51.10. pg_cast

系統目錄 pg_cast 儲存資料型別的轉換方式，包括了內建的和使用者定義的。

需要注意的是，pg_cast 並不代表系統知道如何執行的每一種型別轉換；只有那些不能從某些通用規則中推導出來的。例如，domain 和它的基本型別之間的轉換在 pg_cast 中就沒有明確表示。另一個重要的例外是「自動 I/O 強制轉換」，即使用資料型別自己的 I/O 函數執行的轉換為 text 或其他字串型別或從 text 及其他字串型別轉換的那些，在 pg_cast 中沒有明確表示。

Table 51.10. pg_cast Columns

Column Type Description
oid oid Row identifier
castsource oid (references pg_type.oid) OID of the source data type
casttarget oid (references pg_type.oid) OID of the target data type
castfunc oid (references pg_proc.oid) The OID of the function to use to perform this cast. Zero is stored if the cast method doesn't require a function.
castcontext char Indicates what contexts the cast can 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 means that the function specified in the castfunc field is used. i means that the input/output functions are used. b means that the types are binary-coercible, thus no conversion is required.

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, 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.