Lesson 24 / 30
EXPLAIN at a Glance
Read EXPLAIN output to see whether MySQL uses an index or a full table scan, and fix slow queries.
Reading a query plan
EXPLAIN shows whether MySQL will use an index or scan the whole table.
EXPLAIN SELECT * FROM orders WHERE customer_id = 1;Output:
id | table | type | possible_keys | key | rows 1 | orders | ref | idx_orders_customer | idx_orders_customer | 2
type: ALL is a red flag
type: ALL in EXPLAIN means a full table scan — on a large table, that's usually a sign a useful index is missing.
Quick check: In EXPLAIN output, what does `type: ALL` usually indicate?
- The query used an index perfectly
- The table has no rows
- The query will always be fast
- A full table scan happened
Answer
A full table scan happened — ALL means MySQL had to read every row — often fixable with an index.