SQL Optimization

SQL optimization

Optimize from the plan, not from vibes. Every database ships an EXPLAIN; folklore fixes applied without it make some queries faster and others quietly worse.

Method

1. Measure the actual query with real parameters and production-like data volume. A query fast on 1k dev rows tells you nothing about 10M. Capture: rows examined vs rows returned, time, and the plan (EXPLAIN ANALYZE where available).
2. Read the plan for the expensive truth. The usual suspects, in the order they pay off:
   • Full scan where a seek belongs: filter or join column lacks a usable index, or the predicate defeats it (function on the column, leading wildcard, implicit type cast).
   • Examined ≫ returned: thousands read to return ten: missing composite index, or filtering happens after the join instead of in it.
   • Sort or hash spilling: ORDER BY/GROUP BY/DISTINCT on an unindexed expression over a large set.
   • N+1 at the application seam: one query per row in a loop; the plan looks fine, the trace shows 400 of them.
3. Fix in this order, cheapest first:
   • Rewrite the predicate to be index-friendly (move the function to the constant side; match types exactly).
   • Add or extend a composite index: equality columns first, then the range column, then covering columns if the engine supports them. One good composite beats three single-column indexes.
   • Restructure the query: select only needed columns, filter before joining, replace correlated subqueries with joins or window functions, paginate by keyset (WHERE id > ?) not OFFSET at depth.
   • Only then reach for denormalization, materialized views, or caching: real costs that need the earlier steps ruled out.
4. Verify against the same measurement, same data, same parameters. Then check the write side: every index taxes every insert and update on that table. An index that saves one report and slows every checkout is a bad trade.

Rules

• Never claim a fix without before/after numbers from comparable data.
• Distrust SELECT * on principle: it defeats covering indexes and widens every row on the wire.
• A query that cannot be made fast may be the wrong question: say so and propose the schema or access-pattern change honestly.