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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>61.1. Creating Custom Scan Paths</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="custom-scan.html" title="Chapter 61. Writing a Custom Scan Provider" /><link rel="next" href="custom-scan-plan.html" title="61.2. Creating Custom Scan Plans" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">61.1. Creating Custom Scan Paths</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="custom-scan.html" title="Chapter 61. Writing a Custom Scan Provider">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="custom-scan.html" title="Chapter 61. Writing a Custom Scan Provider">Up</a></td><th width="60%" align="center">Chapter 61. Writing a Custom Scan Provider</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="custom-scan-plan.html" title="61.2. Creating Custom Scan Plans">Next</a></td></tr></table><hr /></div><div class="sect1" id="CUSTOM-SCAN-PATH"><div class="titlepage"><div><div><h2 class="title" style="clear: both">61.1. Creating Custom Scan Paths</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="custom-scan-path.html#CUSTOM-SCAN-PATH-CALLBACKS">61.1.1. Custom Scan Path Callbacks</a></span></dt></dl></div><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 <code class="structname">CustomPath</code> objects and adding
them to <code class="literal">rel</code> using <code class="function">add_path</code>. The custom scan
provider is responsible for initializing the <code class="structname">CustomPath</code>
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>
<code class="structfield">path</code> must be initialized as for any other path, including
the row-count estimate, start and total cost, and sort ordering provided
by this path. <code class="structfield">flags</code> is a bit mask, which
specifies whether the scan provider can support certain optional
capabilities. <code class="structfield">flags</code> should include
<code class="literal">CUSTOMPATH_SUPPORT_BACKWARD_SCAN</code> if the custom path can support
a backward scan, <code class="literal">CUSTOMPATH_SUPPORT_MARK_RESTORE</code> if it
can support mark and restore,
and <code class="literal">CUSTOMPATH_SUPPORT_PROJECTION</code> if it can perform
projections. (If <code class="literal">CUSTOMPATH_SUPPORT_PROJECTION</code> is not
set, the scan node will only be asked to produce Vars of the scanned
relation; while if that flag is set, the scan node must be able to
evaluate scalar expressions over these Vars.)
An optional <code class="structfield">custom_paths</code> is a list of <code class="structname">Path</code>
nodes used by this custom-path node; these will be transformed into
<code class="structname">Plan</code> nodes by planner.
<code class="structfield">custom_private</code> can be used to store the custom path's
private data. Private data should be stored in a form that can be handled
by <code class="literal">nodeToString</code>, so that debugging routines that attempt to
print the custom path will work as designed. <code class="structfield">methods</code> must
point to a (usually statically allocated) object implementing the required
custom path methods, which are further detailed below.
</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 <code class="structname">CustomPath</code> 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" id="CUSTOM-SCAN-PATH-CALLBACKS"><div class="titlepage"><div><div><h3 class="title">61.1.1. Custom Scan Path Callbacks</h3></div></div></div><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 <code class="literal">CustomScan</code> object, which the callback must allocate and
initialize. See <a class="xref" href="custom-scan-plan.html" title="61.2. Creating Custom Scan Plans">Section 61.2</a> for more details.
</p><p>
</p><pre class="programlisting">
List *(*ReparameterizeCustomPathByChild) (PlannerInfo *root,
List *custom_private,
RelOptInfo *child_rel);
</pre><p>
This callback is called while converting a path parameterized by the
top-most parent of the given child relation <code class="literal">child_rel</code>
to be parameterized by the child relation. The callback is used to
reparameterize any paths or translate any expression nodes saved in the
given <code class="literal">custom_private</code> member of a
<code class="structname">CustomPath</code>. The callback may use
<code class="literal">reparameterize_path_by_child</code>,
<code class="literal">adjust_appendrel_attrs</code> or
<code class="literal">adjust_appendrel_attrs_multilevel</code> as required.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="custom-scan.html" title="Chapter 61. Writing a Custom Scan Provider">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="custom-scan.html" title="Chapter 61. Writing a Custom Scan Provider">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="custom-scan-plan.html" title="61.2. Creating Custom Scan Plans">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 61. Writing a Custom Scan Provider </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 61.2. Creating Custom Scan Plans</td></tr></table></div></body></html>
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