Lesson 08 · 12 min
Indexes and EXPLAIN
Why the same query takes 2 ms on one database and 2 seconds on another.
Without an index, the database reads the whole table (Seq Scan) and checks every row. A B-tree index keeps a column's values sorted with pointers to rows, so lookups by =, <, >, BETWEEN and sorting become fast (Index Scan). EXPLAIN ANALYZE runs the query and shows the plan: which method the database chose, how many rows, and how much time.
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