Table name as a PostgreSQL function parameter

Question

I want to pass a table name as a parameter in a Postgres function. I tried this code:

CREATE OR REPLACE FUNCTION some_f(param character varying) RETURNS integer 
AS $$
    BEGIN
    IF EXISTS (select * from quote_ident($1) where quote_ident($1).id=1) THEN
     return 1;
    END IF;
    return 0;
    END;
$$ LANGUAGE plpgsql;

select some_f('table_name');

And I got this:

ERROR:  syntax error at or near "."
LINE 4: ...elect * from quote_ident($1) where quote_ident($1).id=1)...
                                                             ^

********** Error **********

ERROR: syntax error at or near "."

And here is the error I got when changed to this select * from quote_ident($1) tab where tab.id=1:

ERROR:  column tab.id does not exist
LINE 1: ...T EXISTS (select * from quote_ident($1) tab where tab.id...

Probably, quote_ident($1) works, because without the where quote_ident($1).id=1 part I get 1, which means something is selected. Why may the first quote_ident($1) work and the second one not at the same time? And how could this be solved?

Answer 1

This can be further simplified and improved:

CREATE OR REPLACE FUNCTION some_f(_tbl regclass, OUT result integer)
    LANGUAGE plpgsql AS
$func$
BEGIN
   EXECUTE format('SELECT (EXISTS (SELECT FROM %s WHERE id = 1))::int', _tbl)
   INTO result;
END
$func$;

Call with schema-qualified name (see below):

SELECT some_f('myschema.mytable');  -- would fail with quote_ident()

Or:

SELECT some_f('"my very uncommon table name"');

Major points

Use an OUT parameter to simplify the function. You can directly select the result of the dynamic SQL into it and be done. No need for additional variables and code.

EXISTS does exactly what you want. You get true if the row exists or false otherwise. There are various ways to do this, EXISTS is typically most efficient.

You seem to want an integer back, so I cast the boolean result from EXISTS to integer, which yields exactly what you had. I would return boolean instead.

I use the object identifier type regclass as input type for _tbl. That does everything quote_ident(_tbl) or format('%I', _tbl) would do, but better, because:

• .. it prevents SQL injection just as well.

• .. it fails immediately and more gracefully if the table name is invalid / does not exist / is invisible to the current user. (A regclass parameter is only applicable for existing tables.)

• .. it works with schema-qualified table names, where a plain quote_ident(_tbl) or format(%I) would fail because they cannot resolve the ambiguity. You would have to pass and escape schema and table names separately.

It only works for existing tables, obviously.

I still use format(), because it simplifies the syntax (and to demonstrate how it's used), but with %s instead of %I. Typically, queries are more complex so format() helps more. For the simple example we could as well just concatenate:

EXECUTE 'SELECT (EXISTS (SELECT FROM ' || _tbl || ' WHERE id = 1))::int'

No need to table-qualify the id column while there is only a single table in the FROM list. No ambiguity possible in this example. (Dynamic) SQL commands inside EXECUTE have a separate scope, function variables or parameters are not visible there - as opposed to plain SQL commands in the function body.

Here's why you always escape user input for dynamic SQL properly:

db<>fiddle here demonstrating SQL injection
_{Old sqlfiddle}