<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Limitations</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 8.3.6 Documentation" HREF="index.html"><LINK REL="UP" TITLE="GIN Indexes" HREF="gin.html"><LINK REL="PREVIOUS" TITLE="GIN tips and tricks" HREF="gin-tips.html"><LINK REL="NEXT" TITLE="Examples" HREF="gin-examples.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="2009-02-03T04:34:16"></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 8.3.6 Documentation</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="gin-tips.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="gin.html" >Fast Backward</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 52. GIN Indexes</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="gin.html" >Fast Forward</A ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="gin-examples.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="GIN-LIMIT" >52.5. Limitations</A ></H1 ><P > <ACRONYM CLASS="ACRONYM" >GIN</ACRONYM > doesn't support full index scans: because there are often many keys per value, each heap pointer would be returned many times, and there is no easy way to prevent this. </P ><P > When <CODE CLASS="FUNCTION" >extractQuery</CODE > returns zero keys, <ACRONYM CLASS="ACRONYM" >GIN</ACRONYM > will emit an error. Depending on the operator, a void query might match all, some, or none of the indexed values (for example, every array contains the empty array, but does not overlap the empty array), and <ACRONYM CLASS="ACRONYM" >GIN</ACRONYM > cannot determine the correct answer, nor produce a full-index-scan result if it could determine that that was correct. </P ><P > It is not an error for <CODE CLASS="FUNCTION" >extractValue</CODE > to return zero keys, but in this case the indexed value will be unrepresented in the index. This is another reason why full index scan is not useful — it would miss such rows. </P ><P > <ACRONYM CLASS="ACRONYM" >GIN</ACRONYM > searches keys only by equality matching. This might be improved in future. </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="gin-tips.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="gin-examples.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >GIN tips and tricks</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="gin.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Examples</TD ></TR ></TABLE ></DIV ></BODY ></HTML >