How can you improve the performance of a query that uses scalar functions?

Instruction: Discuss the impact of scalar functions on query performance and how you can optimize their usage.

Context: This question evaluates the candidate's understanding of the performance implications of scalar functions in SQL and their ability to optimize their queries accordingly.

Updated

Official answer

Measure where the time is spent before removing functions. A cheap scalar expression in SELECT is different from an expensive user-defined function called for many rows. Functions can take multiple arguments; scalar describes their single-value result.

A function around an indexed predicate column may prevent use of a plain index for that expression. For a timestamp-without-time-zone column, this PostgreSQL day filter leaves the stored column unchanged:

SELECT event_id, created_at
FROM events
WHERE created_at >= TIMESTAMP '2026-01-01 00:00:00'
  AND created_at < TIMESTAMP '2026-01-02 00:00:00';

It can be preferable to casting every created_at to date, while retaining the intended midnight bounds. Zoned timestamps need boundaries in the chosen business timezone, including daylight-saving rules.

Other options include supported expression indexes, valid generated columns or avoiding repeated expensive work. Check determinism, NULL behavior, precision and collation before moving or caching computation. A CTE does not universally guarantee one evaluation. Compare actual plans, CPU and latency across realistic inputs; precomputation adds write/storage costs and application-side work may increase data transfer.

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