Why is this postgres function failing only on one specifc database? - postgresql

Question

I'm trying to fix an issue with a legacy database. The quote_literal function is not working for a specific database on an 8.4 install of postgres.
Here's my results on a fresh test database:
select quote_literal(42);
quote_literal
---------------
'42'
(1 row)
And now the same on the target db
select quote_literal(42);
ERROR: function quote_literal(integer) is not unique
LINE 1: select quote_literal(42);
^
HINT: Could not choose a best candidate function. You might need to add explicit type casts.
AIUI, the quote_literal(anyvalue) function should handle integer values ok, and this seems to be upheld by the first test.
So I figured the quote_literal function must have been overridden in this db but no this doesn't seem to be the case. I could override it with a specific quote_literal(integer) function but I don't see why I should have to.
The question is what is could be causing the failure of this function in this specific database whilst not affecting the fresh db?

Answer 1

Another possibility: Somebody has added implicit casts from text to your database. This was a common workaround for an intentional BC break in 8.3. See the release notes for 8.3, E.57.2. Migration to Version 8.3
Demo:
regress=# \df quote_literal
List of functions
Schema | Name | Result data type | Argument data types | Type
------------+---------------+------------------+---------------------+--------
pg_catalog | quote_literal | text | anyelement | normal
pg_catalog | quote_literal | text | text | normal
(2 rows)
regress=# CREATE FUNCTION pg_catalog.text(integer) RETURNS text STRICT IMMUTABLE LANGUAGE SQL AS 'SELECT textin(int4out($1));';
CREATE FUNCTION
regress=# CREATE CAST (integer AS text) WITH FUNCTION pg_catalog.text(integer) AS IMPLICIT;
CREATE CAST
regress=# SELECT quote_literal(42);
ERROR: function quote_literal(integer) is not unique
LINE 1: SELECT quote_literal(42);
^
HINT: Could not choose a best candidate function. You might need to add explicit type casts.
regress=#
This'll fix it, but probably break other code that's still relying on the cast:
regress=# DROP CAST (integer AS text);
DROP CAST
regress=# SELECT quote_literal(42);
quote_literal
---------------
'42'
(1 row)

Answer 2

Somebody has probably defined another single-argument quote_literal function with an argument type that's assignment-compatible to integer, like bigint.
In psql, connect and run:
\df quote_literal
and you'll see multiple entries, like this:
regress=> \df quote_literal
List of functions
Schema | Name | Result data type | Argument data types | Type
------------+---------------+------------------+---------------------+--------
pg_catalog | quote_literal | text | anyelement | normal
pg_catalog | quote_literal | text | text | normal
public | quote_literal | text | bigint | normal
(3 rows)
You only want the 1st two, in pg_catalog. However, I can't advise you to just:
DROP FUNCTION public.quote_literal(bigint);
... because you might have code that expects it to exist. Time to go digging and see where it's used. Have fun.
Demo showing that this is likely the problem:
regress=> SELECT quote_literal(42);
quote_literal
---------------
'42'
(1 row)
regress=> CREATE OR REPLACE FUNCTION quote_literal(bigint) RETURNS text AS 'SELECT ''borkborkbork''::text;' LANGUAGE sql;
CREATE FUNCTION
regress=> SELECT quote_literal(42);
ERROR: function quote_literal(integer) is not unique
LINE 1: SELECT quote_literal(42);
^
HINT: Could not choose a best candidate function. You might need to add explicit type casts.
regress=>