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><A
NAME="CATALOG-PG-CAST"
>45.10. <TT
CLASS="STRUCTNAME"
>pg_cast</TT
></A
></H1
><P
> The catalog <TT
CLASS="STRUCTNAME"
>pg_cast</TT
> stores data type conversion
paths, both built-in and user-defined.
</P
><P
> It should be noted that <TT
CLASS="STRUCTNAME"
>pg_cast</TT
> 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
<TT
CLASS="STRUCTNAME"
>pg_cast</TT
>. Another important exception is that
<SPAN
CLASS="QUOTE"
>"automatic I/O conversion casts"</SPAN
>, those performed using a data
type's own I/O functions to convert to or from <TT
CLASS="TYPE"
>text</TT
> or other
string types, are not explicitly represented in
<TT
CLASS="STRUCTNAME"
>pg_cast</TT
>.
</P
><DIV
CLASS="TABLE"
><A
NAME="AEN89545"
></A
><P
><B
>Table 45-10. <TT
CLASS="STRUCTNAME"
>pg_cast</TT
> Columns</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Type</TH
><TH
>References</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>castsource</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-type.html"
><TT
CLASS="STRUCTNAME"
>pg_type</TT
></A
>.oid</TT
></TD
><TD
>OID of the source data type</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>casttarget</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-type.html"
><TT
CLASS="STRUCTNAME"
>pg_type</TT
></A
>.oid</TT
></TD
><TD
>OID of the target data type</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>castfunc</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-proc.html"
><TT
CLASS="STRUCTNAME"
>pg_proc</TT
></A
>.oid</TT
></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
><TT
CLASS="STRUCTFIELD"
>castcontext</TT
></TD
><TD
><TT
CLASS="TYPE"
>char</TT
></TD
><TD
> </TD
><TD
> Indicates what contexts the cast can be invoked in.
<TT
CLASS="LITERAL"
>e</TT
> means only as an explicit cast (using
<TT
CLASS="LITERAL"
>CAST</TT
> or <TT
CLASS="LITERAL"
>::</TT
> syntax).
<TT
CLASS="LITERAL"
>a</TT
> means implicitly in assignment
to a target column, as well as explicitly.
<TT
CLASS="LITERAL"
>i</TT
> means implicitly in expressions, as well as the
other cases.
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>castmethod</TT
></TD
><TD
><TT
CLASS="TYPE"
>char</TT
></TD
><TD
> </TD
><TD
> Indicates how the cast is performed.
<TT
CLASS="LITERAL"
>f</TT
> means that the function specified in the <TT
CLASS="STRUCTFIELD"
>castfunc</TT
> field is used.
<TT
CLASS="LITERAL"
>i</TT
> means that the input/output functions are used.
<TT
CLASS="LITERAL"
>b</TT
> means that the types are binary-coercible, thus no conversion is required.
</TD
></TR
></TBODY
></TABLE
></DIV
><P
> The cast functions listed in <TT
CLASS="STRUCTNAME"
>pg_cast</TT
> 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 <TT
CLASS="TYPE"
>integer</TT
>; it receives the type
modifier associated with the destination type, or -1
if there is none. The third argument,
if present, must be type <TT
CLASS="TYPE"
>boolean</TT
>; it receives <TT
CLASS="LITERAL"
>true</TT
>
if the cast is an explicit cast, <TT
CLASS="LITERAL"
>false</TT
> otherwise.
</P
><P
> It is legitimate to create a <TT
CLASS="STRUCTNAME"
>pg_cast</TT
> 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"
>"length coercion functions"</SPAN
> that coerce values of the type
to be legal for a particular type modifier value.
</P
><P
> When a <TT
CLASS="STRUCTNAME"
>pg_cast</TT
> 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.
</P
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