Indexes

An index is a sorted structure, usually a B-tree, that finds rows without scanning the table. It turns a seconds-long query into a millisecond one, but every index slows writes and uses storage.

Where it shows up in real systems

  • Relational database

    The primary handles every write; read replicas add read capacity but lag slightly behind, so a user may not see their own write. A Multi-AZ standby takes over in about a minute if the primary fails. Sharding multiplies write capacity, but a bad shard key creates a hot shard. Without connection pooling, many app instances exhaust the connection limit.

    In production: PostgreSQL (Amazon RDS, Aurora), MySQL, Azure SQL Database, Cloud SQL, Citus, Vitess

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