Adding upstream version 15.4.upstream/15.4upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 69 insertions, 0 deletions

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+<!-- doc/src/sgml/tsm-system-rows.sgml -->
+
+<sect1 id="tsm-system-rows" xreflabel="tsm_system_rows">
+ <title>tsm_system_rows</title>
+
+ <indexterm zone="tsm-system-rows">
+  <primary>tsm_system_rows</primary>
+ </indexterm>
+
+ <para>
+  The <filename>tsm_system_rows</filename> module provides the table sampling method
+  <literal>SYSTEM_ROWS</literal>, which can be used in
+  the <literal>TABLESAMPLE</literal> clause of a <link linkend="sql-select"><command>SELECT</command></link>
+  command.
+ </para>
+
+ <para>
+  This table sampling method accepts a single integer argument that is the
+  maximum number of rows to read.  The resulting sample will always contain
+  exactly that many rows, unless the table does not contain enough rows, in
+  which case the whole table is selected.
+ </para>
+
+ <para>
+  Like the built-in <literal>SYSTEM</literal> sampling
+  method, <literal>SYSTEM_ROWS</literal> performs block-level sampling, so
+  that the sample is not completely random but may be subject to clustering
+  effects, especially if only a small number of rows are requested.
+ </para>
+
+ <para>
+  <literal>SYSTEM_ROWS</literal> does not support
+  the <literal>REPEATABLE</literal> clause.
+ </para>
+
+ <para>
+  This module is considered <quote>trusted</quote>, that is, it can be
+  installed by non-superusers who have <literal>CREATE</literal> privilege
+  on the current database.
+ </para>
+
+ <sect2>
+  <title>Examples</title>
+
+  <para>
+   Here is an example of selecting a sample of a table with
+   <literal>SYSTEM_ROWS</literal>.  First install the extension:
+  </para>
+
+<programlisting>
+CREATE EXTENSION tsm_system_rows;
+</programlisting>
+
+  <para>
+   Then you can use it in a <command>SELECT</command> command, for instance:
+
+<programlisting>
+SELECT * FROM my_table TABLESAMPLE SYSTEM_ROWS(100);
+</programlisting>
+  </para>
+
+  <para>
+   This command will return a sample of 100 rows from the
+   table <structname>my_table</structname> (unless the table does not have 100
+   visible rows, in which case all its rows are returned).
+  </para>
+ </sect2>
+
+</sect1>