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

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.