Should I index every column?

No. Only index columns that are frequently used in WHERE clauses, JOIN conditions, and ORDER BY clauses. Each index adds storage overhead and slows writes. Start with the most critical queries and add indexes based on actual usage patterns.

What is a composite index and does column order matter?

A composite index covers multiple columns (e.g., CREATE INDEX idx ON orders(user_id, created_at)). Column order matters — the index is most effective when queries filter on columns from left to right. The above index helps "WHERE user_id = 5 AND created_at > ..." but not "WHERE created_at > ..." alone.

How do I know if my query is using an index?

Run EXPLAIN ANALYZE before your query. Look for "Index Scan" or "Index Only Scan" in the output. If you see "Seq Scan" on a large table, the query is doing a full table scan and likely needs an index.

Database Indexing Explained

Data structures that turn slow full-table scans into fast lookups — the single biggest lever for query performance.

Database Indexing

Database indexing is the practice of creating data structures (indexes) that allow the database engine to locate rows quickly without scanning the entire table. Indexes dramatically speed up read queries at the cost of additional storage and slightly slower writes.

Explanation

Without an index, a database query must perform a full table scan — reading every row to find matches. For a table with millions of rows, this can take seconds or even minutes. An index creates a sorted data structure (typically a B-tree or hash table) that maps column values to row locations, enabling the database to find matching rows in logarithmic time instead of linear time. Common index types include: B-tree indexes (the default, good for range queries and equality), hash indexes (fast for exact matches but no range support), composite indexes (on multiple columns, order matters), partial indexes (only index rows matching a condition), and full-text indexes (for searching text content). A covering index includes all columns needed by a query, allowing the database to answer the query entirely from the index without touching the table. Over-indexing is as harmful as under-indexing. Every index consumes storage, slows down INSERT/UPDATE/DELETE operations (the index must be updated too), and adds complexity to the query planner. The key is to index columns that appear in WHERE clauses, JOIN conditions, and ORDER BY clauses of frequent queries. Tools like EXPLAIN ANALYZE help identify missing indexes and verify that queries use existing ones.

Bookuvai Implementation

During the database design milestone, Bookuvai engineers analyze expected query patterns and create indexes proactively. Our standard practice includes: primary key indexes (automatic), foreign key indexes (often missed but critical for JOIN performance), and composite indexes for common filter combinations. Post-launch, our monitoring identifies slow queries, and the AI PM creates tickets for missing index optimizations.

Key Facts

B-tree indexes are the default in PostgreSQL and MySQL, handling both equality and range queries
A missing index on a JOIN column can make queries 100x slower
EXPLAIN ANALYZE is the most important tool for diagnosing query performance

Related Terms

Frequently Asked Questions

Should I index every column?: No. Only index columns that are frequently used in WHERE clauses, JOIN conditions, and ORDER BY clauses. Each index adds storage overhead and slows writes. Start with the most critical queries and add indexes based on actual usage patterns.
What is a composite index and does column order matter?: A composite index covers multiple columns (e.g., CREATE INDEX idx ON orders(user_id, created_at)). Column order matters — the index is most effective when queries filter on columns from left to right. The above index helps "WHERE user_id = 5 AND created_at > ..." but not "WHERE created_at > ..." alone.
How do I know if my query is using an index?: Run EXPLAIN ANALYZE before your query. Look for "Index Scan" or "Index Only Scan" in the output. If you see "Seq Scan" on a large table, the query is doing a full table scan and likely needs an index.