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• Obtaining Table Structure Information

• Obtaining Execution Plan Information

• Obtaining Information with EXPLAIN ANALYZE

DESCRIBE provides information about the columns in a table:

mysql> DESCRIBE City;+------------+----------+------+-----+---------+----------------+| Field      | Type     | Null | Key | Default | Extra          |+------------+----------+------+-----+---------+----------------+| Id         | int(11)  | NO   | PRI | NULL    | auto_increment || Name       | char(35) | NO   |     |         |                || Country    | char(3)  | NO   | UNI |         |                || District   | char(20) | YES  | MUL |         |                || Population | int(11)  | NO   |     | 0       |                |+------------+----------+------+-----+---------+----------------+

DESCRIBE is a shortcut forSHOW COLUMNS. These statements also display information for views. The description forSHOW COLUMNS provides more information about the output columns. SeeSection 15.7.7.5, “SHOW COLUMNS Statement”.

By default,DESCRIBE displays information about all columns in the table.col_name, if given, is the name of a column in the table. In this case, the statement displays information only for the named column.wild, if given, is a pattern string. It can contain the SQL% and_ wildcard characters. In this case, the statement displays output only for the columns with names matching the string. There is no need to enclose the string within quotation marks unless it contains spaces or other special characters.

TheDESCRIBE statement is provided for compatibility with Oracle.

TheSHOW CREATE TABLE,SHOW TABLE STATUS, andSHOW INDEX statements also provide information about tables. SeeSection 15.7.7, “SHOW Statements”.

Theexplain_format system variable, added in MySQL 8.0.32, has no effect on the output ofEXPLAIN when used to obtain information about table columns.

TheEXPLAIN statement provides information about how MySQL executes statements:

EXPLAIN requires the same privileges required to execute the explained statement. Additionally,EXPLAIN also requires theSHOW VIEW privilege for any explained view.EXPLAIN ... FOR CONNECTION also requires thePROCESS privilege if the specified connection belongs to a different user.

Theexplain_format system variable introduced in MySQL 8.0.32 determines the format of the output fromEXPLAIN when used to display a query execution plan. This variable can take any of the values used with theFORMAT option, with the addition ofDEFAULT as a synonym forTRADITIONAL. The following example uses thecountry table from theworld database which can be obtained fromMySQL: Other Downloads:

mysql> USE world; # Make world the current databaseDatabase changed

Checking the value ofexplain_format, we see that it has the default value, and thatEXPLAIN (with noFORMAT option) therefore uses the traditional tabular output:

mysql> SELECT @@explain_format;+------------------+| @@explain_format |+------------------+| TRADITIONAL      |+------------------+1 row in set (0.00 sec)mysql> EXPLAIN SELECT Name FROM country WHERE Code Like 'A%';+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+| id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+|  1 | SIMPLE      | country | NULL       | range | PRIMARY       | PRIMARY | 12      | NULL |   17 |   100.00 | Using where |+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+1 row in set, 1 warning (0.00 sec)

If we set the value ofexplain_format toTREE, then rerun the sameEXPLAIN statement, the output uses the tree-like format:

mysql> SET @@explain_format=TREE;Query OK, 0 rows affected (0.00 sec)mysql> SELECT @@explain_format;+------------------+| @@explain_format |+------------------+| TREE             |+------------------+1 row in set (0.00 sec)mysql> EXPLAIN SELECT Name FROM country WHERE Code LIKE 'A%';+--------------------------------------------------------------------------------------------------------------+| EXPLAIN                                                                                                      |+--------------------------------------------------------------------------------------------------------------+| -> Filter: (country.`Code` like 'A%')  (cost=3.67 rows=17)    -> Index range scan on country using PRIMARY over ('A' <= Code <= 'A????????')  (cost=3.67 rows=17)  |+--------------------------------------------------------------------------------------------------------------+1 row in set, 1 warning (0.00 sec)

As stated previously, theFORMAT option overrides this setting. Executing the sameEXPLAIN statement usingFORMAT=JSON instead ofFORMAT=TREE shows that this is the case:

mysql> EXPLAIN FORMAT=JSON SELECT Name FROM country WHERE Code LIKE 'A%';+------------------------------------------------------------------------------+| EXPLAIN                                                                      |+------------------------------------------------------------------------------+| {  "query_block": {    "select_id": 1,    "cost_info": {      "query_cost": "3.67"    },    "table": {      "table_name": "country",      "access_type": "range",      "possible_keys": [        "PRIMARY"      ],      "key": "PRIMARY",      "used_key_parts": [        "Code"      ],      "key_length": "12",      "rows_examined_per_scan": 17,      "rows_produced_per_join": 17,      "filtered": "100.00",      "cost_info": {        "read_cost": "1.97",        "eval_cost": "1.70",        "prefix_cost": "3.67",        "data_read_per_join": "16K"      },      "used_columns": [        "Code",        "Name"      ],      "attached_condition": "(`world`.`country`.`Code` like 'A%')"    }  }}                                                                              |+------------------------------------------------------------------------------+1 row in set, 1 warning (0.00 sec)

To return the default output ofEXPLAIN to the tabular format, setexplain_format toTRADITIONAL. Alternatively, you can set it toDEFAULT, which has the same effect, as shown here:

mysql> SET @@explain_format=DEFAULT;Query OK, 0 rows affected (0.00 sec)mysql> SELECT @@explain_format;+------------------+| @@explain_format |+------------------+| TRADITIONAL      |+------------------+1 row in set (0.00 sec)

With the help ofEXPLAIN, you can see where you should add indexes to tables so that the statement executes faster by using indexes to find rows. You can also useEXPLAIN to check whether the optimizer joins the tables in an optimal order. To give a hint to the optimizer to use a join order corresponding to the order in which the tables are named in aSELECT statement, begin the statement withSELECT STRAIGHT_JOIN rather than justSELECT. (SeeSection 15.2.13, “SELECT Statement”.)

