Casanova
Soporte y capacitación de PostgreSQL
Asesoría y desarrollo de sistemas
Guayaquil - Ecuador
Cel. +59387171157
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