PostgreSQL throws an ambiguous function error when it cannot determine which overloaded function to call due to unclear argument types or multiple matching candidates. Resolve this by using explicit type casts or qualifying function calls.
PostgreSQL allows multiple functions with the same name but different parameter types (function overloading). When you call a function, PostgreSQL uses a resolution algorithm to determine which version to execute. If two or more function versions match your arguments equally well with no clear winner, PostgreSQL raises an "ambiguous function" error. This happens because the database cannot reliably choose between competing candidates without more information about which function you intended to call.
Explicitly cast ambiguous arguments to the exact types expected by the function. This tells PostgreSQL precisely which function version to call.
-- Ambiguous call
SELECT generate_series($1, $2, $3);
-- Fixed with explicit casts
SELECT generate_series($1::integer, $2::integer, $3::integer);If functions with the same name exist in multiple schemas, prefix the function name with the schema name to remove ambiguity.
-- Ambiguous
SELECT my_function(arg1, arg2);
-- Clear schema ownership
SELECT public.my_function(arg1, arg2);
-- or
SELECT custom_schema.my_function(arg1, arg2);Use the \df command in psql to list all overloaded versions of a function and their exact signatures.
-- List all versions of a function
\df function_name
-- More detailed output
\df+ function_nameReview the parameter types listed to understand which signature your call should match.
In PL/pgSQL functions, ambiguity can occur between variable names and column names. Use a naming convention like prefixing variables with v_.
CREATE FUNCTION my_func() AS $$
DECLARE
v_word text; -- prefix variables
BEGIN
SELECT id FROM emp a WHERE a.id = v_word; -- qualify table references
END;
$$ LANGUAGE plpgsql;If multiple function versions have different sets of default parameters, PostgreSQL may not be able to distinguish them. Consider renaming functions or simplifying parameter sets.
-- Problem: two versions with overlapping defaults
CREATE FUNCTION process(a int, b int DEFAULT 0) ...
CREATE FUNCTION process(a int, b int DEFAULT 1) ... -- ambiguous!
-- Solution: use different function names
CREATE FUNCTION process_default(a int, b int DEFAULT 0) ...
CREATE FUNCTION process_custom(a int, b int) ...PostgreSQL uses a multi-step resolution algorithm for function overloading: it first checks for exact type matches, then preferred types within a category, then falls back to implicit type conversions. If multiple candidates remain after all steps, the error is raised. The search_path setting controls which schema is searched first; if you have identically-named functions in different schemas, ensure your search_path is ordered to prefer trusted schemas and prevent ambiguity. For C-language functions, overloaded variants must have different C function names internally. In PL/pgSQL, use the plpgsql.variable_conflict setting (values: error, use_variable, use_column) to control how ambiguous names are resolved between variables and columns.
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