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Scalar functions look innocent…
…until your query goes from 50 ms → 12 seconds.

-- ❌ Never do this inside SELECT
SELECT Id, dbo.GetUserScore(Id)
FROM Users;

💥 Why They’re Slow

• ⛓ Executed row-by-row

• 🚫 Breaks parallelism

• 📉 Kills execution plans

• 🐌 Turns queries into RBAR (“Row By Agonizing Row”)

✔ The Fix

Use Inline Table-Valued Functions instead:

CREATE FUNCTION dbo.GetUserScore(@id INT)
RETURNS TABLE
AS
RETURN (SELECT Score FROM Scores WHERE UserId = @id);

💡 Bonus

Scalar UDF Inlining in SQL 2019 helps — but don’t rely on magic.