<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >tsm_system_rows</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.6.21 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Additional Supplied Modules" HREF="contrib.html"><LINK REL="PREVIOUS" TITLE="tsearch2" HREF="tsearch2.html"><LINK REL="NEXT" TITLE="tsm_system_time" HREF="tsm-system-time.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2021-02-27T18:26:08"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="4" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.6.21 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="tsearch2" HREF="tsearch2.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="contrib.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Appendix F. Additional Supplied Modules</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="tsm_system_time" HREF="tsm-system-time.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="TSM-SYSTEM-ROWS" >F.42. tsm_system_rows</A ></H1 ><P > The <TT CLASS="FILENAME" >tsm_system_rows</TT > module provides the table sampling method <TT CLASS="LITERAL" >SYSTEM_ROWS</TT >, which can be used in the <TT CLASS="LITERAL" >TABLESAMPLE</TT > clause of a <A HREF="sql-select.html" >SELECT</A > command. </P ><P > 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. </P ><P > Like the built-in <TT CLASS="LITERAL" >SYSTEM</TT > sampling method, <TT CLASS="LITERAL" >SYSTEM_ROWS</TT > 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. </P ><P > <TT CLASS="LITERAL" >SYSTEM_ROWS</TT > does not support the <TT CLASS="LITERAL" >REPEATABLE</TT > clause. </P ><DIV CLASS="SECT2" ><H2 CLASS="SECT2" ><A NAME="AEN144578" >F.42.1. Examples</A ></H2 ><P > Here is an example of selecting a sample of a table with <TT CLASS="LITERAL" >SYSTEM_ROWS</TT >. First install the extension: </P ><PRE CLASS="PROGRAMLISTING" >CREATE EXTENSION tsm_system_rows;</PRE ><P > Then you can use it in a <TT CLASS="COMMAND" >SELECT</TT > command, for instance: </P><PRE CLASS="PROGRAMLISTING" >SELECT * FROM my_table TABLESAMPLE SYSTEM_ROWS(100);</PRE ><P> </P ><P > This command will return a sample of 100 rows from the table <TT CLASS="STRUCTNAME" >my_table</TT > (unless the table does not have 100 visible rows, in which case all its rows are returned). </P ></DIV ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="tsearch2.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="tsm-system-time.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >tsearch2</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="contrib.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >tsm_system_time</TD ></TR ></TABLE ></DIV ></BODY ></HTML >