MySQL + SQL · Lesson 91

EXPLAIN Query Plan in MySQL

What is EXPLAIN?

EXPLAIN shows how MySQL will run a query — which indexes it uses and how many rows it scans. It is the first tool for fixing slow queries.

Example

EXPLAIN SELECT * FROM students WHERE marks > 80;

Look at the type and key columns: type=ALL means a full table scan (slow); a named key means an index is used (fast).

Summary

  • EXPLAIN reveals the query plan: indexes used and rows scanned.
  • type=ALL = full scan (add an index); a key = index used.
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