MySQL :: MySQL 8.4 Reference Manual :: 15.8.2 EXPLAIN Statement (original) (raw)

The DESCRIBE andEXPLAIN statements are synonyms. In practice, the DESCRIBE keyword is more often used to obtain information about table structure, whereas EXPLAIN is used to obtain a query execution plan (that is, an explanation of how MySQL would execute a query).

The following discussion uses theDESCRIBE andEXPLAIN keywords in accordance with those uses, but the MySQL parser treats them as completely synonymous.

Obtaining Table Structure Information

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 15.7.7.6, “SHOW COLUMNS Statement”.

By default, DESCRIBE displays information about all columns in the table.colname, 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.

The DESCRIBE statement is provided for compatibility with Oracle.

The SHOW CREATE TABLE,SHOW TABLE STATUS, andSHOW INDEX statements also provide information about tables. See Section 15.7.7, “SHOW Statements”.

The explain_format system variable has no effect on the output ofEXPLAIN when used to obtain information about table columns.

Obtaining Execution Plan Information

The EXPLAIN statement provides information about how MySQL executes statements:

mysql> EXPLAIN FORMAT=JSON INTO @myselect  
    ->     SELECT name FROM a WHERE id = 2;  
Query OK, 0 rows affected (0.00 sec)  
mysql> SELECT @myselect\G  
*************************** 1. row ***************************  
@myex: {  
  "query_block": {  
    "select_id": 1,  
    "cost_info": {  
      "query_cost": "1.00"  
    },  
    "table": {  
      "table_name": "a",  
      "access_type": "const",  
      "possible_keys": [  
        "PRIMARY"  
      ],  
      "key": "PRIMARY",  
      "used_key_parts": [  
        "id"  
      ],  
      "key_length": "4",  
      "ref": [  
        "const"  
      ],  
      "rows_examined_per_scan": 1,  
      "rows_produced_per_join": 1,  
      "filtered": "100.00",  
      "cost_info": {  
        "read_cost": "0.00",  
        "eval_cost": "0.10",  
        "prefix_cost": "0.00",  
        "data_read_per_join": "408"  
      },  
      "used_columns": [  
        "id",  
        "name"  
      ]  
    }  
  }  
}  
1 row in set (0.00 sec)

This works with any explainable statement (SELECT,TABLE,INSERT,UPDATE,REPLACE, orDELETE). Examples usingUPDATE and DELETE statements are shown here: 

mysql> EXPLAIN FORMAT=JSON INTO @myupdate  
    ->   UPDATE a SET name2 = "garcia" WHERE id = 3;  
Query OK, 0 rows affected (0.00 sec)  
mysql> EXPLAIN FORMAT=JSON INTO @mydelete  
    ->     DELETE FROM a WHERE name1 LIKE '%e%';  
Query OK, 0 rows affected (0.00 sec)  
mysql> SELECT @myupdate, @mydelete\G  
*************************** 1. row ***************************  
@myupdate: {  
  "query_block": {  
    "select_id": 1,  
    "table": {  
      "update": true,  
      "table_name": "a",  
      "access_type": "range",  
      "possible_keys": [  
        "PRIMARY"  
      ],  
      "key": "PRIMARY",  
      "used_key_parts": [  
        "id"  
      ],  
      "key_length": "4",  
      "ref": [  
        "const"  
      ],  
      "rows_examined_per_scan": 1,  
      "filtered": "100.00",  
      "attached_condition": "(`db`.`a`.`id` = 3)"  
    }  
  }  
}  
@mydelete: {  
  "query_block": {  
    "select_id": 1,  
    "table": {  
      "delete": true,  
      "table_name": "a",  
      "access_type": "ALL",  
      "rows_examined_per_scan": 2,  
      "filtered": "100.00",  
      "attached_condition": "(`db`.`a`.`name1` like '%e%')"  
    }  
  }  
}  
1 row in set (0.00 sec)

