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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>41.2. Views and the Rule System</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="querytree.html" title="41.1. The Query Tree" /><link rel="next" href="rules-materializedviews.html" title="41.3. Materialized Views" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">41.2. Views and the Rule System</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="querytree.html" title="41.1. The Query Tree">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="rules.html" title="Chapter 41. The Rule System">Up</a></td><th width="60%" align="center">Chapter 41. The Rule System</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="rules-materializedviews.html" title="41.3. Materialized Views">Next</a></td></tr></table><hr /></div><div class="sect1" id="RULES-VIEWS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">41.2. Views and the Rule System <a href="#RULES-VIEWS" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="rules-views.html#RULES-SELECT">41.2.1. How <code class="command">SELECT</code> Rules Work</a></span></dt><dt><span class="sect2"><a href="rules-views.html#RULES-VIEWS-NON-SELECT">41.2.2. View Rules in Non-<code class="command">SELECT</code> Statements</a></span></dt><dt><span class="sect2"><a href="rules-views.html#RULES-VIEWS-POWER">41.2.3. The Power of Views in <span class="productname">PostgreSQL</span></a></span></dt><dt><span class="sect2"><a href="rules-views.html#RULES-VIEWS-UPDATE">41.2.4. Updating a View</a></span></dt></dl></div><a id="id-1.8.6.7.2" class="indexterm"></a><a id="id-1.8.6.7.3" class="indexterm"></a><p>
Views in <span class="productname">PostgreSQL</span> are implemented
using the rule system. A view is basically an empty table (having no
actual storage) with an <code class="literal">ON SELECT DO INSTEAD</code> rule.
Conventionally, that rule is named <code class="literal">_RETURN</code>.
So a view like
</p><pre class="programlisting">
CREATE VIEW myview AS SELECT * FROM mytab;
</pre><p>
is very nearly the same thing as
</p><pre class="programlisting">
CREATE TABLE myview (<em class="replaceable"><code>same column list as mytab</code></em>);
CREATE RULE "_RETURN" AS ON SELECT TO myview DO INSTEAD
SELECT * FROM mytab;
</pre><p>
although you can't actually write that, because tables are not
allowed to have <code class="literal">ON SELECT</code> rules.
</p><p>
A view can also have other kinds of <code class="literal">DO INSTEAD</code>
rules, allowing <code class="command">INSERT</code>, <code class="command">UPDATE</code>,
or <code class="command">DELETE</code> commands to be performed on the view
despite its lack of underlying storage.
This is discussed further below, in
<a class="xref" href="rules-views.html#RULES-VIEWS-UPDATE" title="41.2.4. Updating a View">Section 41.2.4</a>.
</p><div class="sect2" id="RULES-SELECT"><div class="titlepage"><div><div><h3 class="title">41.2.1. How <code class="command">SELECT</code> Rules Work <a href="#RULES-SELECT" class="id_link">#</a></h3></div></div></div><a id="id-1.8.6.7.6.2" class="indexterm"></a><p>
Rules <code class="literal">ON SELECT</code> are applied to all queries as the last step, even
if the command given is an <code class="command">INSERT</code>,
<code class="command">UPDATE</code> or <code class="command">DELETE</code>. And they
have different semantics from rules on the other command types in that they modify the
query tree in place instead of creating a new one. So
<code class="command">SELECT</code> rules are described first.
</p><p>
Currently, there can be only one action in an <code class="literal">ON SELECT</code> rule, and it must
be an unconditional <code class="command">SELECT</code> action that is <code class="literal">INSTEAD</code>. This restriction was
required to make rules safe enough to open them for ordinary users, and
it restricts <code class="literal">ON SELECT</code> rules to act like views.
</p><p>
The examples for this chapter are two join views that do some
calculations and some more views using them in turn. One of the
two first views is customized later by adding rules for
<code class="command">INSERT</code>, <code class="command">UPDATE</code>, and
<code class="command">DELETE</code> operations so that the final result will
be a view that behaves like a real table with some magic
functionality. This is not such a simple example to start from and
this makes things harder to get into. But it's better to have one
example that covers all the points discussed step by step rather
than having many different ones that might mix up in mind.
