pg_cast stores data type conversion paths, both built-in and user-defined.
It should be noted that
pg_cast 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
pg_cast. Another important exception is that “automatic I/O conversion casts”, those performed using a data type's own I/O functions to convert to or from
text or other string types, are not explicitly represented in
|Row identifier (hidden attribute; must be explicitly selected)|
|OID of the source data type|
|OID of the target data type|
|The OID of the function to use to perform this cast. Zero is stored if the cast method doesn't require a function.|
| Indicates what contexts the cast can be invoked in. |
| Indicates how the cast is performed. |
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,
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.
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.