You can work with this value using MySQL JSON functions as you would with any other JSON value, as in these examples using JSON_EXTRACT(): 

mysql> SELECT JSON_EXTRACT(@myselect, "$.query_block.table.key");  
+----------------------------------------------------+  
| JSON_EXTRACT(@myselect, "$.query_block.table.key") |  
+----------------------------------------------------+  
| "PRIMARY"                                          |  
+----------------------------------------------------+  
1 row in set (0.01 sec)  
mysql> SELECT JSON_EXTRACT(@myupdate, "$.query_block.table.access_type") AS U_acc,  
    ->        JSON_EXTRACT(@mydelete, "$.query_block.table.access_type") AS D_acc;  
+---------+-------+  
| U_acc   | D_acc |  
+---------+-------+  
| "range" | "ALL" |  
+---------+-------+  
1 row in set (0.00 sec)

For complex statements, the JSON output can be quite large; in particular, it can be difficult when reading it to pair the closing bracket and opening brackets; to cause the JSON structure's key, if it has one, to be repeated near the closing bracket, setend_markers_in_json=ON. You should be aware that while this makes the output easier to read, it also renders the JSON invalid, causing JSON functions to raise an error.
See also Section 14.17, “JSON Functions”.
Trying to use an INTO clause without explicitly including FORMAT=JSON causesEXPLAIN to be rejected withER_EXPLAIN_INTO_IMPLICIT_FORMAT_NOT_SUPPORTED. This is true regardless of the current value of theexplain_format system variable.
The INTO clause is not supported withFOR CONNECTION.
INTO is also not supported withEXPLAIN ANALYZE whenexplain_json_format_version=1.
Important
If, for any reason, the statement to be analyzed is rejected, the user variable is not updated.

mysql> USE b;  
Database changed  
mysql> CREATE SCHEMA s1;  
Query OK, 1 row affected (0.01 sec)  
mysql> CREATE SCHEMA s2;  
Query OK, 1 row affected (0.01 sec)  
mysql> USE s1;  
Database changed  
mysql> CREATE TABLE t (c1 INT NOT NULL AUTO_INCREMENT PRIMARY KEY, c2 INT NOT NULL);  
Query OK, 0 rows affected (0.04 sec)  
mysql> USE s2;  
Database changed  
mysql> CREATE TABLE t (c1 INT NOT NULL AUTO_INCREMENT PRIMARY KEY, c2 INT NOT NULL, KEY i1 (c2));  
Query OK, 0 rows affected (0.04 sec)  
mysql> USE b;  
Database changed  
mysql> EXPLAIN FORMAT=TREE FOR SCHEMA s1 SELECT * FROM t WHERE c2 > 50\G  
*************************** 1. row ***************************  
EXPLAIN: -> Filter: (t.c2 > 50)  (cost=0.35 rows=1)  
    -> Table scan on t  (cost=0.35 rows=1)  
1 row in set (0.00 sec)  
mysql> EXPLAIN FORMAT=TREE FOR SCHEMA s2 SELECT * FROM t WHERE c2 > 50\G  
*************************** 1. row ***************************  
EXPLAIN: -> Filter: (t.c2 > 50)  (cost=0.35 rows=1)  
    -> Covering index scan on t using i1  (cost=0.35 rows=1)  
1 row in set (0.00 sec)

If the database does not exist, the statement is rejected with ER_BAD_DB_ERROR. If the user does not have the necessary privileges to run the statement, it is rejected withER_DBACCESS_DENIED_ERROR.
FOR SCHEMA is not compatible withFOR CONNECTION.

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

The explain_format system variable 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 the FORMAT 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 database
Database changed

Checking the value of explain_format, we see that it has the default value, and thatEXPLAIN (with no FORMAT 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 of explain_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, the FORMAT 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 of EXPLAIN to the tabular format, set explain_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)

MySQL 8.4 supports two versions of the JSON output format. Version 1 is the linear format always used in MySQL 8.2 and earlier; this remains the default in MySQL 8.4, and is used in the examples already shown in this section. Version 2 of the JSON output format is based on access paths, and is intended to provide compatibility with future versions of the MySQL Optimizer. You can switch to the Version 2 format by setting the value of theexplain_json_format_version server system variable to 2, as shown here for the same EXPLAIN statement used in the previous example:

mysql> SELECT @@explain_json_format_version;
+-------------------------------+
| @@explain_json_format_version |
+-------------------------------+
|                             1 |
+-------------------------------+
1 row in set (0.00 sec)

