Syntax
------

EXPLAIN tbl_name [col_name | wild]

Or

EXPLAIN [EXTENDED | PARTITIONS | FORMAT=JSON] 
 {SELECT select_options | UPDATE update_options | DELETE delete_options}

Description
-----------

The EXPLAIN statement can be used either as a synonym for DESCRIBE or as a way
to obtain information about how MariaDB executes a SELECT, UPDATE or DELETE
statement:

* 'EXPLAIN tbl_name' is synonymous with 
 'DESCRIBE tbl_name' or
 'SHOW COLUMNS FROM tbl_name'.
* When you precede a SELECT, UPDATE or a DELETE statement with the keyword 
 EXPLAIN, MariaDB displays information from the optimizer
 about the query execution plan. That is, MariaDB explains how it would
 process the SELECT, UPDATE or DELETE, including information about how tables
 are joined and in which order. EXPLAIN EXTENDED can be
 used to provide additional information.
* EXPLAIN PARTITIONS is useful only when examining queries involving
partitioned tables. For details, see Partition pruning and selection.
* ANALYZE statement performs the query as well as producing EXPLAIN output,
and provides actual as well as estimated statistics.
* EXPLAIN output can be printed in the slow query log. See EXPLAIN in the Slow
Query Log for details.

SHOW EXPLAIN shows the output of a running statement. In some cases, its
output can be closer to reality than EXPLAIN.

The ANALYZE statement runs a statement and returns information about its
execution plan. It also shows additional columns, to check how much the
optimizer's estimation about filtering and found rows are close to reality.

There is an online EXPLAIN Analyzer that you can use to share EXPLAIN and
EXPLAIN EXTENDED output with others.

EXPLAIN can acquire metadata locks in the same way that SELECT does, as it
needs to know table metadata and, sometimes, data as well.

Columns in EXPLAIN ... SELECT
-----------------------------

+--------------------------------------+--------------------------------------+
| Column name                          | Description                          |
+--------------------------------------+--------------------------------------+
| id                                   | Sequence number that shows in which  |
|                                      | order tables are joined.             |
+--------------------------------------+--------------------------------------+
| select_type                          | What kind of SELECT the table comes  |
|                                      | from.                                |
+--------------------------------------+--------------------------------------+
| table                                | Alias name of table. Materialized    |
|                                      | temporary tables for sub queries     |
|                                      | are named <subquery#>                |
+--------------------------------------+--------------------------------------+
| type                                 | How rows are found from the table    |
|                                      | (join type).                         |
+--------------------------------------+--------------------------------------+
| possible_keys                        | keys in table that could be used to  |
|                                      | find rows in the table               |
+--------------------------------------+--------------------------------------+
| key                                  | The name of the key that is used to  |
|                                      | retrieve rows. NULL is no key was    |
|                                      | used.                                |
+--------------------------------------+--------------------------------------+
| key_len                              | How many bytes of the key that was   |
|                                      | used (shows if we are using only     |
|                                      | parts of the multi-column key).      |
+--------------------------------------+--------------------------------------+
| ref                                  | The reference that is used as the    |
|                                      | key value.                           |
+--------------------------------------+--------------------------------------+
| rows                                 | An estimate of how many rows we      |
|                                      | will find in the table for each key  |
|                                      | lookup.                              |
+--------------------------------------+--------------------------------------+
| Extra                                | Extra information about this join.   |
+--------------------------------------+--------------------------------------+

Here are descriptions of the values for some of the more complex columns in
EXPLAIN ... SELECT:

"Select_type" Column
--------------------

The select_type column can have the following values:

