An Index Is a Trade-off, Not a Free Speedup
Every index speeds up the reads it’s designed for and slows down every write to that table, while consuming additional storage. Effective indexing strategy is about matching indexes to your actual query patterns, not indexing every column defensively.
Index Columns Used in WHERE, JOIN, and ORDER BY
The most direct win: any column frequently used to filter, join, or sort a large table is a strong indexing candidate. A query doing a full table scan on a million-row table because a WHERE clause column lacks an index is one of the most common and easily fixed performance problems in production databases.
Composite Indexes and Column Order Matters
An index on multiple columns is only useful for queries that filter on a prefix of those columns in the same order the index was defined — an index on (status, created_at) helps a query filtering by status alone or by status and created_at together, but doesn’t help a query filtering by created_at alone. Understanding this ordering requirement is essential to designing composite indexes that actually get used.
Covering Indexes Eliminate Table Lookups Entirely
When an index includes every column a query needs — both the filter columns and the selected columns — the database can satisfy the entire query from the index alone, without a separate lookup into the table itself. This “index-only scan” is often dramatically faster than a standard index lookup followed by a table row fetch.
Watch for Unused and Redundant Indexes
Indexes created speculatively “just in case” and never actually used by any query still cost write performance and storage with zero benefit. Most databases provide tools to identify unused indexes (like PostgreSQL’s pg_stat_user_indexes) — periodically auditing and removing genuinely unused indexes is worthwhile maintenance.
Use EXPLAIN to Verify, Don’t Guess
Whether an index is actually being used for a specific query is not something to assume — run EXPLAIN (or EXPLAIN ANALYZE) and read the actual query plan. A query that “should” use an index based on your mental model sometimes doesn’t, for reasons ranging from stale statistics to the query planner correctly determining a full scan is actually faster for a small table.
Practical Recommendations
- Index based on actual slow-query logs and real query patterns, not speculation.
- Design composite index column order to match your most common query filter patterns.
- Regularly audit for unused indexes and remove them.
- Always verify with EXPLAIN rather than assuming an index is being used.