</p><p>
The real tables we need in the first two rule system descriptions
are these:
</p><pre class="programlisting">
CREATE TABLE shoe_data (
shoename text, -- primary key
sh_avail integer, -- available number of pairs
slcolor text, -- preferred shoelace color
slminlen real, -- minimum shoelace length
slmaxlen real, -- maximum shoelace length
slunit text -- length unit
);
CREATE TABLE shoelace_data (
sl_name text, -- primary key
sl_avail integer, -- available number of pairs
sl_color text, -- shoelace color
sl_len real, -- shoelace length
sl_unit text -- length unit
);
CREATE TABLE unit (
un_name text, -- primary key
un_fact real -- factor to transform to cm
);
</pre><p>
As you can see, they represent shoe-store data.
</p><p>
The views are created as:
</p><pre class="programlisting">
CREATE VIEW shoe AS
SELECT sh.shoename,
sh.sh_avail,
sh.slcolor,
sh.slminlen,
sh.slminlen * un.un_fact AS slminlen_cm,
sh.slmaxlen,
sh.slmaxlen * un.un_fact AS slmaxlen_cm,
sh.slunit
FROM shoe_data sh, unit un
WHERE sh.slunit = un.un_name;
CREATE VIEW shoelace AS
SELECT s.sl_name,
s.sl_avail,
s.sl_color,
s.sl_len,
s.sl_unit,
s.sl_len * u.un_fact AS sl_len_cm
FROM shoelace_data s, unit u
WHERE s.sl_unit = u.un_name;
CREATE VIEW shoe_ready AS
SELECT rsh.shoename,
rsh.sh_avail,
rsl.sl_name,
rsl.sl_avail,
least(rsh.sh_avail, rsl.sl_avail) AS total_avail
FROM shoe rsh, shoelace rsl
WHERE rsl.sl_color = rsh.slcolor
AND rsl.sl_len_cm >= rsh.slminlen_cm
AND rsl.sl_len_cm <= rsh.slmaxlen_cm;
</pre><p>
The <code class="command">CREATE VIEW</code> command for the
<code class="literal">shoelace</code> view (which is the simplest one we
have) will create a relation <code class="literal">shoelace</code> and an entry in
<code class="structname">pg_rewrite</code> that tells that there is a
rewrite rule that must be applied whenever the relation <code class="literal">shoelace</code>
is referenced in a query's range table. The rule has no rule
qualification (discussed later, with the non-<code class="command">SELECT</code> rules, since
<code class="command">SELECT</code> rules currently cannot have them) and it is <code class="literal">INSTEAD</code>. Note
that rule qualifications are not the same as query qualifications.
The action of our rule has a query qualification.
The action of the rule is one query tree that is a copy of the
<code class="command">SELECT</code> statement in the view creation command.
</p><div class="note"><h3 class="title">Note</h3><p>
The two extra range
table entries for <code class="literal">NEW</code> and <code class="literal">OLD</code> that you can see in
the <code class="structname">pg_rewrite</code> entry aren't of interest
for <code class="command">SELECT</code> rules.
</p></div><p>
Now we populate <code class="literal">unit</code>, <code class="literal">shoe_data</code>
and <code class="literal">shoelace_data</code> and run a simple query on a view:
</p><pre class="programlisting">
INSERT INTO unit VALUES ('cm', 1.0);
INSERT INTO unit VALUES ('m', 100.0);
INSERT INTO unit VALUES ('inch', 2.54);
INSERT INTO shoe_data VALUES ('sh1', 2, 'black', 70.0, 90.0, 'cm');
INSERT INTO shoe_data VALUES ('sh2', 0, 'black', 30.0, 40.0, 'inch');
INSERT INTO shoe_data VALUES ('sh3', 4, 'brown', 50.0, 65.0, 'cm');
INSERT INTO shoe_data VALUES ('sh4', 3, 'brown', 40.0, 50.0, 'inch');
INSERT INTO shoelace_data VALUES ('sl1', 5, 'black', 80.0, 'cm');
INSERT INTO shoelace_data VALUES ('sl2', 6, 'black', 100.0, 'cm');
INSERT INTO shoelace_data VALUES ('sl3', 0, 'black', 35.0 , 'inch');
INSERT INTO shoelace_data VALUES ('sl4', 8, 'black', 40.0 , 'inch');
INSERT INTO shoelace_data VALUES ('sl5', 4, 'brown', 1.0 , 'm');
INSERT INTO shoelace_data VALUES ('sl6', 0, 'brown', 0.9 , 'm');
INSERT INTO shoelace_data VALUES ('sl7', 7, 'brown', 60 , 'cm');
INSERT INTO shoelace_data VALUES ('sl8', 1, 'brown', 40 , 'inch');
SELECT * FROM shoelace;
sl_name | sl_avail | sl_color | sl_len | sl_unit | sl_len_cm
-----------+----------+----------+--------+---------+-----------
sl1 | 5 | black | 80 | cm | 80
sl2 | 6 | black | 100 | cm | 100
sl7 | 7 | brown | 60 | cm | 60
sl3 | 0 | black | 35 | inch | 88.9
sl4 | 8 | black | 40 | inch | 101.6
sl8 | 1 | brown | 40 | inch | 101.6
sl5 | 4 | brown | 1 | m | 100
sl6 | 0 | brown | 0.9 | m | 90
(8 rows)
</pre><p>
</p><p>
This is the simplest <code class="command">SELECT</code> you can do on our
views, so we take this opportunity to explain the basics of view
rules. The <code class="literal">SELECT * FROM shoelace</code> was
interpreted by the parser and produced the query tree:
</p><pre class="programlisting">
SELECT shoelace.sl_name, shoelace.sl_avail,
shoelace.sl_color, shoelace.sl_len,
shoelace.sl_unit, shoelace.sl_len_cm
FROM shoelace shoelace;
</pre><p>
and this is given to the rule system. The rule system walks through the
range table and checks if there are rules
for any relation. When processing the range table entry for
<code class="literal">shoelace</code> (the only one up to now) it finds the
<code class="literal">_RETURN</code> rule with the query tree:
</p><pre class="programlisting">
SELECT s.sl_name, s.sl_avail,
s.sl_color, s.sl_len, s.sl_unit,
s.sl_len * u.un_fact AS sl_len_cm
FROM shoelace old, shoelace new,
shoelace_data s, unit u
WHERE s.sl_unit = u.un_name;
</pre><p>
</p><p>
To expand the view, the rewriter simply creates a subquery range-table
entry containing the rule's action query tree, and substitutes this
range table entry for the original one that referenced the view. The
resulting rewritten query tree is almost the same as if you had typed:
</p><pre class="programlisting">
SELECT shoelace.sl_name, shoelace.sl_avail,
shoelace.sl_color, shoelace.sl_len,
shoelace.sl_unit, shoelace.sl_len_cm
FROM (SELECT s.sl_name,
s.sl_avail,
s.sl_color,
s.sl_len,
s.sl_unit,
s.sl_len * u.un_fact AS sl_len_cm
FROM shoelace_data s, unit u
WHERE s.sl_unit = u.un_name) shoelace;
</pre><p>
There is one difference however: the subquery's range table has two
extra entries <code class="literal">shoelace old</code> and <code class="literal">shoelace new</code>. These entries don't
participate directly in the query, since they aren't referenced by
the subquery's join tree or target list. The rewriter uses them
to store the access privilege check information that was originally present
in the range-table entry that referenced the view. In this way, the
executor will still check that the user has proper privileges to access
the view, even though there's no direct use of the view in the rewritten
query.
</p><p>
That was the first rule applied. The rule system will continue checking
the remaining range-table entries in the top query (in this example there
are no more), and it will recursively check the range-table entries in
the added subquery to see if any of them reference views. (But it
won't expand <code class="literal">old</code> or <code class="literal">new</code> — otherwise we'd have infinite recursion!)
In this example, there are no rewrite rules for <code class="literal">shoelace_data</code> or <code class="literal">unit</code>,
so rewriting is complete and the above is the final result given to
the planner.
</p><p>
Now we want to write a query that finds out for which shoes currently in the store
we have the matching shoelaces (color and length) and where the
total number of exactly matching pairs is greater than or equal to two.
