Performance
Indexes and query plans — how to make a query fast, and how to check whether it actually worked.
Every page up to this point is about getting the right answer. This section is about getting it back quickly, and — just as important — knowing how to check.
Indexes
What an index actually does, and the patterns that quietly stop the planner from using one.
Reading query plans
EXPLAIN across engines — what a scan vs. a search actually means, and what to look for.
Playground note
The playground can run CREATE INDEX and EXPLAIN QUERY PLAN for real —
SQLite's query planner behaves like any other engine's at a conceptual level,
choosing a table scan or an index the same way Postgres, MySQL, or SQL Server
would. Every example here builds its own scratch copy of a seed table before
adding an index to it, so nothing here changes how any other page's examples
behave.