Extra in EXPLAIN in MYSQL

In the EXPLAIN output in MySQL, the “Extra” column provides additional information about how MySQL is processing the query. When the “Extra” column shows NULL, it generally means that there are no additional special considerations or operations beyond the usual execution of the query.

Here are some common scenarios where the “Extra” column might have a value other than NULL and what they mean:

  1. Using Where:
    If the “Extra” column includes the phrase “Using where,” it indicates that MySQL is applying the WHERE clause to filter rows during the query execution. Example:
   EXPLAIN SELECT * FROM orders WHERE status = 'Shipped';
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
   | id | select_type | table | type | possible_keys   | key         | ref     | rows  | Extra       |
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
   |  1 | SIMPLE      | orders| ref  | idx_status      | idx_status  | const   | 50   | Using where |
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
  1. Using Index:
    If the “Extra” column includes the phrase “Using index,” it means that the query is able to retrieve all required information from the index without having to access the actual table. This is often seen in covering index scenarios. Example:
   EXPLAIN SELECT id FROM orders WHERE status = 'Shipped';
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
   | id | select_type | table | type | possible_keys   | key         | ref     | rows  | Extra       |
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
   |  1 | SIMPLE      | orders| ref  | idx_status      | idx_status  | const   | 50   | Using index |
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
  1. Using Temporary; Using Filesort:
    If the “Extra” column includes the phrases “Using temporary” or “Using filesort,” it indicates that MySQL needs to create a temporary table or use a filesort operation to fulfill the query. This might happen when dealing with ORDER BY or GROUP BY clauses. Example:
   EXPLAIN SELECT * FROM orders ORDER BY order_date;
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------+
   | id | select_type | table | type | possible_keys   | key         | ref     | rows  | Extra          |
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------+
   |  1 | SIMPLE      | orders| ALL  | NULL            | NULL        | NULL    | 50   | Using filesort |
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------+

When the “Extra” column is NULL, it generally means that there are no special considerations beyond the basic execution of the query. The absence of “Using where,” “Using index,” or other phrases indicates a straightforward query execution.

Yes, the “Extra” column in the EXPLAIN output can have several possible cases, and it provides additional information about how MySQL is handling the query. Here are some common scenarios you might encounter in the “Extra” column:

  1. Using where:
  • Indicates that MySQL is applying the WHERE clause to filter rows during the query execution. Example:
   EXPLAIN SELECT * FROM orders WHERE status = 'Shipped';
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
   | id | select_type | table | type | possible_keys   | key         | ref     | rows  | Extra       |
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
   |  1 | SIMPLE      | orders| ref  | idx_status      | idx_status  | const   | 50   | Using where |
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
  1. Using index:
  • Indicates that the query is able to retrieve all required information from the index without accessing the actual table. Example:
   EXPLAIN SELECT id FROM orders WHERE status = 'Shipped';
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
   | id | select_type | table | type | possible_keys   | key         | ref     | rows  | Extra       |
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
   |  1 | SIMPLE      | orders| ref  | idx_status      | idx_status  | const   | 50   | Using index |
   +----+-------------+-------+------+-----------------+-------------+---------+------+-------------+
  1. Using temporary; Using filesort:
  • Indicates that MySQL needs to create a temporary table or use a filesort operation to fulfill the query. This might happen with ORDER BY or GROUP BY clauses. Example:
   EXPLAIN SELECT * FROM orders ORDER BY order_date;
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------+
   | id | select_type | table | type | possible_keys   | key         | ref     | rows  | Extra          |
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------+
   |  1 | SIMPLE      | orders| ALL  | NULL            | NULL        | NULL    | 50   | Using filesort |
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------+
  1. Using index condition:
  • Indicates that the index is being used for filtering, but some conditions are applied afterward. Example:
   EXPLAIN SELECT * FROM orders WHERE status = 'Shipped' AND order_date > '2022-01-01';
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------------+
   | id | select_type | table | type | possible_keys   | key         | ref     | rows  | Extra                |
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------------+
   |  1 | SIMPLE      | orders| range| idx_status_date | idx_status  | NULL    | 25   | Using index condition|
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------------+

These are just a few examples, and the “Extra” column can include other information based on the specific optimizations or operations performed by MySQL during the query execution. Understanding the “Extra” column can provide insights into how MySQL is processing your queries and help in optimizing them for better performance.