The optimizer trace may sometimes provide information complementary to that ofEXPLAIN. However, the optimizer trace format and content are subject to change between versions. For details, seeSection 10.15, “Tracing the Optimizer”.

If you have a problem with indexes not being used when you believe that they should be, runANALYZE TABLE to update table statistics, such as cardinality of keys, that can affect the choices the optimizer makes. SeeSection 15.7.3.1, “ANALYZE TABLE Statement”.

MySQL 8.0.18 introducesEXPLAIN ANALYZE, which runs a statement and producesEXPLAIN output along with timing and additional, iterator-based, information about how the optimizer's expectations matched the actual execution. For each iterator, the following information is provided:

The query execution information is displayed using theTREE output format, in which nodes represent iterators.EXPLAIN ANALYZE always uses theTREE output format. In MySQL 8.0.21 and later, this can optionally be specified explicitly usingFORMAT=TREE; formats other thanTREE remain unsupported.

EXPLAIN ANALYZE can be used withSELECT statements, as well as with multi-tableUPDATE andDELETE statements. Beginning with MySQL 8.0.19, it can also be used withTABLE statements.

Beginning with MySQL 8.0.20, you can terminate this statement usingKILL QUERY orCTRL-C.

EXPLAIN ANALYZE cannot be used withFOR CONNECTION.

Example output:

mysql> EXPLAIN ANALYZE SELECT * FROM t1 JOIN t2 ON (t1.c1 = t2.c2)\G*************************** 1. row ***************************EXPLAIN: -> Inner hash join (t2.c2 = t1.c1)  (cost=4.70 rows=6)(actual time=0.032..0.035 rows=6 loops=1)    -> Table scan on t2  (cost=0.06 rows=6)(actual time=0.003..0.005 rows=6 loops=1)    -> Hash        -> Table scan on t1  (cost=0.85 rows=6)(actual time=0.018..0.022 rows=6 loops=1)mysql> EXPLAIN ANALYZE SELECT * FROM t3 WHERE i > 8\G*************************** 1. row ***************************EXPLAIN: -> Filter: (t3.i > 8)  (cost=1.75 rows=5)(actual time=0.019..0.021 rows=6 loops=1)    -> Table scan on t3  (cost=1.75 rows=15)(actual time=0.017..0.019 rows=15 loops=1)mysql> EXPLAIN ANALYZE SELECT * FROM t3 WHERE pk > 17\G*************************** 1. row ***************************EXPLAIN: -> Filter: (t3.pk > 17)  (cost=1.26 rows=5)(actual time=0.013..0.016 rows=5 loops=1)    -> Index range scan on t3 using PRIMARY  (cost=1.26 rows=5)(actual time=0.012..0.014 rows=5 loops=1)

The tables used in the example output were created by the statements shown here:

CREATE TABLE t1 (    c1 INTEGER DEFAULT NULL,    c2 INTEGER DEFAULT NULL);CREATE TABLE t2 (    c1 INTEGER DEFAULT NULL,    c2 INTEGER DEFAULT NULL);CREATE TABLE t3 (    pk INTEGER NOT NULL PRIMARY KEY,    i INTEGER DEFAULT NULL);

Values shown foractual time in the output of this statement are expressed in milliseconds.

As of MySQL 8.0.32, theexplain_format system variable has the following effects onEXPLAIN ANALYZE:

We illustrate the behavior described in the second point here, re-using the lastEXPLAIN ANALYZE statement from the previous example:

mysql> SET @@explain_format=JSON;Query OK, 0 rows affected (0.00 sec)mysql> SELECT @@explain_format;+------------------+| @@explain_format |+------------------+| JSON             |+------------------+1 row in set (0.00 sec)mysql> EXPLAIN ANALYZE SELECT * FROM t3 WHERE pk > 17\GERROR 1235 (42000): This version of MySQL doesn't yet support 'EXPLAIN ANALYZE with JSON format'mysql> EXPLAIN ANALYZE FORMAT=TRADITIONAL SELECT * FROM t3 WHERE pk > 17\GERROR 1235 (42000): This version of MySQL doesn't yet support 'EXPLAIN ANALYZE with TRADITIONAL format'mysql> EXPLAIN ANALYZE FORMAT=TREE SELECT * FROM t3 WHERE pk > 17\G*************************** 1. row ***************************EXPLAIN: -> Filter: (t3.pk > 17)  (cost=1.26 rows=5)(actual time=0.013..0.016 rows=5 loops=1)    -> Index range scan on t3 using PRIMARY  (cost=1.26 rows=5)(actual time=0.012..0.014 rows=5 loops=1)

UsingFORMAT=TRADITIONAL orFORMAT=JSON withEXPLAIN ANALYZE always raises an error, regardless of the value ofexplain_format.

Beginning with MySQL 8.0.33, numbers in the output ofEXPLAIN ANALYZE andEXPLAIN FORMAT=TREE are formatted according to the following rules: