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Eugene Pakhomov's questions

Martin Hope
Eugene Pakhomov
Asked: 2020-07-26 21:13:17 +0800 CST
  • 2

设置（仅在 PostgreSQL 9.6 上尝试过）：

CREATE TYPE MY_ENUM AS ENUM ('a', 'b');
CREATE CAST (CHARACTER VARYING AS MY_ENUM) WITH INOUT AS IMPLICIT;
CREATE TABLE t (x MY_ENUM);
INSERT INTO t VALUES ('a'::MY_ENUM), ('b'::MY_ENUM);

正如预期的那样，这些工作正常：

INSERT INTO t VALUES ('a');
SELECT * FROM t WHERE x = 'a';

但这些不会：

PREPARE p(CHARACTER VARYING) AS SELECT * FROM t WHERE x = $1;
;; error: operator does not exist: my_enum = character varying

CREATE FUNCTION f(ix CHARACTER VARYING, OUT ox MY_ENUM) AS
$$
    SELECT * FROM t WHERE x = ix
$$ LANGUAGE sql;
;; error: operator does not exist: my_enum = character varying

CREATE FUNCTION f(ix CHARACTER VARYING) RETURNS VOID AS
$$
BEGIN
    SELECT * FROM t WHERE input_type = ix;
END;
$$ LANGUAGE plpgsql;

SELECT f('piano');
;; error: operator does not exist: my_enum = character varying

根据文档：

如果强制转换标记为 AS IMPLICIT，则可以在任何上下文中隐式调用它，无论是赋值还是在表达式内部。

那么为什么会出现错误呢？

postgresql cast