Cases:

Certainly! The “Extra” column in the EXPLAIN output can contain various additional information depending on the specific characteristics of the query and how MySQL is executing it. Here are a few more scenarios you might encounter in the “Extra” column:

  1. Using join buffer (Block Nested Loop):
  • Indicates that MySQL is using a join buffer to process the join between tables. Example:
   EXPLAIN SELECT * FROM orders JOIN customers ON orders.customer_id = customers.customer_id;
   +----+-------------+----------+------+---------------------+-------------+---------+-------+------+-------------------------+
   | id | select_type | table    | type | possible_keys       | key         | ref     | rows  | ...  | Extra                   |
   +----+-------------+----------+------+---------------------+-------------+---------+-------+------+-------------------------+
   |  1 | SIMPLE      | orders   | ALL  | NULL                | NULL        | NULL    | 50    | ...  |                         |
   |  1 | SIMPLE      | customers| ALL  | PRIMARY             | NULL        | NULL    | 100   | ...  | Using join buffer (BNL)|
   +----+-------------+----------+------+---------------------+-------------+---------+-------+------+-------------------------+
  1. Using where; Using join buffer (Batched Key Access):
  • Indicates that MySQL is using a join buffer with a batched key access strategy for better performance. Example:
   EXPLAIN SELECT * FROM orders JOIN customers USING (customer_id) WHERE status = 'Shipped';
   +----+-------------+----------+------+---------------------+-------------+---------+-------+------+-------------------------------+
   | id | select_type | table    | type | possible_keys       | key         | ref     | rows  | ...  | Extra                         |
   +----+-------------+----------+------+---------------------+-------------+---------+-------+------+-------------------------------+
   |  1 | SIMPLE      | orders   | ref  | idx_status           | idx_status  | const   | 50    | ...  | Using where; Using join buffer|
   |  1 | SIMPLE      | customers| eq_ref| PRIMARY             | PRIMARY     | orders.customer_id | ... | Using join buffer (Batched Key Access)|
   +----+-------------+----------+------+---------------------+-------------+---------+-------+------+-------------------------------+
  1. Using sort_union:
  • Indicates that MySQL is using a sort_union optimization for UNION queries. Example:
   EXPLAIN SELECT * FROM orders WHERE status = 'Shipped'
   UNION
   SELECT * FROM orders WHERE status = 'Pending';
   +----+--------------+----------+------+---------------------+-------------+---------+------+------------------+
   | id | select_type  | table    | type | possible_keys       | key         | ref     | rows | ...  | Extra            |
   +----+--------------+----------+------+---------------------+-------------+---------+------+------------------+
   |  1 | PRIMARY      | orders   | ref  | idx_status           | idx_status  | const   | 25   | ...  | Using sort_union |
   |  2 | UNION        | orders   | ref  | idx_status           | idx_status  | const   | 25   | ...  | Using sort_union |
   +----+--------------+----------+------+---------------------+-------------+---------+------+------------------+

These are just a few examples, and the “Extra” column can contain various other information based on the specific characteristics of the query. Understanding the “Extra” column is valuable for query optimization and gaining insights into how MySQL is processing your queries. If you encounter specific terms in the “Extra” column that you’d like further clarification on, feel free to ask!

case:

Certainly! Here are a few more scenarios you might encounter in the “Extra” column of the EXPLAIN output:

  1. Using intersect:
  • Indicates that MySQL is using an intersect operation for certain types of queries involving multiple tables. Example:
   EXPLAIN SELECT * FROM orders WHERE status = 'Shipped'
   INTERSECT
   SELECT * FROM orders WHERE order_date > '2022-01-01';
   +----+-------------+-------+------+---------------------+-------------+---------+------+----------------------+
   | id | select_type | table | type | possible_keys       | key         | ref     | rows | ...  | Extra                |
   +----+-------------+-------+------+---------------------+-------------+---------+------+----------------------+
   |  1 | PRIMARY     | orders| ref  | idx_status_date     | idx_status  | const   | 25   | ...  | Using intersect      |
   |  2 | PRIMARY     | orders| ref  | idx_status_date     | idx_status  | const   | 25   | ...  | Using intersect      |
   +----+-------------+-------+------+---------------------+-------------+---------+------+----------------------+
  1. Using index for group-by:
  • Indicates that MySQL is using an index to optimize a GROUP BY operation. Example:
   EXPLAIN SELECT status, COUNT(*) FROM orders GROUP BY status;
   +----+-------------+-------+-------+---------------+-------------+---------+------+-------------------------+
   | id | select_type | table | type  | possible_keys | key         | ref     | rows | Extra                   |
   +----+-------------+-------+-------+---------------+-------------+---------+------+-------------------------+
   |  1 | SIMPLE      | orders| index | NULL          | idx_status  | NULL    | 50   | Using index for group-by |
   +----+-------------+-------+-------+---------------+-------------+---------+------+-------------------------+
  1. Using where; Using join buffer (Batched Key Access) with GROUP BY:
  • Indicates a combination of WHERE, join buffer, and GROUP BY optimizations. Example:
   EXPLAIN SELECT customers.customer_id, COUNT(*) FROM orders
   JOIN customers USING (customer_id)
   WHERE orders.status = 'Shipped'
   GROUP BY customers.customer_id;
   +----+-------------+----------+--------+---------------------+-------------+---------+------+--------------------------------------+
   | id | select_type | table    | type   | possible_keys       | key         | ref     | rows | Extra                                |
   +----+-------------+----------+--------+---------------------+-------------+---------+------+--------------------------------------+
   |  1 | SIMPLE      | orders   | ref    | idx_status          | idx_status  | const   | 25   | Using where; Using temporary; Using join buffer (Batched Key Access)|
   |  1 | SIMPLE      | customers| eq_ref | PRIMARY             | PRIMARY     | orders.customer_id | 1    |                                      |
   +----+-------------+----------+--------+---------------------+-------------+---------+------+--------------------------------------+
  1. Using intersect with sort_union:
  • Indicates a combination of intersect and sort_union optimizations. Example:
   EXPLAIN SELECT * FROM orders WHERE status = 'Shipped'
   INTERSECT
   SELECT * FROM orders WHERE order_date > '2022-01-01'
   ORDER BY order_date;
   +----+-------------+-------+------+---------------------+-------------+---------+------+-----------------------------+
   | id | select_type | table | type | possible_keys       | key         | ref     | rows | Extra                       |
   +----+-------------+-------+------+---------------------+-------------+---------+------+-----------------------------+
   |  1 | PRIMARY     | orders| ref  | idx_status_date     | idx_status  | const   | 25   | Using intersect             |
   |  2 | PRIMARY     | orders| ref  | idx_status_date     | idx_status  | const   | 25   | Using intersect; Using where|
   |  3 | PRIMARY     | orders| ref  | idx_status_date     | idx_status  | NULL    | 50   | Using intersect; Using where; Using filesort |
   +----+-------------+-------+------+---------------------+-------------+---------+------+-----------------------------+