+-----------------+-----------------------------------+-----------------------+
| Value           | Description                       | Comment               |
+-----------------+-----------------------------------+-----------------------+
| DEPENDENT       | The SUBQUERY is DEPENDENT.        |                       |
| SUBQUERY        |                                   |                       |
+-----------------+-----------------------------------+-----------------------+
| DEPENDENT UNION | The UNION is DEPENDENT.           |                       |
+-----------------+-----------------------------------+-----------------------+
| DERIVED         | The SELECT is DERIVED from the    |                       |
|                 | PRIMARY.                          |                       |
+-----------------+-----------------------------------+-----------------------+
| MATERIALIZED    | The SUBQUERY is MATERIALIZED.     | Materialized tables   |
|                 |                                   | will be populated at  |
|                 |                                   | first access and      |
|                 |                                   | will be accessed by   |
|                 |                                   | the primary key (=    |
|                 |                                   | one key lookup).      |
|                 |                                   | Number of rows in     |
|                 |                                   | EXPLAIN shows the     |
|                 |                                   | cost of populating    |
|                 |                                   | the table             |
+-----------------+-----------------------------------+-----------------------+
| PRIMARY         | The SELECT is in the outermost    |                       |
|                 | query, but there is also a        |                       |
|                 | SUBQUERY within it.               |                       |
+-----------------+-----------------------------------+-----------------------+
| SIMPLE          | It is a simple SELECT query       |                       |
|                 | without any SUBQUERY or UNION.    |                       |
+-----------------+-----------------------------------+-----------------------+
| SUBQUERY        | The SELECT is a SUBQUERY of the   |                       |
|                 | PRIMARY.                          |                       |
+-----------------+-----------------------------------+-----------------------+
| UNCACHEABLE     | The SUBQUERY is UNCACHEABLE.      |                       |
| SUBQUERY        |                                   |                       |
+-----------------+-----------------------------------+-----------------------+
| UNCACHEABLE     | The UNION is UNCACHEABLE.         |                       |
| UNION           |                                   |                       |
+-----------------+-----------------------------------+-----------------------+
| UNION           | The SELECT is a UNION of the      |                       |
|                 | PRIMARY.                          |                       |
+-----------------+-----------------------------------+-----------------------+
| UNION RESULT    | The result of the UNION.          |                       |
+-----------------+-----------------------------------+-----------------------+
| LATERAL DERIVED | The SELECT uses a Lateral         |                       |
|                 | Derived optimization              |                       |
+-----------------+-----------------------------------+-----------------------+

"Type" Column
-------------

This column contains information on how the table is accessed.

+------------------------+---------------------------------------------------+
| Value                  | Description                                       |
+------------------------+---------------------------------------------------+
| ALL                    | A full table scan is done for the table (all      |
|                        | rows are read). This is bad if the table is       |
|                        | large and the table is joined against a previous  |
|                        | table!  This happens when the optimizer could     |
|                        | not find any usable index to access rows.         |
+------------------------+---------------------------------------------------+
| const                  | There is only one possibly matching row in the    |
|                        | table. The row is read before the optimization    |
|                        | phase and all columns in the table are treated    |
|                        | as constants.                                     |
+------------------------+---------------------------------------------------+
| eq_ref                 | A unique index is used to find the rows. This is  |
|                        | the best possible plan to find the row.           |
+------------------------+---------------------------------------------------+
| fulltext               | A fulltext index is used to access the rows.      |
+------------------------+---------------------------------------------------+
| index_merge            | A 'range' access is done for for several index    |
|                        | and the found rows are merged. The key column     |
|                        | shows which keys are used.                        |
+------------------------+---------------------------------------------------+
| index_subquery         | This is similar as ref, but used for sub queries  |
|                        | that are transformed to key lookups.              |
+------------------------+---------------------------------------------------+
| index                  | A full scan over the used index.  Better than     |
|                        | ALL but still bad if index is large and the       |
|                        | table is joined against a previous table.         |
+------------------------+---------------------------------------------------+
| range                  | The table will be accessed with a key over one    |
|                        | or more value ranges.                             |
+------------------------+---------------------------------------------------+
| ref_or_null            | Like 'ref' but in addition another search for     |
|                        | the 'null' value is done if the first value was   |
|                        | not found. This happens usually with sub queries. |
+------------------------+---------------------------------------------------+
| ref                    | A non unique index or prefix of an unique index   |
|                        | is used to find the rows. Good if the prefix      |
|                        | doesn't match many rows.                          |
+------------------------+---------------------------------------------------+
| system                 | The table has 0 or 1 rows.                        |
+------------------------+---------------------------------------------------+
| unique_subquery        | This is similar as eq_ref, but used for sub       |
|                        | queries that are transformed to key lookups       |
+------------------------+---------------------------------------------------+

"Extra" Column
--------------

This column consists of one or more of the following values, separated by ';'

Note that some of these values are detected after the optimization phase.

The optimization phase can do the following changes to the WHERE clause:

* Add the expressions from the ON and USING clauses to the WHERE
 clause.
* Constant propagation:  If there is column=constant, replace all column
 instances with this constant.
* Replace all columns from 'const' tables with their values.
* Remove the used key columns from the WHERE (as this will be tested as
 part of the key lookup).
* Remove impossible constant sub expressions.
 For example WHERE '(a=1 and a=2) OR b=1' becomes 'b=1'.
* Replace columns with other columns that has identical values:
 Example:  WHERE a=b and a=c may be treated
 as 'WHERE a=b and a=c and b=c'.
* Add extra conditions to detect impossible row conditions earlier. This
 happens mainly with OUTER JOIN where we in some cases add detection
 of NULL values in the WHERE (Part of 'Not exists' optimization).
 This can cause an unexpected 'Using where' in the Extra column.
* For each table level we remove expressions that have already been tested when
 we read the previous row. Example: When joining tables t1 with t2
 using the following WHERE 't1.a=1 and t1.a=t2.b', we don't have to
 test 't1.a=1' when checking rows in t2 as we already know that this
 expression is true.

