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TABLESAMPLE, SQL Standard and extensible

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Add a TABLESAMPLE clause to SELECT statements that allows
user to specify random BERNOULLI sampling or block level
SYSTEM sampling. Implementation allows for extensible
sampling functions to be written, using a standard API.
Basic version follows SQLStandard exactly. Usable
concrete use cases for the sampling API follow in later
commits.

Petr Jelinek

Reviewed by Michael Paquier and Simon Riggs
This commit is contained in:
Simon Riggs
2015-05-15 14:37:10 -04:00
parent 11a83bbedd
commit f6d208d6e5
66 changed files with 2756 additions and 40 deletions
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61
doc/src/sgml/ref/select.sgml
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@@ -49,7 +49,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="parameter">expression</replac
<phrase>where <replaceable class="parameter">from_item</replaceable> can be one of:</phrase>
[ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ] [ [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ] ]
[ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ] [ [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ] ] [ TABLESAMPLE <replaceable class="parameter">sampling_method</replaceable> ( <replaceable class="parameter">argument</replaceable> [, ...] ) [ REPEATABLE ( <replaceable class="parameter">seed</replaceable> ) ] ]
[ LATERAL ] ( <replaceable class="parameter">select</replaceable> ) [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ]
<replaceable class="parameter">with_query_name</replaceable> [ [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ] ]
[ LATERAL ] <replaceable class="parameter">function_name</replaceable> ( [ <replaceable class="parameter">argument</replaceable> [, ...] ] )
@@ -316,6 +316,50 @@ TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ]
</listitem>
</varlistentry>
<varlistentry>
<term>TABLESAMPLE <replaceable class="parameter">sampling_method</replaceable> ( <replaceable class="parameter">argument</replaceable> [, ...] ) [ REPEATABLE ( <replaceable class="parameter">seed</replaceable> ) ]</term>
<listitem>
<para>
Table sample clause after
<replaceable class="parameter">table_name</replaceable> indicates that
a <replaceable class="parameter">sampling_method</replaceable> should
be used to retrieve subset of rows in the table.
The <replaceable class="parameter">sampling_method</replaceable> can be
any sampling method installed in the database. There are currently two
sampling methods available in the standard
<productname>PostgreSQL</productname> distribution:
<itemizedlist>
<listitem>
<para><literal>SYSTEM</literal></para>
</listitem>
<listitem>
<para><literal>BERNOULLI</literal></para>
</listitem>
</itemizedlist>
Both of these sampling methods currently accept only single argument
which is the percent (floating point from 0 to 100) of the rows to
be returned.
The <literal>SYSTEM</literal> sampling method does block level
sampling with each block having the same chance of being selected and
returns all rows from each selected block.
The <literal>BERNOULLI</literal> scans whole table and returns
individual rows with equal probability. Additional sampling methods
may be installed in the database via extensions.
</para>
<para>
The optional parameter <literal>REPEATABLE</literal> uses the seed
parameter, which can be a number or expression producing a number, as
a random seed for sampling. Note that subsequent commands may return
different results even if same <literal>REPEATABLE</literal> clause was
specified. This happens because <acronym>DML</acronym> statements and
maintenance operations such as <command>VACUUM</> may affect physical
distribution of data. The <function>setseed()</> function will not
affect the sampling result when the <literal>REPEATABLE</literal>
parameter is used.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">alias</replaceable></term>
<listitem>
@@ -1927,5 +1971,20 @@ SELECT distributors.* WHERE distributors.name = 'Westward';
<literal>ROWS FROM( ... )</> is an extension of the SQL standard.
</para>
</refsect2>
<refsect2>
<title><literal>TABLESAMPLE</literal> clause</title>
<para>
The <literal>TABLESAMPLE</> clause is currently accepted only on physical
relations and materialized views.
</para>
<para>
Additional modules allow you to install custom sampling methods and use
them instead of the SQL standard methods.
</para>
</refsect2>
</refsect1>
</refentry>