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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>52.6. Executor</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="planner-optimizer.html" title="52.5. Planner/Optimizer" /><link rel="next" href="catalogs.html" title="Chapter 53. System Catalogs" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">52.6. Executor</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="planner-optimizer.html" title="52.5. Planner/Optimizer">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="overview.html" title="Chapter 52. Overview of PostgreSQL Internals">Up</a></td><th width="60%" align="center">Chapter 52. Overview of PostgreSQL Internals</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="catalogs.html" title="Chapter 53. System Catalogs">Next</a></td></tr></table><hr /></div><div class="sect1" id="EXECUTOR"><div class="titlepage"><div><div><h2 class="title" style="clear: both">52.6. Executor <a href="#EXECUTOR" class="id_link">#</a></h2></div></div></div><p>
The <em class="firstterm">executor</em> takes the plan created by the
planner/optimizer and recursively processes it to extract the required set
of rows. This is essentially a demand-pull pipeline mechanism.
Each time a plan node is called, it must deliver one more row, or
report that it is done delivering rows.
</p><p>
To provide a concrete example, assume that the top
node is a <code class="literal">MergeJoin</code> node.
Before any merge can be done two rows have to be fetched (one from
each subplan). So the executor recursively calls itself to
process the subplans (it starts with the subplan attached to
<code class="literal">lefttree</code>). The new top node (the top node of the left
subplan) is, let's say, a
<code class="literal">Sort</code> node and again recursion is needed to obtain
an input row. The child node of the <code class="literal">Sort</code> might
be a <code class="literal">SeqScan</code> node, representing actual reading of a table.
Execution of this node causes the executor to fetch a row from the
table and return it up to the calling node. The <code class="literal">Sort</code>
node will repeatedly call its child to obtain all the rows to be sorted.
When the input is exhausted (as indicated by the child node returning
a NULL instead of a row), the <code class="literal">Sort</code> code performs
the sort, and finally is able to return its first output row, namely
the first one in sorted order. It keeps the remaining rows stored so
that it can deliver them in sorted order in response to later demands.
</p><p>
The <code class="literal">MergeJoin</code> node similarly demands the first row
from its right subplan. Then it compares the two rows to see if they
can be joined; if so, it returns a join row to its caller. On the next
call, or immediately if it cannot join the current pair of inputs,
it advances to the next row of one table
or the other (depending on how the comparison came out), and again
checks for a match. Eventually, one subplan or the other is exhausted,
and the <code class="literal">MergeJoin</code> node returns NULL to indicate that
no more join rows can be formed.
</p><p>
Complex queries can involve many levels of plan nodes, but the general
approach is the same: each node computes and returns its next output
row each time it is called. Each node is also responsible for applying
any selection or projection expressions that were assigned to it by
the planner.
</p><p>
The executor mechanism is used to evaluate all five basic SQL query
types: <code class="command">SELECT</code>, <code class="command">INSERT</code>,
<code class="command">UPDATE</code>, <code class="command">DELETE</code>, and
<code class="command">MERGE</code>.
For <code class="command">SELECT</code>, the top-level executor code
only needs to send each row returned by the query plan tree
off to the client. <code class="command">INSERT ... SELECT</code>,
<code class="command">UPDATE</code>, <code class="command">DELETE</code>, and
<code class="command">MERGE</code>
are effectively <code class="command">SELECT</code>s under a special
top-level plan node called <code class="literal">ModifyTable</code>.
</p><p>
<code class="command">INSERT ... SELECT</code> feeds the rows up
to <code class="literal">ModifyTable</code> for insertion. For
<code class="command">UPDATE</code>, the planner arranges that each
computed row includes all the updated column values, plus the
<em class="firstterm">TID</em> (tuple ID, or row ID) of the original
target row; this data is fed up to the <code class="literal">ModifyTable</code>
node, which uses the information to create a new updated row and
mark the old row deleted. For <code class="command">DELETE</code>, the only
column that is actually returned by the plan is the TID, and the
<code class="literal">ModifyTable</code> node simply uses the TID to visit each
target row and mark it deleted. For <code class="command">MERGE</code>, the
planner joins the source and target relations, and includes all
column values required by any of the <code class="literal">WHEN</code> clauses,
plus the TID of the target row; this data is fed up to the
<code class="literal">ModifyTable</code> node, which uses the information to
work out which <code class="literal">WHEN</code> clause to execute, and then
inserts, updates or deletes the target row, as required.
</p><p>
A simple <code class="command">INSERT ... VALUES</code> command creates a
trivial plan tree consisting of a single <code class="literal">Result</code>
node, which computes just one result row, feeding that up
to <code class="literal">ModifyTable</code> to perform the insertion.
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