+------------------------+---------------------------------------------------+
| Value                  | Description                                       |
+------------------------+---------------------------------------------------+
| const row not found    | The table was a system table (a table with        |
|                        | should exactly one row), but no row was found.    |
+------------------------+---------------------------------------------------+
| Distinct               | If distinct optimization (remove duplicates) was  |
|                        | used. This is marked only for the last table in   |
|                        | the SELECT.                                       |
+------------------------+---------------------------------------------------+
| Full scan on NULL key  | The table is a part of the sub query and if the   |
|                        | value that is used to match the sub query will    |
|                        | be NULL, we will do a full table scan.            |
+------------------------+---------------------------------------------------+
| Impossible HAVING      | The used HAVING clause is always false so the     |
|                        | SELECT will return no rows.                       |
+------------------------+---------------------------------------------------+
| Impossible WHERE       | The used WHERE clause is always false so the      |
| noticed after reading  | SELECT will return no rows. This case was         |
| const tables.          | detected after we had read all 'const' tables     |
|                        | and used the column values as constant in the     |
|                        | WHERE clause. For example: WHERE const_column=5   |
|                        | and const_column had a value of 4.                |
+------------------------+---------------------------------------------------+
| Impossible WHERE       | The used WHERE clause is always false so the      |
|                        | SELECT will return no rows. For example: WHERE    |
|                        | 1=2                                               |
+------------------------+---------------------------------------------------+
| No matching min/max    | During early optimization of MIN()/MAX() values   |
| row                    | it was detected that no row could match the       |
|                        | WHERE clause. The MIN()/MAX() function will       |
|                        | return NULL.                                      |
+------------------------+---------------------------------------------------+
| no matching row in     | The table was a const table (a table with only    |
| const table            | one possible matching row), but no row was found. |
+------------------------+---------------------------------------------------+
| No tables used         | The SELECT was a sub query that did not use any   |
|                        | tables. For example a there was no FROM clause    |
|                        | or a FROM DUAL clause.                            |
+------------------------+---------------------------------------------------+
| Not exists             | Stop searching after more row if we find one      |
|                        | single matching row. This optimization is used    |
|                        | with LEFT JOIN where one is explicitly searching  |
|                        | for rows that doesn't exists in the LEFT JOIN     |
|                        | TABLE. Example: SELECT * FROM t1 LEFT JOIN t2 on  |
|                        | (...) WHERE t2.not_null_column IS NULL.  As       |
|                        | t2.not_null_column can only be NULL if there was  |
|                        | no matching row for on condition, we can stop     |
|                        | searching if we find a single matching row.       |
+------------------------+---------------------------------------------------+
| Open_frm_only          | For information_schema tables.  Only the frm      |
|                        | (table definition file was opened) was opened     |
|                        | for each matching row.                            |
+------------------------+---------------------------------------------------+
| Open_full_table        | For information_schema tables. A full table open  |
|                        | for each matching row is done to retrieve the     |
|                        | requested information. (Slow)                     |
+------------------------+---------------------------------------------------+
| Open_trigger_only      | For information_schema tables. Only the trigger   |
|                        | file definition was opened for each matching row. |
+------------------------+---------------------------------------------------+
| Range checked for      | This only happens when there was no good default  |
| each record (index     | index to use but there may some index that could  |
| map: ...)              | be used when we can treat all columns from        |
|                        | previous table as constants.  For each row        |
|                        | combination the optimizer will decide which       |
|                        | index to use (if any) to fetch a row from this    |
|                        | table. This is not fast, but faster than a full   |
|                        | table scan that is the only other choice. The     |
|                        | index map is a bitmask that shows which index     |
|                        | are considered for each row condition.            |
+------------------------+---------------------------------------------------+
| Scanned 0/1/all        | For information_schema tables. Shows how many     |