</p><pre class="programlisting">
SELECT * FROM shoe_ready WHERE total_avail >= 2;
shoename | sh_avail | sl_name | sl_avail | total_avail
----------+----------+---------+----------+-------------
sh1 | 2 | sl1 | 5 | 2
sh3 | 4 | sl7 | 7 | 4
(2 rows)
</pre><p>
</p><p>
The output of the parser this time is the query tree:
</p><pre class="programlisting">
SELECT shoe_ready.shoename, shoe_ready.sh_avail,
shoe_ready.sl_name, shoe_ready.sl_avail,
shoe_ready.total_avail
FROM shoe_ready shoe_ready
WHERE shoe_ready.total_avail >= 2;
</pre><p>
The first rule applied will be the one for the
<code class="literal">shoe_ready</code> view and it results in the
query tree:
</p><pre class="programlisting">
SELECT shoe_ready.shoename, shoe_ready.sh_avail,
shoe_ready.sl_name, shoe_ready.sl_avail,
shoe_ready.total_avail
FROM (SELECT rsh.shoename,
rsh.sh_avail,
rsl.sl_name,
rsl.sl_avail,
least(rsh.sh_avail, rsl.sl_avail) AS total_avail
FROM shoe rsh, shoelace rsl
WHERE rsl.sl_color = rsh.slcolor
AND rsl.sl_len_cm >= rsh.slminlen_cm
AND rsl.sl_len_cm <= rsh.slmaxlen_cm) shoe_ready
WHERE shoe_ready.total_avail >= 2;
</pre><p>
Similarly, the rules for <code class="literal">shoe</code> and
<code class="literal">shoelace</code> are substituted into the range table of
the subquery, leading to a three-level final query tree:
</p><pre class="programlisting">
SELECT shoe_ready.shoename, shoe_ready.sh_avail,
shoe_ready.sl_name, shoe_ready.sl_avail,
shoe_ready.total_avail
FROM (SELECT rsh.shoename,
rsh.sh_avail,
rsl.sl_name,
rsl.sl_avail,
least(rsh.sh_avail, rsl.sl_avail) AS total_avail
FROM (SELECT sh.shoename,
sh.sh_avail,
sh.slcolor,
sh.slminlen,
sh.slminlen * un.un_fact AS slminlen_cm,
sh.slmaxlen,
sh.slmaxlen * un.un_fact AS slmaxlen_cm,
sh.slunit
FROM shoe_data sh, unit un
WHERE sh.slunit = un.un_name) rsh,
(SELECT s.sl_name,
s.sl_avail,
s.sl_color,
s.sl_len,
s.sl_unit,
s.sl_len * u.un_fact AS sl_len_cm
FROM shoelace_data s, unit u
WHERE s.sl_unit = u.un_name) rsl
WHERE rsl.sl_color = rsh.slcolor
AND rsl.sl_len_cm >= rsh.slminlen_cm
AND rsl.sl_len_cm <= rsh.slmaxlen_cm) shoe_ready
WHERE shoe_ready.total_avail > 2;
</pre><p>
</p><p>
This might look inefficient, but the planner will collapse this into a
single-level query tree by <span class="quote">“<span class="quote">pulling up</span>”</span> the subqueries,
and then it will plan the joins just as if we'd written them out
manually. So collapsing the query tree is an optimization that the
rewrite system doesn't have to concern itself with.
</p></div><div class="sect2" id="RULES-VIEWS-NON-SELECT"><div class="titlepage"><div><div><h3 class="title">41.2.2. View Rules in Non-<code class="command">SELECT</code> Statements <a href="#RULES-VIEWS-NON-SELECT" class="id_link">#</a></h3></div></div></div><p>
Two details of the query tree aren't touched in the description of
view rules above. These are the command type and the result relation.
In fact, the command type is not needed by view rules, but the result
relation may affect the way in which the query rewriter works, because
special care needs to be taken if the result relation is a view.
</p><p>
There are only a few differences between a query tree for a
<code class="command">SELECT</code> and one for any other
command. Obviously, they have a different command type and for a
command other than a <code class="command">SELECT</code>, the result
relation points to the range-table entry where the result should
go. Everything else is absolutely the same. So having two tables
<code class="literal">t1</code> and <code class="literal">t2</code> with columns <code class="literal">a</code> and
<code class="literal">b</code>, the query trees for the two statements:
</p><pre class="programlisting">
SELECT t2.b FROM t1, t2 WHERE t1.a = t2.a;
UPDATE t1 SET b = t2.b FROM t2 WHERE t1.a = t2.a;
</pre><p>
are nearly identical. In particular:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
The range tables contain entries for the tables <code class="literal">t1</code> and <code class="literal">t2</code>.
</p></li><li class="listitem"><p>
The target lists contain one variable that points to column
<code class="literal">b</code> of the range table entry for table <code class="literal">t2</code>.
