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<!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>59.3. Executing Custom Scans</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-plan.html" title="59.2. Creating Custom Scan Plans" /><link rel="next" href="geqo.html" title="Chapter 60. Genetic Query Optimizer" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">59.3. Executing Custom Scans</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="custom-scan-plan.html" title="59.2. Creating Custom Scan Plans">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="custom-scan.html" title="Chapter 59. Writing A Custom Scan Provider">Up</a></td><th width="60%" align="center">Chapter 59. Writing A Custom Scan Provider</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="geqo.html" title="Chapter 60. Genetic Query Optimizer">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="CUSTOM-SCAN-EXECUTION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">59.3. Executing Custom Scans</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="custom-scan-execution.html#CUSTOM-SCAN-EXECUTION-CALLBACKS">59.3.1. Custom Scan Execution Callbacks</a></span></dt></dl></div><p>
When a <code class="structfield">CustomScan</code> is executed, its execution state is
represented by a <code class="structfield">CustomScanState</code>, which is declared as
follows:
</p><pre class="programlisting">
typedef struct CustomScanState
{
ScanState ss;
uint32 flags;
const CustomExecMethods *methods;
} CustomScanState;
</pre><p>
</p><p>
<code class="structfield">ss</code> is initialized as for any other scan state,
except that if the scan is for a join rather than a base relation,
<code class="literal">ss.ss_currentRelation</code> is left NULL.
<code class="structfield">flags</code> is a bit mask with the same meaning as in
<code class="structname">CustomPath</code> and <code class="structname">CustomScan</code>.
<code class="structfield">methods</code> must point to a (usually statically allocated)
object implementing the required custom scan state methods, which are
further detailed below. Typically, a <code class="structname">CustomScanState</code>, which
need not support <code class="function">copyObject</code>, will actually be a larger
structure embedding the above as its first member.
</p><div class="sect2" id="CUSTOM-SCAN-EXECUTION-CALLBACKS"><div class="titlepage"><div><div><h3 class="title">59.3.1. Custom Scan Execution Callbacks</h3></div></div></div><p>
</p><pre class="programlisting">
void (*BeginCustomScan) (CustomScanState *node,
EState *estate,
int eflags);
</pre><p>
Complete initialization of the supplied <code class="structname">CustomScanState</code>.
Standard fields have been initialized by <code class="function">ExecInitCustomScan</code>,
but any private fields should be initialized here.
</p><p>
</p><pre class="programlisting">
TupleTableSlot *(*ExecCustomScan) (CustomScanState *node);
</pre><p>
Fetch the next scan tuple. If any tuples remain, it should fill
<code class="literal">ps_ResultTupleSlot</code> with the next tuple in the current scan
direction, and then return the tuple slot. If not,
<code class="literal">NULL</code> or an empty slot should be returned.
</p><p>
</p><pre class="programlisting">
void (*EndCustomScan) (CustomScanState *node);
</pre><p>
Clean up any private data associated with the <code class="literal">CustomScanState</code>.
This method is required, but it does not need to do anything if there is
no associated data or it will be cleaned up automatically.
</p><p>
</p><pre class="programlisting">
void (*ReScanCustomScan) (CustomScanState *node);
</pre><p>
Rewind the current scan to the beginning and prepare to rescan the
relation.
</p><p>
</p><pre class="programlisting">
void (*MarkPosCustomScan) (CustomScanState *node);
</pre><p>
Save the current scan position so that it can subsequently be restored
by the <code class="function">RestrPosCustomScan</code> callback. This callback is
optional, and need only be supplied if the
<code class="literal">CUSTOMPATH_SUPPORT_MARK_RESTORE</code> flag is set.
</p><p>
</p><pre class="programlisting">
void (*RestrPosCustomScan) (CustomScanState *node);
</pre><p>
Restore the previous scan position as saved by the
<code class="function">MarkPosCustomScan</code> callback. This callback is optional,
and need only be supplied if the
<code class="literal">CUSTOMPATH_SUPPORT_MARK_RESTORE</code> flag is set.
</p><p>
</p><pre class="programlisting">
Size (*EstimateDSMCustomScan) (CustomScanState *node,
ParallelContext *pcxt);
</pre><p>
Estimate the amount of dynamic shared memory that will be required
for parallel operation. This may be higher than the amount that will
actually be used, but it must not be lower. The return value is in bytes.
This callback is optional, and need only be supplied if this custom
scan provider supports parallel execution.
</p><p>
</p><pre class="programlisting">
void (*InitializeDSMCustomScan) (CustomScanState *node,
ParallelContext *pcxt,
void *coordinate);
</pre><p>
Initialize the dynamic shared memory that will be required for parallel
operation. <code class="literal">coordinate</code> points to a shared memory area of
size equal to the return value of <code class="function">EstimateDSMCustomScan</code>.
This callback is optional, and need only be supplied if this custom
scan provider supports parallel execution.
</p><p>
</p><pre class="programlisting">
void (*ReInitializeDSMCustomScan) (CustomScanState *node,
ParallelContext *pcxt,
void *coordinate);
</pre><p>
Re-initialize the dynamic shared memory required for parallel operation
when the custom-scan plan node is about to be re-scanned.
This callback is optional, and need only be supplied if this custom
scan provider supports parallel execution.
Recommended practice is that this callback reset only shared state,
while the <code class="function">ReScanCustomScan</code> callback resets only local
state. Currently, this callback will be called
before <code class="function">ReScanCustomScan</code>, but it's best not to rely on
that ordering.
</p><p>
</p><pre class="programlisting">
void (*InitializeWorkerCustomScan) (CustomScanState *node,
shm_toc *toc,
void *coordinate);
</pre><p>
Initialize a parallel worker's local state based on the shared state
set up by the leader during <code class="function">InitializeDSMCustomScan</code>.
This callback is optional, and need only be supplied if this custom
scan provider supports parallel execution.
</p><p>
</p><pre class="programlisting">
void (*ShutdownCustomScan) (CustomScanState *node);
</pre><p>
Release resources when it is anticipated the node will not be executed
to completion. This is not called in all cases; sometimes,
<code class="literal">EndCustomScan</code> may be called without this function having
been called first. Since the DSM segment used by parallel query is
destroyed just after this callback is invoked, custom scan providers that
wish to take some action before the DSM segment goes away should implement
this method.
</p><p>
</p><pre class="programlisting">
void (*ExplainCustomScan) (CustomScanState *node,
List *ancestors,
ExplainState *es);
</pre><p>
Output additional information for <code class="command">EXPLAIN</code> of a custom-scan
plan node. This callback is optional. Common data stored in the
<code class="structname">ScanState</code>, such as the target list and scan relation, will
be shown even without this callback, but the callback allows the display
of additional, private state.
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