Why should you care?

PostgresVerse pricing API calls calculate_gst(amount) in SQL so web and batch jobs share one tax rule.

See it live — copy this example

Run in pgAdmin or psql.

CREATE OR REPLACE FUNCTION calculate_gst(amount numeric)
RETURNS numeric
LANGUAGE sql
IMMUTABLE
AS $$
  SELECT round(amount * 0.18, 2);
$$;

SELECT product_id, price, calculate_gst(price) AS gst
FROM products;

What happened?

• SQL-language function multiplies price by 18% GST and rounds.
• IMMUTABLE tells planner result depends only on input — safe to index expressions using it.

Practice next

1. CREATE FUNCTION in PostgresVerse.
2. SELECT gst for several products.
3. Try CREATE FUNCTION in pgAdmin and inspect Properties.
4. Add function masking email: split_part(email,'@',1) || '@***'.
5. Use function in CHECK: CHECK (calculate_gst(price) >= 0).

Remember

Functions encapsulate reusable SQL logic. LANGUAGE sql is simple; plpgsql adds variables and control flow. Volatility category affects optimization.

PostgresVerse GST helper

Finance changes GST rate in one function; invoices and app quotes update together.

Outcome: No hunt for magic number 0.18 across microservices.