| databases              | times we had to do a directory scan.              |
+------------------------+---------------------------------------------------+
| Select tables          | All tables in the join was optimized away. This   |
| optimized away         | happens when we are only using COUNT(*), MIN()    |
|                        | and MAX() functions in the SELECT and we where    |
|                        | able to replace all of these with constants.      |
+------------------------+---------------------------------------------------+
| Skip_open_table        | For information_schema tables. The queried table  |
|                        | didn't need to be opened.                         |
+------------------------+---------------------------------------------------+
| unique row not found   | The table was detected to be a const table (a     |
|                        | table with only one possible matching row)        |
|                        | during the early optimization phase, but no row   |
|                        | was found.                                        |
+------------------------+---------------------------------------------------+
| Using filesort         | Filesort is needed to resolve the query. This     |
|                        | means an extra phase where we first collect all   |
|                        | columns to sort, sort them with a disk based      |
|                        | merge sort and then use the sorted set to         |
|                        | retrieve the rows in sorted order. If the column  |
|                        | set is small, we store all the columns in the     |
|                        | sort file to not have to go to the database to    |
|                        | retrieve them again.                              |
+------------------------+---------------------------------------------------+
| Using index            | Only the index is used to retrieve the needed     |
|                        | information from the table. There is no need to   |
|                        | perform an extra seek to retrieve the actual      |
|                        | record.                                           |
+------------------------+---------------------------------------------------+
| Using index condition  | Like 'Using where' but the where condition is     |
|                        | pushed down to the table engine for internal      |
|                        | optimization at the index level.                  |
+------------------------+---------------------------------------------------+
| Using index            | Like 'Using index condition' but in addition we   |
| condition(BKA)         | use batch key access to retrieve rows.            |
+------------------------+---------------------------------------------------+
| Using index for        | The index is being used to resolve a GROUP BY or  |
| group-by               | DISTINCT query. The rows are not read.  This is   |
|                        | very efficient if the table has a lot of          |
|                        | identical index entries as duplicates are         |
|                        | quickly jumped over.                              |
+------------------------+---------------------------------------------------+
| Using intersect(...)   | For index_merge joins. Shows which index are      |
|                        | part of the intersect.                            |
+------------------------+---------------------------------------------------+
| Using join buffer      | We store previous row combinations in a row       |
|                        | buffer to be able to match each row against all   |
|                        | of the rows combinations in the join buffer at    |
|                        | one go.                                           |
+------------------------+---------------------------------------------------+
| Using sort_union(...)  | For index_merge joins. Shows which index are      |
|                        | part of the union.                                |
+------------------------+---------------------------------------------------+
| Using temporary        | A temporary table is created to hold the result.  |
|                        | This typically happens if you are using GROUP     |
|                        | BY, DISTINCT or ORDER BY.                         |
+------------------------+---------------------------------------------------+
| Using where            | A WHERE expression (in additional to the          |
|                        | possible key lookup) is used to check if the row  |
|                        | should be accepted. If you don't have 'Using      |
|                        | where' together with a join type of ALL, you are  |
|                        | probably doing something wrong!                   |
+------------------------+---------------------------------------------------+
| Using where with       | Like 'Using where' but the where condition is     |
| pushed condition       | pushed down to the table engine for internal      |
|                        | optimization at the row level.                    |
+------------------------+---------------------------------------------------+
| Using buffer           | The UPDATE statement will first buffer the rows,  |
|                        | and then run the updates, rather than do updates  |
|                        | on the fly. See Using Buffer UPDATE Algorithm     |
|                        | for a detailed explanation.                       |
+------------------------+---------------------------------------------------+

