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The pg_cast table stores data type conversion paths, both built-in paths and those defined with CREATE CAST.

Note that pg_cast does not represent every type conversion known to the system, 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 pg_cast.

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.

When a pg_cast entry has different source and target types and a function that takes more than one argument, the entry converts from one type to another and applies 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.

Table 1. pg_catalog.pg_cast
column type references description
castsource oid pg_type.oid OID of the source data type.
casttarget oid pg_type.oid OID of the target data type.
castfunc oid pg_proc.oid The OID of the function to use to perform this cast. Zero is stored if the cast method does not require a function.
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.
castmethod char   Indicates how the cast is performed:

f - The function identified in the castfunc field is used.

i - The input/output functions are used.

b - The types are binary-coercible, and no conversion is required.