How to diagnose and fix the 42723 duplicate_function error code in Postgres. 

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The 42723 error code in PostgreSQL indicates a duplicate_function error. This error arises when you attempt to create a function that already exists with the same name and argument data types in the same schema. PostgreSQL uses the combination of function name and argument types (not return type) to uniquely identify a function.

Here are some scenarios where a duplicate_function error might occur, along with examples and sample code to diagnose and fix the issue:

Example 1: Creating a Function That Already Exists

Diagnosis:
You try to create a function without realizing that a function with the same name and argument types already exists.

CREATE FUNCTION add_numbers(a integer, b integer) RETURNS integer AS $$
BEGIN
    RETURN a + b;
END;
$$ LANGUAGE plpgsql;

If you run this command again or a function with the same name and signature already exists, PostgreSQL will raise a duplicate_function error.

Fix:
Before creating a new function, check if a function with the same name and parameter list already exists. You can either:

  • Use a different name for the new function.
  • Drop the existing function before creating the new one (if applicable).
  • Use the CREATE OR REPLACE FUNCTION statement to update the existing function.
CREATE OR REPLACE FUNCTION add_numbers(a integer, b integer) RETURNS integer AS $$
BEGIN
    RETURN a + b;
END;
$$ LANGUAGE plpgsql;

Example 2: Overloading Functions

Diagnosis:
PostgreSQL allows function overloading, which means you can have multiple functions with the same name but different argument types. However, if you try to create an overloaded function with the same argument types as an existing function, you’ll get a duplicate_function error.

CREATE FUNCTION compute(value double precision) RETURNS double precision AS $$
BEGIN
    RETURN value * value;
END;
$$ LANGUAGE plpgsql;

If a function named compute with a single double precision argument already exists, this will cause a duplicate_function error.

Fix:
Ensure that the new function has a different signature (different number of parameters or different parameter types):

CREATE FUNCTION compute(value integer) RETURNS integer AS $$
BEGIN
    RETURN value * value;
END;
$$ LANGUAGE plpgsql;

Example 3: Function Signature Clashes Due to Type Aliases

Diagnosis:
PostgreSQL has various aliases for data types (e.g., int for integer, float for double precision). If you create functions using these aliases, you might unintentionally create a function with a signature that already exists.

CREATE FUNCTION calculate(value int) RETURNS int AS $$
BEGIN
    RETURN value + 10;
END;
$$ LANGUAGE plpgsql;

If a function named calculate with a single integer parameter already exists, this will lead to a duplicate_function error because int is an alias for integer.

Fix:
Be aware of the type aliases and ensure that the new function’s signature is unique:

CREATE FUNCTION calculate(value bigint) RETURNS bigint AS $$
BEGIN
    RETURN value + 10;
END;
$$ LANGUAGE plpgsql;

General Tips:

  • Always verify the existing functions in your database using the \df command in psql or query the pg_proc system catalog.
  • Consider using CREATE OR REPLACE FUNCTION carefully, as it can change the behavior of the existing function.
  • Keep in mind type aliases to avoid unintentional signature clashes.

For more information on function creation and management in PostgreSQL, you can refer to the official PostgreSQL documentation on functions. This documentation provides detailed information on how to create and modify functions, including handling duplicate functions.

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