</p></li><li class="listitem"><p>
The qualification expressions compare the columns <code class="literal">a</code> of both
range-table entries for equality.
</p></li><li class="listitem"><p>
The join trees show a simple join between <code class="literal">t1</code> and <code class="literal">t2</code>.
</p></li></ul></div><p>
</p><p>
The consequence is, that both query trees result in similar
execution plans: They are both joins over the two tables. For the
<code class="command">UPDATE</code> the missing columns from <code class="literal">t1</code> are added to
the target list by the planner and the final query tree will read
as:
</p><pre class="programlisting">
UPDATE t1 SET a = t1.a, b = t2.b FROM t2 WHERE t1.a = t2.a;
</pre><p>
and thus the executor run over the join will produce exactly the
same result set as:
</p><pre class="programlisting">
SELECT t1.a, t2.b FROM t1, t2 WHERE t1.a = t2.a;
</pre><p>
But there is a little problem in
<code class="command">UPDATE</code>: the part of the executor plan that does
the join does not care what the results from the join are
meant for. It just produces a result set of rows. The fact that
one is a <code class="command">SELECT</code> command and the other is an
<code class="command">UPDATE</code> is handled higher up in the executor, where
it knows that this is an <code class="command">UPDATE</code>, and it knows that
this result should go into table <code class="literal">t1</code>. But which of the rows
that are there has to be replaced by the new row?
</p><p>
To resolve this problem, another entry is added to the target list
in <code class="command">UPDATE</code> (and also in
<code class="command">DELETE</code>) statements: the current tuple ID
(<acronym class="acronym">CTID</acronym>).<a id="id-1.8.6.7.7.5.4" class="indexterm"></a>
This is a system column containing the
file block number and position in the block for the row. Knowing
the table, the <acronym class="acronym">CTID</acronym> can be used to retrieve the
original row of <code class="literal">t1</code> to be updated. After adding the
<acronym class="acronym">CTID</acronym> to the target list, the query actually looks like:
</p><pre class="programlisting">
SELECT t1.a, t2.b, t1.ctid FROM t1, t2 WHERE t1.a = t2.a;
</pre><p>
Now another detail of <span class="productname">PostgreSQL</span> enters
the stage. Old table rows aren't overwritten, and this
is why <code class="command">ROLLBACK</code> is fast. In an <code class="command">UPDATE</code>,
the new result row is inserted into the table (after stripping the
<acronym class="acronym">CTID</acronym>) and in the row header of the old row, which the
<acronym class="acronym">CTID</acronym> pointed to, the <code class="literal">cmax</code> and
<code class="literal">xmax</code> entries are set to the current command counter
and current transaction ID. Thus the old row is hidden, and after
the transaction commits the vacuum cleaner can eventually remove
the dead row.
</p><p>
Knowing all that, we can simply apply view rules in absolutely
the same way to any command. There is no difference.
</p></div><div class="sect2" id="RULES-VIEWS-POWER"><div class="titlepage"><div><div><h3 class="title">41.2.3. The Power of Views in <span class="productname">PostgreSQL</span> <a href="#RULES-VIEWS-POWER" class="id_link">#</a></h3></div></div></div><p>
The above demonstrates how the rule system incorporates view
definitions into the original query tree. In the second example, a
simple <code class="command">SELECT</code> from one view created a final
query tree that is a join of 4 tables (<code class="literal">unit</code> was used twice with
different names).
</p><p>
The benefit of implementing views with the rule system is
that the planner has all
the information about which tables have to be scanned plus the
relationships between these tables plus the restrictive
qualifications from the views plus the qualifications from
the original query
in one single query tree. And this is still the situation
when the original query is already a join over views.
The planner has to decide which is
the best path to execute the query, and the more information
the planner has, the better this decision can be. And
the rule system as implemented in <span class="productname">PostgreSQL</span>
ensures that this is all information available about the query
up to that point.
</p></div><div class="sect2" id="RULES-VIEWS-UPDATE"><div class="titlepage"><div><div><h3 class="title">41.2.4. Updating a View <a href="#RULES-VIEWS-UPDATE" class="id_link">#</a></h3></div></div></div><p>
What happens if a view is named as the target relation for an
<code class="command">INSERT</code>, <code class="command">UPDATE</code>, or
<code class="command">DELETE</code>? Doing the substitutions
described above would give a query tree in which the result
relation points at a subquery range-table entry, which will not
work. There are several ways in which <span class="productname">PostgreSQL</span>
can support the appearance of updating a view, however.
In order of user-experienced complexity those are: automatically substitute
in the underlying table for the view, execute a user-defined trigger,
or rewrite the query per a user-defined rule.
These options are discussed below.
</p><p>
If the subquery selects from a single base relation and is simple
enough, the rewriter can automatically replace the subquery with the
underlying base relation so that the <code class="command">INSERT</code>,
<code class="command">UPDATE</code>, or <code class="command">DELETE</code> is applied to
the base relation in the appropriate way. Views that are
<span class="quote">“<span class="quote">simple enough</span>”</span> for this are called <em class="firstterm">automatically
updatable</em>. For detailed information on the kinds of view that can
be automatically updated, see <a class="xref" href="sql-createview.html" title="CREATE VIEW"><span class="refentrytitle">CREATE VIEW</span></a>.
</p><p>
Alternatively, the operation may be handled by a user-provided
<code class="literal">INSTEAD OF</code> trigger on the view
(see <a class="xref" href="sql-createtrigger.html" title="CREATE TRIGGER"><span class="refentrytitle">CREATE TRIGGER</span></a>).
Rewriting works slightly differently
in this case. For <code class="command">INSERT</code>, the rewriter does
nothing at all with the view, leaving it as the result relation
for the query. For <code class="command">UPDATE</code> and
<code class="command">DELETE</code>, it's still necessary to expand the
view query to produce the <span class="quote">“<span class="quote">old</span>”</span> rows that the command will
attempt to update or delete. So the view is expanded as normal,
but another unexpanded range-table entry is added to the query
to represent the view in its capacity as the result relation.
</p><p>
The problem that now arises is how to identify the rows to be
updated in the view. Recall that when the result relation
is a table, a special <acronym class="acronym">CTID</acronym> entry is added to the target
list to identify the physical locations of the rows to be updated.
This does not work if the result relation is a view, because a view
does not have any <acronym class="acronym">CTID</acronym>, since its rows do not have
actual physical locations. Instead, for an <code class="command">UPDATE</code>
or <code class="command">DELETE</code> operation, a special <code class="literal">wholerow</code>
entry is added to the target list, which expands to include all
columns from the view. The executor uses this value to supply the
<span class="quote">“<span class="quote">old</span>”</span> row to the <code class="literal">INSTEAD OF</code> trigger. It is
up to the trigger to work out what to update based on the old and
new row values.
</p><p>
Another possibility is for the user to define <code class="literal">INSTEAD</code>
rules that specify substitute actions for <code class="command">INSERT</code>,
<code class="command">UPDATE</code>, and <code class="command">DELETE</code> commands on
a view. These rules will rewrite the command, typically into a command
that updates one or more tables, rather than views. That is the topic
of <a class="xref" href="rules-update.html" title="41.4. Rules on INSERT, UPDATE, and DELETE">Section 41.4</a>.
</p><p>
Note that rules are evaluated first, rewriting the original query
before it is planned and executed. Therefore, if a view has
<code class="literal">INSTEAD OF</code> triggers as well as rules on <code class="command">INSERT</code>,
<code class="command">UPDATE</code>, or <code class="command">DELETE</code>, then the rules will be
evaluated first, and depending on the result, the triggers may not be
used at all.
</p><p>
Automatic rewriting of an <code class="command">INSERT</code>,
<code class="command">UPDATE</code>, or <code class="command">DELETE</code> query on a
simple view is always tried last. Therefore, if a view has rules or
triggers, they will override the default behavior of automatically
updatable views.
</p><p>
If there are no <code class="literal">INSTEAD</code> rules or <code class="literal">INSTEAD OF</code>
triggers for the view, and the rewriter cannot automatically rewrite
the query as an update on the underlying base relation, an error will
be thrown because the executor cannot update a view as such.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="querytree.html" title="41.1. The Query Tree">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="rules.html" title="Chapter 41. The Rule System">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="rules-materializedviews.html" title="41.3. Materialized Views">Next</a></td></tr><tr><td width="40%" align="left" valign="top">41.1. The Query Tree </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 41.3. Materialized Views</td></tr></table></div></body></html>
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