EXPLAIN query analysis in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the EXPLAIN statement do in MySQL?

EXPLAIN shows how MySQL executes a query. It helps understand the query plan and performance.

intermediate

What is the meaning of the 'type' column in EXPLAIN output?

The 'type' column shows the join type or how MySQL finds rows. Types range from 'ALL' (slow) to 'const' (fast).

beginner

What does the 'rows' column in EXPLAIN output represent?

It estimates how many rows MySQL will examine to execute the query part.

beginner

Why is it important to check the 'key' column in EXPLAIN output?

The 'key' column shows which index MySQL uses. Using indexes speeds up queries.

intermediate

What does it mean if EXPLAIN shows 'Using temporary' in the 'Extra' column?

It means MySQL creates a temporary table to process the query, which can slow down performance.

What is the main purpose of the EXPLAIN statement in MySQL?

A To show how MySQL executes a query

B To delete data from a table

C To create a new database

D To backup the database

EXPLAIN helps understand the query execution plan, not data modification or database management.

In EXPLAIN output, which 'type' value indicates the slowest way to read data?

A const

B ref

C ALL

D eq_ref

'ALL' means a full table scan, which is the slowest method.

What does the 'key' column in EXPLAIN output tell you?

A The query execution time

B The number of rows returned

C The table name

D The index MySQL uses for the query

'key' shows which index is used to speed up the query.

If EXPLAIN shows 'Using temporary' in the Extra column, what does it mean?

A The query is invalid

B MySQL is using a temporary table

C No indexes are used

D The query is very fast

'Using temporary' means MySQL creates a temporary table to process the query.

Which EXPLAIN column estimates how many rows MySQL will check?

A rows

B select_type

C id

D Extra

The 'rows' column estimates the number of rows MySQL will examine.

Explain what information the EXPLAIN statement provides about a MySQL query.

Think about how EXPLAIN helps you understand query performance.

Describe why checking the EXPLAIN output is useful before optimizing a query.

Consider what problems EXPLAIN can help you find.