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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>38.7. Function Volatility Categories</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="xfunc-overload.html" title="38.6. Function Overloading" /><link rel="next" href="xfunc-pl.html" title="38.8. Procedural Language Functions" /></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">38.7. Function Volatility Categories</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="xfunc-overload.html" title="38.6. Function Overloading">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="extend.html" title="Chapter 38. Extending SQL">Up</a></td><th width="60%" align="center">Chapter 38. Extending <acronym xmlns="http://www.w3.org/1999/xhtml" class="acronym">SQL</acronym></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="xfunc-pl.html" title="38.8. Procedural Language Functions">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="XFUNC-VOLATILITY"><div class="titlepage"><div><div><h2 class="title" style="clear: both">38.7. Function Volatility Categories</h2></div></div></div><a id="id-1.8.3.10.2" class="indexterm"></a><a id="id-1.8.3.10.3" class="indexterm"></a><a id="id-1.8.3.10.4" class="indexterm"></a><a id="id-1.8.3.10.5" class="indexterm"></a><p>
Every function has a <em class="firstterm">volatility</em> classification, with
the possibilities being <code class="literal">VOLATILE</code>, <code class="literal">STABLE</code>, or
<code class="literal">IMMUTABLE</code>. <code class="literal">VOLATILE</code> is the default if the
<a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a>
command does not specify a category. The volatility category is a
promise to the optimizer about the behavior of the function:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
A <code class="literal">VOLATILE</code> function can do anything, including modifying
the database. It can return different results on successive calls with
the same arguments. The optimizer makes no assumptions about the
behavior of such functions. A query using a volatile function will
re-evaluate the function at every row where its value is needed.
</p></li><li class="listitem"><p>
A <code class="literal">STABLE</code> function cannot modify the database and is
guaranteed to return the same results given the same arguments
for all rows within a single statement. This category allows the
optimizer to optimize multiple calls of the function to a single
call. In particular, it is safe to use an expression containing
such a function in an index scan condition. (Since an index scan
will evaluate the comparison value only once, not once at each
row, it is not valid to use a <code class="literal">VOLATILE</code> function in an
index scan condition.)
</p></li><li class="listitem"><p>
An <code class="literal">IMMUTABLE</code> function cannot modify the database and is
guaranteed to return the same results given the same arguments forever.
This category allows the optimizer to pre-evaluate the function when
a query calls it with constant arguments. For example, a query like
<code class="literal">SELECT ... WHERE x = 2 + 2</code> can be simplified on sight to
<code class="literal">SELECT ... WHERE x = 4</code>, because the function underlying
the integer addition operator is marked <code class="literal">IMMUTABLE</code>.
</p></li></ul></div><p>
</p><p>
For best optimization results, you should label your functions with the
strictest volatility category that is valid for them.
</p><p>
Any function with side-effects <span class="emphasis"><em>must</em></span> be labeled
<code class="literal">VOLATILE</code>, so that calls to it cannot be optimized away.
Even a function with no side-effects needs to be labeled
<code class="literal">VOLATILE</code> if its value can change within a single query;
some examples are <code class="literal">random()</code>, <code class="literal">currval()</code>,
<code class="literal">timeofday()</code>.
</p><p>
Another important example is that the <code class="function">current_timestamp</code>
family of functions qualify as <code class="literal">STABLE</code>, since their values do
not change within a transaction.
</p><p>
There is relatively little difference between <code class="literal">STABLE</code> and
<code class="literal">IMMUTABLE</code> categories when considering simple interactive
queries that are planned and immediately executed: it doesn't matter
a lot whether a function is executed once during planning or once during
query execution startup. But there is a big difference if the plan is
saved and reused later. Labeling a function <code class="literal">IMMUTABLE</code> when
it really isn't might allow it to be prematurely folded to a constant during
planning, resulting in a stale value being re-used during subsequent uses
of the plan. This is a hazard when using prepared statements or when
using function languages that cache plans (such as
<span class="application">PL/pgSQL</span>).
</p><p>
For functions written in SQL or in any of the standard procedural
languages, there is a second important property determined by the
volatility category, namely the visibility of any data changes that have
been made by the SQL command that is calling the function. A
<code class="literal">VOLATILE</code> function will see such changes, a <code class="literal">STABLE</code>
or <code class="literal">IMMUTABLE</code> function will not. This behavior is implemented
using the snapshotting behavior of MVCC (see <a class="xref" href="mvcc.html" title="Chapter 13. Concurrency Control">Chapter 13</a>):
<code class="literal">STABLE</code> and <code class="literal">IMMUTABLE</code> functions use a snapshot
established as of the start of the calling query, whereas
<code class="literal">VOLATILE</code> functions obtain a fresh snapshot at the start of
each query they execute.
</p><div class="note"><h3 class="title">Note</h3><p>
Functions written in C can manage snapshots however they want, but it's
usually a good idea to make C functions work this way too.
</p></div><p>
Because of this snapshotting behavior,
a function containing only <code class="command">SELECT</code> commands can safely be
marked <code class="literal">STABLE</code>, even if it selects from tables that might be
undergoing modifications by concurrent queries.
<span class="productname">PostgreSQL</span> will execute all commands of a
<code class="literal">STABLE</code> function using the snapshot established for the
calling query, and so it will see a fixed view of the database throughout
that query.
</p><p>
The same snapshotting behavior is used for <code class="command">SELECT</code> commands
within <code class="literal">IMMUTABLE</code> functions. It is generally unwise to select
from database tables within an <code class="literal">IMMUTABLE</code> function at all,
since the immutability will be broken if the table contents ever change.
However, <span class="productname">PostgreSQL</span> does not enforce that you
do not do that.
</p><p>
A common error is to label a function <code class="literal">IMMUTABLE</code> when its
results depend on a configuration parameter. For example, a function
that manipulates timestamps might well have results that depend on the
<a class="xref" href="runtime-config-client.html#GUC-TIMEZONE">TimeZone</a> setting. For safety, such functions should
be labeled <code class="literal">STABLE</code> instead.
</p><div class="note"><h3 class="title">Note</h3><p>
<span class="productname">PostgreSQL</span> requires that <code class="literal">STABLE</code>
and <code class="literal">IMMUTABLE</code> functions contain no SQL commands other
than <code class="command">SELECT</code> to prevent data modification.
(This is not a completely bulletproof test, since such functions could
still call <code class="literal">VOLATILE</code> functions that modify the database.
If you do that, you will find that the <code class="literal">STABLE</code> or
<code class="literal">IMMUTABLE</code> function does not notice the database changes
applied by the called function, since they are hidden from its snapshot.)
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