How to get the schema name of a table of type regclass in PostgreSQL? In writing a function to test if a column col_name exists in a table _tbl, I'd like to extract the table's schema name, which is passed into the function as a regclass parameter (for security??).

CREATE OR REPLACE FUNCTION column_exists(_tbl regclass, col_name text)
  RETURNS bool AS
$func$
  SELECT EXISTS (
    SELECT 1 FROM information_schema.columns 
    WHERE table_schema=get_schema($1) 
      AND table_name=get_table($1) 
      AND column_name=$2
    );
$func$  LANGUAGE sql;


So if the table name is 'staging.my_table'::regclass, I'd like to get staging from an imaginary function get_schema.

Can I just implement this function with e.g. split_part(_tbl::text, '.', 1)?

In addition, is it guaranteed that the table name _tbl, when converted to text, will always have a schema name? (i.e. not omitting things such as public.)

I'm not very familiar with the regclass type. I searched but couldn't find how to extract the schema name, and just wanted to ask first before re-inventing wheels.
Can I just implement this function with e.g. split_part(_tbl::text, '.', 1)?


I'm not sure that would be safe. Instead, I would use,

SELECT nspname
FROM pg_catalog.pg_class AS c
JOIN pg_catalog.pg_namespace AS ns
  ON c.relnamespace = ns.oid
WHERE c.oid = _tbl;


That is guaranteed to work.


  In writing a function to test if a column col_name exists in a table _tbl, I'd like to extract the table's schema name, which is passed into the function as a regclass parameter (for security??).


So you could do something like,

SELECT nspname
FROM pg_catalog.pg_attribute AS a
JOIN pg_catalog.pg_class AS c
  ON a.attrelid = c.oid
JOIN pg_catalog.pg_namespace AS ns
  ON c.relnamespace = ns.oid
WHERE c.oid = _tbl
  AND a.attname = col_name;


Update

If you have a fully-qualified identifier, one with the table name, and schema name inside, You can use parse_ident to parse it safely.