These are additional examples of situations you might encounter in the “Extra” column. Understanding these details can help you optimize your queries and gain insights into how MySQL is processing them. If you have specific queries or cases you’d like more information about, feel free to share them!

CASES:

Certainly! Here are a few more scenarios you might encounter in the “Extra” column of the EXPLAIN output:

  1. Using index condition for where clause:
  • Indicates that an index is being used for part of the WHERE clause conditions. Example:
   EXPLAIN SELECT * FROM orders WHERE status = 'Shipped' AND order_date > '2022-01-01';
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------------+
   | id | select_type | table | type | possible_keys   | key         | ref     | rows | Extra                |
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------------+
   |  1 | SIMPLE      | orders| range| idx_status_date | idx_status  | NULL    | 25   | Using index condition|
   +----+-------------+-------+------+-----------------+-------------+---------+------+----------------------+
  1. Using where; Using join buffer (Block Nested Loop) with GROUP BY:
  • Indicates a combination of WHERE, join buffer, and GROUP BY optimizations. Example:
   EXPLAIN SELECT customers.customer_id, COUNT(*) FROM orders
   JOIN customers USING (customer_id)
   WHERE orders.status = 'Shipped'
   GROUP BY customers.customer_id;
   +----+-------------+----------+--------+---------------------+-------------+---------+------+--------------------------------------+
   | id | select_type | table    | type   | possible_keys       | key         | ref     | rows | Extra                                |
   +----+-------------+----------+--------+---------------------+-------------+---------+------+--------------------------------------+
   |  1 | SIMPLE      | orders   | ref    | idx_status          | idx_status  | const   | 25   | Using where; Using temporary; Using join buffer (BNL)|
   |  1 | SIMPLE      | customers| eq_ref | PRIMARY             | PRIMARY     | orders.customer_id | 1    |                                      |
   +----+-------------+----------+--------+---------------------+-------------+---------+------+--------------------------------------+
  1. Using index for group-by (skip-scan):
  • Indicates that MySQL is using an index to optimize a GROUP BY operation, possibly employing skip-scan optimization. Example:
   EXPLAIN SELECT status, COUNT(*) FROM orders GROUP BY status, order_date;
   +----+-------------+-------+-------+-------------------+-------------+---------+------+-----------------------------+
   | id | select_type | table | type  | possible_keys     | key         | ref     | rows | Extra                       |
   +----+-------------+-------+-------+-------------------+-------------+---------+------+-----------------------------+
   |  1 | SIMPLE      | orders| index | idx_status_date   | idx_status  | NULL    | 50   | Using index for group-by     |
   +----+-------------+-------+-------+-------------------+-------------+---------+------+-----------------------------+
  1. Using index for distinct:
  • Indicates that MySQL is using an index to optimize a query with the DISTINCT keyword. Example:
   EXPLAIN SELECT DISTINCT status FROM orders;
   +----+-------------+-------+-------+---------------+-------------+---------+------+------------------------+
   | id | select_type | table | type  | possible_keys | key         | ref     | rows | Extra                  |
   +----+-------------+-------+-------+---------------+-------------+---------+------+------------------------+
   |  1 | SIMPLE      | orders| index | idx_status    | idx_status  | NULL    | 50   | Using index for distinct|
   +----+-------------+-------+-------+---------------+-------------+---------+------+------------------------+

These are additional examples of scenarios you might encounter in the “Extra” column. Understanding these details can help you optimize your queries and gain insights into how MySQL is processing them. If you have specific queries or cases you’d like more information about, feel free to share them!

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