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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>52.10. pg_cast</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="catalog-pg-auth-members.html" title="52.9. pg_auth_members" /><link rel="next" href="catalog-pg-class.html" title="52.11. pg_class" /></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">52.10. <code xmlns="http://www.w3.org/1999/xhtml" class="structname">pg_cast</code></th></tr><tr><td width="10%" align="left"><a accesskey="p" href="catalog-pg-auth-members.html" title="52.9. pg_auth_members">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="catalogs.html" title="Chapter 52. System Catalogs">Up</a></td><th width="60%" align="center">Chapter 52. System Catalogs</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="catalog-pg-class.html" title="52.11. pg_class">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="CATALOG-PG-CAST"><div class="titlepage"><div><div><h2 class="title" style="clear: both">52.10. <code class="structname">pg_cast</code></h2></div></div></div><a id="id-1.10.4.12.2" class="indexterm"></a><p>
The catalog <code class="structname">pg_cast</code> stores data type conversion
paths, both built-in and user-defined.
</p><p>
It should be noted that <code class="structname">pg_cast</code> does not represent
every type conversion that the system knows how to perform; only those that
cannot be deduced from some generic rule. For example, casting between a
domain and its base type is not explicitly represented in
<code class="structname">pg_cast</code>. Another important exception is that
<span class="quote">“<span class="quote">automatic I/O conversion casts</span>”</span>, those performed using a data
type's own I/O functions to convert to or from <code class="type">text</code> or other
string types, are not explicitly represented in
<code class="structname">pg_cast</code>.
</p><div class="table" id="id-1.10.4.12.5"><p class="title"><strong>Table 52.10. <code class="structname">pg_cast</code> Columns</strong></p><div class="table-contents"><table class="table" summary="pg_cast Columns" border="1"><colgroup><col /><col /><col /><col /></colgroup><thead><tr><th>Name</th><th>Type</th><th>References</th><th>Description</th></tr></thead><tbody><tr><td><code class="structfield">oid</code></td><td><code class="type">oid</code></td><td> </td><td>Row identifier (hidden attribute; must be explicitly selected)</td></tr><tr><td><code class="structfield">castsource</code></td><td><code class="type">oid</code></td><td><code class="literal"><a class="link" href="catalog-pg-type.html" title="52.62. pg_type"><code class="structname">pg_type</code></a>.oid</code></td><td>OID of the source data type</td></tr><tr><td><code class="structfield">casttarget</code></td><td><code class="type">oid</code></td><td><code class="literal"><a class="link" href="catalog-pg-type.html" title="52.62. pg_type"><code class="structname">pg_type</code></a>.oid</code></td><td>OID of the target data type</td></tr><tr><td><code class="structfield">castfunc</code></td><td><code class="type">oid</code></td><td><code class="literal"><a class="link" href="catalog-pg-proc.html" title="52.39. pg_proc"><code class="structname">pg_proc</code></a>.oid</code></td><td>
The OID of the function to use to perform this cast. Zero is
stored if the cast method doesn't require a function.
</td></tr><tr><td><code class="structfield">castcontext</code></td><td><code class="type">char</code></td><td> </td><td>
Indicates what contexts the cast can be invoked in.
<code class="literal">e</code> means only as an explicit cast (using
<code class="literal">CAST</code> or <code class="literal">::</code> syntax).
<code class="literal">a</code> means implicitly in assignment
to a target column, as well as explicitly.
<code class="literal">i</code> means implicitly in expressions, as well as the
other cases.
</td></tr><tr><td><code class="structfield">castmethod</code></td><td><code class="type">char</code></td><td> </td><td>
Indicates how the cast is performed.
<code class="literal">f</code> means that the function specified in the <code class="structfield">castfunc</code> field is used.
<code class="literal">i</code> means that the input/output functions are used.
<code class="literal">b</code> means that the types are binary-coercible, thus no conversion is required.
</td></tr></tbody></table></div></div><br class="table-break" /><p>
The cast functions listed in <code class="structname">pg_cast</code> must
always take the cast source type as their first argument type, and
return the cast destination type as their result type. A cast
function can have up to three arguments. The second argument,
if present, must be type <code class="type">integer</code>; it receives the type
modifier associated with the destination type, or -1
if there is none. The third argument,
if present, must be type <code class="type">boolean</code>; it receives <code class="literal">true</code>
if the cast is an explicit cast, <code class="literal">false</code> otherwise.
</p><p>
It is legitimate to create a <code class="structname">pg_cast</code> entry
in which the source and target types are the same, if the associated
function takes more than one argument. Such entries represent
<span class="quote">“<span class="quote">length coercion functions</span>”</span> that coerce values of the type
to be legal for a particular type modifier value.
</p><p>
When a <code class="structname">pg_cast</code> entry has different source and
target types and a function that takes more than one argument, it
represents converting from one type to another and applying a length
coercion in a single step. When no such entry is available, coercion
to a type that uses a type modifier involves two steps, one to
convert between data types and a second to apply the modifier.
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