mysql> SET @@explain_json_format_version = 2;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @@explain_json_format_version;
+-------------------------------+
| @@explain_json_format_version |
+-------------------------------+
|                             2 |
+-------------------------------+
1 row in set (0.00 sec)

mysql> EXPLAIN FORMAT=JSON SELECT Name FROM country WHERE Code LIKE 'A%';
+------------------------------------------------------------------------------+
| EXPLAIN                                                                      |
+------------------------------------------------------------------------------+
| {
  "query": "/* select#1 */ select `world`.`country`.`Name` AS `Name` from `world`.`country` where (`world`.`country`.`Code` like 'A%')",
  "inputs": [
    {
      "ranges": [
        "('A' <= Code <= 'A????????')"
      ],
      "covering": false,
      "operation": "Index range scan on country using PRIMARY over ('A' <= Code < 'A????????')",
      "index_name": "PRIMARY",
      "table_name": "country",
      "access_type": "index",
      "estimated_rows": 17.0,
      "index_access_type": "index_range_scan",
      "estimated_total_cost": 3.668778400708174
    }
  ],
  "condition": "(country.`Code` like 'A%')",
  "operation": "Filter: (country.`Code` like 'A%')",
  "access_type": "filter",
  "estimated_rows": 17.0,
  "estimated_total_cost": 3.668778400708174
}                                                                              |
+------------------------------------------------------------------------------+

After using the Version 2 format, you can cause the JSON output from all subsequent EXPLAIN FORMAT=JSON statements to revert to the Version 1 format by settingexplain_json_format_version back to1 (the default).

The value of explain_json_format_version determines the version of the JSON output format employed by allEXPLAIN statements which use it, whether the JSON format is used because a given EXPLAIN statement includes an explicit FORMAT=JSON option, or because the JSON format is used automatically due to the explain_format system variable being set to JSON.

With the help of EXPLAIN, 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 with SELECT STRAIGHT_JOIN rather than just SELECT. (SeeSection 15.2.13, “SELECT Statement”.)

The optimizer trace may sometimes provide information complementary to that of EXPLAIN. 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, run ANALYZE 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”.

Obtaining Information with EXPLAIN ANALYZE

EXPLAIN ANALYZE 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. This can optionally be specified explicitly using FORMAT=TREE; formats other than TREE remain unsupported.

EXPLAIN ANALYZE can be used withSELECT statements, multi-tableUPDATE andDELETE statements, andTABLE statements.

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=3.5 rows=5) 
(actual time=0.121..0.131 rows=1 loops=1) 
    -> Table scan on t2  (cost=0.07 rows=5) 
(actual time=0.0126..0.0221 rows=5 loops=1) 
    -> Hash
        -> Table scan on t1  (cost=0.75 rows=5) 
(actual time=0.0372..0.0534 rows=5 loops=1)

mysql> EXPLAIN ANALYZE SELECT * FROM t3 WHERE i > 8\G
*************************** 1. row ***************************
EXPLAIN: -> Filter: (t3.i > 8)  (cost=0.75 rows=1.67) 
(actual time=0.0168..0.0182 rows=1 loops=1)
    -> Table scan on t3  (cost=0.75 rows=5) 
(actual time=0.015..0.0167 rows=5 loops=1)

mysql> EXPLAIN ANALYZE FORMAT=TREE SELECT * FROM t3 WHERE pk > 17\G
*************************** 1. row ***************************
EXPLAIN: -> Filter: (t3.pk > 17)  (cost=0.91 rows=2) 
(actual time=0.0334..0.042 rows=2 loops=1)
    -> Index range scan on t3 using PRIMARY over (17 < pk)  
(cost=0.91 rows=2) (actual time=0.0306..0.0384 rows=2 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 for actual time in the output of this statement are expressed in milliseconds.

explain_format has the following effects on EXPLAIN ANALYZE:

We illustrate the behavior described in the second point here, re-using the last EXPLAIN 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\G
ERROR 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\G
ERROR 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)

Using FORMAT=TRADITIONAL orFORMAT=JSON with EXPLAIN ANALYZE always raises an error, regardless of the value of explain_format.

In MySQL 8.4, numbers in the output ofEXPLAIN ANALYZE and EXPLAIN FORMAT=TREE are formatted according to the following rules: