Postgresql：错误：查询结构与使用 DbLink 的函数结果类型不匹配

Question

所以我编写了这个方法，旨在使用 dblink 查询另一个具有相同结构的远程数据库（灵感来自这篇文章 Specify dblink column definition list from a local existing type 和这篇文章 Refactor a PL/pgSQL function to return the output of various SELECT queries）

CREATE OR REPLACE FUNCTION select_remote(_table anyelement)
  RETURNS SETOF anyelement 
  AS $func$
DECLARE
        _dblink_schema text;
        _cols          text; 
        _server        text := 'host=ngrok.com port=45790 user=postgres password=postgres dbname=backup-28-08';
        _table_name    text := pg_typeof(_table);
BEGIN

        SELECT nspname INTO _dblink_schema
        FROM pg_namespace n, pg_extension e
        WHERE e.extname = 'dblink' AND e.extnamespace = n.oid;

    SELECT array_to_string(array_agg(column_name || ' ' || udt_name), ', ')  INTO _cols
        FROM (select column_name, udt_name from information_schema.columns
        WHERE table_name = _table_name
        order by ordinal_position) as sub;

        RETURN QUERY EXECUTE format('SELECT * FROM %I.dblink(%L, %L) AS remote (%s)',
            _dblink_schema,
            _server,
            format('SELECT * FROM %I', _table_name),
            _cols
        );


END;
$func$ LANGUAGE plpgsql;

但是当我执行select * from select_remote(NULL::my_table) 时，我收到了这个错误：

ERROR:  structure of query does not match function result type
DETAIL:  Returned type character varying does not match expected type character varying(255) in column 2.
CONTEXT:  PL/pgSQL function select_remote(anyelement) line 18 at RETURN QUERY

********** Erreur **********

ERROR: structure of query does not match function result type
État SQL :42804
Détail :Returned type character varying does not match expected type character varying(255) in column 2.
Contexte : PL/pgSQL function select_remote(anyelement) line 18 at RETURN QUERY

这让我抓狂，因为远程表和本地表确实具有相同的结构。

例如。如果我只返回查询字符串，我可以UNION它到本地表，它工作得很好：

SELECT * FROM public.dblink('host=ngrok.com port=45790 user=postgres password=postgres dbname=backup-28-08', 'SELECT * FROM my_table') AS remote (id int4, fname varchar, lname varchar, email varchar, slug varchar) 
UNION 
SELECT * FROM my_table

我做错了什么？我如何强制anyelement 接受这些数据，即使它来自远程表？或者返回一些不同的东西来让它工作？

谢谢

Answer 1

以下基于对my question 的已接受答案：

CREATE OR REPLACE FUNCTION select_remote(_table anyelement)
RETURNS SETOF anyelement 
AS $func$
DECLARE
    _dblink_schema text;
    _cols          text; 
    _server        text := 'host=ngrok.com port=45790 user=postgres password=postgres dbname=backup-28-08';
    _table_name    text := pg_typeof(_table);
BEGIN

    SELECT nspname INTO _dblink_schema
    FROM pg_namespace n, pg_extension e
    WHERE e.extname = 'dblink' AND e.extnamespace = n.oid;

SELECT array_to_string(array_agg(column_name || ' ' || udt_name), ', ')  INTO _cols
    FROM (select column_name, udt_name from information_schema.columns
    WHERE table_name = _table_name
    order by ordinal_position) as sub;

    RETURN QUERY EXECUTE format('SELECT (remote::%I).* FROM %I.dblink(%L, %L) AS remote (%s)',
        _table_name,
        _dblink_schema,
        _server,
        format('SELECT * FROM %I', _table_name),
        _cols
    );


END;
$func$ LANGUAGE plpgsql;

请注意，dblink 调用的“远程”的选定表/列被投射到本地表

SELECT (remote::%I).* FROM %I.dblink(%L, %L) AS remote (%s)