How to use the EXPLAIN command in MySQL?
EXPLAIN in MySQL reveals query execution plans, showing index usage, table read order, and row filtering to optimize performance; use it before SELECT to analyze steps, check key columns like type and rows, identify inefficiencies in Extra, and combine with indexing strategies for faster queries.
The EXPLAIN command in MySQL helps you understand how the database executes a query. It shows the execution plan, including which indexes are used, the order of table reads, and how rows are filtered. This is essential for optimizing slow queries.
How to Use EXPLAIN
Simply add EXPLAIN before your SELECT (or other supported) statement:
EXPLAIN SELECT * FROM users WHERE id = 1;This returns a result set with information about how MySQL would execute the query, not the actual query results.
Understanding the Output Columns
Each row in the EXPLAIN output represents a step in the query execution. Key columns include:
- id: The query's unique identifier. Values may repeat if there are subqueries or unions.
- select_type: Type of the SELECT (e.g., SIMPLE, PRIMARY, SUBQUERY).
- table: The table being accessed.
- partitions: Which partitions are matched (if partitioning is used).
- type: Join type. Important for performance: system, const, eq_ref, ref, range, index, ALL (ALL is usually bad).
- possible_keys: Indexes MySQL could use.
- key: The actual index used.
- key_len: Length of the key used. Helps determine if full or partial index is used.
- ref: Shows which column or constant is compared to the index.
- rows: Estimated number of rows MySQL must examine.
- filtered: Percentage of rows filtered by table condition.
- Extra: Additional info like "Using where", "Using index", "Using temporary", "Using filesort". These can indicate inefficiencies.
Using EXPLAIN with JOINs and Complex Queries
For queries involving multiple tables, EXPLAIN shows one row per table access. The order indicates join sequence. Look at:
- Whether type is efficient (avoid ALL when possible).
- If proper indexes are used (key column).
- High rows values, which suggest full table scans.
- Warnings in Extra, like "Using temporary" (temporary table created) or "Using filesort" (expensive sort operation).
Example:
EXPLAIN SELECT u.name, o.total FROM users u JOIN orders o ON u.id = o.user_id WHERE u.status = 'active';EXPLAIN FORMAT Options
You can request different output formats:
- EXPLAIN FORMAT=TRADITIONAL: Default tabular format.
- EXPLAIN FORMAT=JSON: Provides detailed structured info, including cost estimates and used indexes.
JSON format is helpful for deeper analysis:
EXPLAIN FORMAT=JSON SELECT * FROM users WHERE age > 30;EXPLAIN vs EXPLAIN ANALYZE
In MySQL 8.0 , EXPLAIN ANALYZE runs the query and shows actual execution statistics, comparing them to estimates. This gives real insight into performance issues:
EXPLAIN ANALYZE SELECT * FROM posts WHERE author_id = 5;It displays actual runtime, loops, and row counts—very useful for tuning.
Basically, use EXPLAIN whenever you're checking query performance. It’s a fast way to spot full table scans, missing indexes, or inefficient joins. Combine it with proper indexing and query restructuring to improve speed.
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