How to Create Oracle-Style UDFs in PostgreSQL

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How do I create Oracle-style user-defined functions in PostgreSQL?

User-defined functions (UDFs) let you package SQL and PL/pgSQL logic into reusable, callable objects.

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Description

Example H2

Example H3

What are user-defined functions in PostgreSQL?

User-defined functions (UDFs) are database objects that encapsulate SQL or procedural logic and return a value or result set. They behave like Oracle functions but use PostgreSQL’s CREATE FUNCTION syntax.

Why replicate Oracle UDFs in PostgreSQL?

Teams migrating from Oracle keep business logic close to data. Re-implementing Oracle UDFs in PostgreSQL preserves familiar call patterns while leveraging open-source tooling.

How do I create a basic function?

Use CREATE OR REPLACE FUNCTION, define parameters, return type, language, and body. LANGUAGE plpgsql is closest to Oracle PL/SQL.

CREATE OR REPLACE FUNCTION get_customer_total(p_customer_id INT) RETURNS NUMERIC AS $$ DECLARE v_total NUMERIC; BEGIN SELECT COALESCE(SUM(total_amount),0) INTO v_total FROM Orders WHERE customer_id = p_customer_id; RETURN v_total; END; $$ LANGUAGE plpgsql IMMUTABLE;

Step-by-step explanation

Parameters receive input, DECLARE reserves variables, BEGIN…END houses the logic, and RETURN outputs the value.

How do I call the function?

SELECT get_customer_total(42);

Calls work in SELECT lists, WHERE clauses, or FROM … CROSS JOIN LATERAL blocks.

When should I use IMMUTABLE, STABLE, or VOLATILE?

Mark a UDF IMMUTABLE when the same input always yields the same output (ideal for sums on static tables). Use STABLE when reads but no writes occur. Default VOLATILE if writes or nondeterministic logic exist.

How do I update or replace a UDF?

Use CREATE OR REPLACE FUNCTION to redeploy logic without dropping dependent objects. Ensure signature (name + arg types) stays constant to avoid breakage.

Can I drop a UDF safely?

Run DROP FUNCTION IF EXISTS function_name(arg_types) [CASCADE]. Add CASCADE only when you’re certain dependent objects can also be removed.

Best practices for writing UDFs

Prefix parameters with p_ and variables with v_ for readability.
Set SET search_path at the top to avoid schema confusion.
Use STRICT to auto-return NULL when any argument is NULL.
Document purpose, parameters, and return type in a COMMENT.

Why How to Create Oracle-Style UDFs in PostgreSQL is important

How to Create Oracle-Style UDFs in PostgreSQL Example Usage


-- Calculate a customer’s lifetime value using Orders and OrderItems
CREATE OR REPLACE FUNCTION customer_lifetime_value(p_customer_id INT)
RETURNS NUMERIC AS $$
DECLARE
    v_ltv NUMERIC;
BEGIN
    SELECT COALESCE(SUM(oi.quantity * pr.price),0)
    INTO v_ltv
    FROM Orders o
    JOIN OrderItems oi  ON oi.order_id = o.id
    JOIN Products pr    ON pr.id        = oi.product_id
    WHERE o.customer_id = p_customer_id;

    RETURN v_ltv;
END;
$$ LANGUAGE plpgsql STABLE;

-- Usage
SELECT name, customer_lifetime_value(id) AS ltv
FROM Customers
ORDER BY ltv DESC
LIMIT 5;

How to Create Oracle-Style UDFs in PostgreSQL Syntax


CREATE [OR REPLACE] FUNCTION function_name ( [ [ arg_name ] arg_type [, ...] ] )
RETURNS return_type
LANGUAGE plpgsql | sql | c | internal | ...
[ IMMUTABLE | STABLE | VOLATILE ]
[ STRICT ]
[ SECURITY INVOKER | SECURITY DEFINER ]
AS $$
DECLARE
    -- variable declarations
BEGIN
    -- SQL or plpgsql logic
END;
$$;

-- Example in ecommerce context
CREATE OR REPLACE FUNCTION customer_last_order_date(p_customer_id INT)
RETURNS DATE AS $$
BEGIN
    RETURN (
        SELECT MAX(order_date)
        FROM Orders
        WHERE customer_id = p_customer_id
    );
END;
$$ LANGUAGE plpgsql STABLE;

Common Mistakes

Omitting the LANGUAGE clause. Without specifying LANGUAGE plpgsql, PostgreSQL assumes SQL language which rejects BEGIN…END blocks. Always append <code>LANGUAGE plpgsql</code>.
Marking data-changing functions as IMMUTABLE. PostgreSQL’s planner may cache results, causing stale data. Use VOLATILE when the function writes or reads frequently changing tables.