EXPLAIN EXTENDED
----------------

The EXTENDED keyword adds another column, filtered, to the output. This is a
percentage estimate of the table rows that will be filtered by the condition.

An EXPLAIN EXTENDED will always throw a warning, as it adds extra Message
information to a subsequent SHOW WARNINGS statement. This includes what the
SELECT query would look like after optimizing and rewriting rules are applied
and how the optimizer qualifies columns and tables.

Examples
--------

As synonym for DESCRIBE or SHOW COLUMNS FROM:

DESCRIBE city;
+------------+----------+------+-----+---------+----------------+
| Field      | Type     | Null | Key | Default | Extra          |
+------------+----------+------+-----+---------+----------------+
| Id         | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name       | char(35) | YES  |     | NULL    |                |
| Country    | char(3)  | NO   | UNI |         |                |
| District   | char(20) | YES  | MUL |         |                |
| Population | int(11)  | YES  |     | NULL    |                |
+------------+----------+------+-----+---------+----------------+

A simple set of examples to see how EXPLAIN can identify poor index usage:

CREATE TABLE IF NOT EXISTS `employees_example` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `first_name` varchar(30) NOT NULL,
 `last_name` varchar(40) NOT NULL,
 `position` varchar(25) NOT NULL,
 `home_address` varchar(50) NOT NULL,
 `home_phone` varchar(12) NOT NULL,
 `employee_code` varchar(25) NOT NULL,
 PRIMARY KEY (`id`),
 UNIQUE KEY `employee_code` (`employee_code`),
 KEY `first_name` (`first_name`,`last_name`)
) ENGINE=Aria;

INSERT INTO `employees_example` (`first_name`, `last_name`, `position`,
`home_address`, `home_phone`, `employee_code`)
 VALUES
 ('Mustapha', 'Mond', 'Chief Executive Officer', '692 Promiscuous Plaza',
'326-555-3492', 'MM1'),
 ('Henry', 'Foster', 'Store Manager', '314 Savage Circle', '326-555-3847',
'HF1'),
 ('Bernard', 'Marx', 'Cashier', '1240 Ambient Avenue', '326-555-8456', 'BM1'),
 ('Lenina', 'Crowne', 'Cashier', '281 Bumblepuppy Boulevard', '328-555-2349',
'LC1'),
 ('Fanny', 'Crowne', 'Restocker', '1023 Bokanovsky Lane', '326-555-6329',
'FC1'),
 ('Helmholtz', 'Watson', 'Janitor', '944 Soma Court', '329-555-2478', 'HW1');

SHOW INDEXES FROM employees_example;
+-------------------+------------+---------------+--------------+--------------
+-----------+-------------+----------+--------+------+------------+---------+--
------------+
| Table             | Non_unique | Key_name      | Seq_in_index | Column_name 
 | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
Index_comment |
+-------------------+------------+---------------+--------------+--------------
+-----------+-------------+----------+--------+------+------------+---------+--
------------+
| employees_example |          0 | PRIMARY       |            1 | id          
 | A         |           7 |     NULL | NULL   |      | BTREE      |         |
       |
| employees_example |          0 | employee_code |            1 |
employee_code | A         |           7 |     NULL | NULL   |      | BTREE    
 |         |               |
| employees_example |          1 | first_name    |            1 | first_name  
 | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
       |
| employees_example |          1 | first_name    |            2 | last_name   
 | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
       |
+-------------------+------------+---------------+--------------+--------------
+-----------+-------------+----------+--------+------+------------+---------+--
------------+

SELECT on a primary key:

EXPLAIN SELECT * FROM employees_example WHERE id=1;
+------+-------------+-------------------+-------+---------------+---------+---
-----+-------+------+-------+
| id   | select_type | table             | type  | possible_keys | key     |
key_len | ref   | rows | Extra |
+------+-------------+-------------------+-------+---------------+---------+---
-----+-------+------+-------+
|    1 | SIMPLE      | employees_example | const | PRIMARY       | PRIMARY | 4
   | const |    1 |       |
+------+-------------+-------------------+-------+---------------+---------+---
-----+-------+------+-------+

The type is const, which means that only one possible result could be
returned. Now, returning the same record but searching by their phone number:

EXPLAIN SELECT * FROM employees_example WHERE home_phone='326-555-3492';
+------+-------------+-------------------+------+---------------+------+-------
-+------+------+-------------+
| id   | select_type | table             | type | possible_keys | key  |
key_len | ref  | rows | Extra       |
+------+-------------+-------------------+------+---------------+------+-------
-+------+------+-------------+
|    1 | SIMPLE      | employees_example | ALL  | NULL          | NULL | NULL 
 | NULL |    6 | Using where |
+------+-------------+-------------------+------+---------------+------+-------
-+------+------+-------------+

Here, the type is All, which means no index could be used. Looking at the rows
count, a full table scan (all six rows) had to be performed in order to
retrieve the record. If it's a requirement to search by phone number, an index
will have to be created.

SHOW EXPLAIN example:

SHOW EXPLAIN FOR 1;
+------+-------------+-------+-------+---------------+------+---------+------+-
-------+-------------+
| id   | select_type | table | type  | possible_keys | key  | key_len | ref  |
rows    | Extra       |
+------+-------------+-------+-------+---------------+------+---------+------+-
-------+-------------+
|    1 | SIMPLE      | tbl   | index | NULL          | a    | 5       | NULL |
1000107 | Using index |
+------+-------------+-------+-------+---------------+------+---------+------+-
-------+-------------+
1 row in set, 1 warning (0.00 sec)

Example of ref_or_null Optimization
-----------------------------------

SELECT * FROM table_name
 WHERE key_column=expr OR key_column IS NULL;

ref_or_null is something that often happens when you use subqueries with NOT
IN as then one has to do an extra check for NULL values if the first value
didn't have a matching row.

URL: https://mariadb.com/kb/en/explain/