<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Creating Custom Scan Paths</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 9.6.19 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Writing A Custom Scan Provider" HREF="custom-scan.html"><LINK REL="PREVIOUS" TITLE="Writing A Custom Scan Provider" HREF="custom-scan.html"><LINK REL="NEXT" TITLE="Creating Custom Scan Plans" HREF="custom-scan-plan.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="2020-09-02T07:27:19"></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 9.6.19 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Writing A Custom Scan Provider" HREF="custom-scan.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="custom-scan.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 57. Writing A Custom Scan Provider</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="Creating Custom Scan Plans" HREF="custom-scan-plan.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="CUSTOM-SCAN-PATH" >57.1. Creating Custom Scan Paths</A ></H1 ><P > A custom scan provider will typically add paths for a base relation by setting the following hook, which is called after the core code has generated all the access paths it can for the relation (except for Gather paths, which are made after this call so that they can use partial paths added by the hook): </P><PRE CLASS="PROGRAMLISTING" >typedef void (*set_rel_pathlist_hook_type) (PlannerInfo *root, RelOptInfo *rel, Index rti, RangeTblEntry *rte); extern PGDLLIMPORT set_rel_pathlist_hook_type set_rel_pathlist_hook;</PRE ><P> </P ><P > Although this hook function can be used to examine, modify, or remove paths generated by the core system, a custom scan provider will typically confine itself to generating <TT CLASS="STRUCTNAME" >CustomPath</TT > objects and adding them to <TT CLASS="LITERAL" >rel</TT > using <CODE CLASS="FUNCTION" >add_path</CODE >. The custom scan provider is responsible for initializing the <TT CLASS="STRUCTNAME" >CustomPath</TT > object, which is declared like this: </P><PRE CLASS="PROGRAMLISTING" >typedef struct CustomPath { Path path; uint32 flags; List *custom_paths; List *custom_private; const CustomPathMethods *methods; } CustomPath;</PRE ><P> </P ><P > <TT CLASS="STRUCTFIELD" >path</TT > must be initialized as for any other path, including the row-count estimate, start and total cost, and sort ordering provided by this path. <TT CLASS="STRUCTFIELD" >flags</TT > is a bit mask, which should include <TT CLASS="LITERAL" >CUSTOMPATH_SUPPORT_BACKWARD_SCAN</TT > if the custom path can support a backward scan and <TT CLASS="LITERAL" >CUSTOMPATH_SUPPORT_MARK_RESTORE</TT > if it can support mark and restore. Both capabilities are optional. An optional <TT CLASS="STRUCTFIELD" >custom_paths</TT > is a list of <TT CLASS="STRUCTNAME" >Path</TT > nodes used by this custom-path node; these will be transformed into <TT CLASS="STRUCTNAME" >Plan</TT > nodes by planner. <TT CLASS="STRUCTFIELD" >custom_private</TT > can be used to store the custom path's private data. Private data should be stored in a form that can be handled by <TT CLASS="LITERAL" >nodeToString</TT >, so that debugging routines that attempt to print the custom path will work as designed. <TT CLASS="STRUCTFIELD" >methods</TT > must point to a (usually statically allocated) object implementing the required custom path methods, of which there is currently only one. </P ><P > A custom scan provider can also provide join paths. Just as for base relations, such a path must produce the same output as would normally be produced by the join it replaces. To do this, the join provider should set the following hook, and then within the hook function, create <TT CLASS="STRUCTNAME" >CustomPath</TT > path(s) for the join relation. </P><PRE CLASS="PROGRAMLISTING" >typedef void (*set_join_pathlist_hook_type) (PlannerInfo *root, RelOptInfo *joinrel, RelOptInfo *outerrel, RelOptInfo *innerrel, JoinType jointype, JoinPathExtraData *extra); extern PGDLLIMPORT set_join_pathlist_hook_type set_join_pathlist_hook;</PRE ><P> This hook will be invoked repeatedly for the same join relation, with different combinations of inner and outer relations; it is the responsibility of the hook to minimize duplicated work. </P ><DIV CLASS="SECT2" ><H2 CLASS="SECT2" ><A NAME="CUSTOM-SCAN-PATH-CALLBACKS" >57.1.1. Custom Scan Path Callbacks</A ></H2 ><P ></P><PRE CLASS="PROGRAMLISTING" >Plan *(*PlanCustomPath) (PlannerInfo *root, RelOptInfo *rel, CustomPath *best_path, List *tlist, List *clauses, List *custom_plans);</PRE ><P> Convert a custom path to a finished plan. The return value will generally be a <TT CLASS="LITERAL" >CustomScan</TT > object, which the callback must allocate and initialize. See <A HREF="custom-scan-plan.html" >Section 57.2</A > for more details. </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="custom-scan.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="custom-scan-plan.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Writing A Custom Scan Provider</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="custom-scan.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Creating Custom Scan Plans</TD ></TR ></TABLE ></DIV ></BODY ></HTML >