The catalog pg_cast
stores data type conversion paths,
both built-in paths and those defined with
CREATE CAST.
Table 43.11. pg_cast
Columns
Name | Type | References | Description |
---|---|---|---|
castsource |
oid |
|
OID of the source data type |
casttarget |
oid |
|
OID of the target data type |
castfunc |
oid |
|
The OID of the function to use to perform this cast. Zero is stored if the data types are binary compatible (that is, no run-time operation is needed to perform the cast) |
castcontext |
char |
Indicates what contexts the cast may be invoked in.
e means only as an explicit cast (using
CAST or :: syntax).
a means implicitly in assignment
to a target column, as well as explicitly.
i means implicitly in expressions, as well as the
other cases
|
The cast functions listed in pg_cast
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 integer
; it receives the type
modifier associated with the destination type, or -1
if there is none. The third argument,
if present, must be type boolean
; it receives true
if the cast is an explicit cast, false
otherwise.
It is legitimate to create a pg_cast
entry
in which the source and target types are the same, if the associated
function takes more than one argument. Such entries represent
“length coercion functions” that coerce values of the type
to be legal for a particular type modifier value. Note however that
at present there is no support for associating non-default type
modifiers with user-created data types, and so this facility is only
of use for the small number of built-in types that have type modifier
syntax built into the grammar.
When a pg_cast
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.