<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Visibility of Data Changes</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 8.0.11 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Triggers" HREF="triggers.html"><LINK REL="PREVIOUS" TITLE="Triggers" HREF="triggers.html"><LINK REL="NEXT" TITLE="Writing Trigger Functions in C" HREF="trigger-interface.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="2007-02-02T03:57:22"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" >PostgreSQL 8.0.11 Documentation</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="triggers.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="triggers.html" >Fast Backward</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 32. Triggers</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="triggers.html" >Fast Forward</A ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="trigger-interface.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="TRIGGER-DATACHANGES" >32.2. Visibility of Data Changes</A ></H1 ><P > If you execute SQL commands in your trigger function, and these commands access the table that the trigger is for, then you need to be aware of the data visibility rules, because they determine whether these SQL commands will see the data change that the trigger is fired for. Briefly: <P ></P ></P><UL ><LI ><P > Statement-level triggers follow simple visibility rules: none of the changes made by a statement are visible to statement-level triggers that are invoked before the statement, whereas all modifications are visible to statement-level after triggers. </P ></LI ><LI ><P > The data change (insertion, update, or deletion) causing the trigger to fire is naturally <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >not</I ></SPAN > visible to SQL commands executed in a row-level before trigger, because it hasn't happened yet. </P ></LI ><LI ><P > However, SQL commands executed in a row-level before trigger <SPAN CLASS="emphasis" ><I CLASS="EMPHASIS" >will</I ></SPAN > see the effects of data changes for rows previously processed in the same outer command. This requires caution, since the ordering of these change events is not in general predictable; a SQL command that affects multiple rows may visit the rows in any order. </P ></LI ><LI ><P > When a row-level after trigger is fired, all data changes made by the outer command are already complete, and are visible to the invoked trigger function. </P ></LI ></UL ><P> </P ><P > Further information about data visibility rules can be found in <A HREF="spi-visibility.html" >Section 39.4</A >. The example in <A HREF="trigger-example.html" >Section 32.4</A > contains a demonstration of these rules. </P ></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="triggers.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="trigger-interface.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Triggers</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="triggers.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Writing Trigger Functions in C</TD ></TR ></TABLE ></DIV ></BODY ></HTML >