15.8.2 EXPLAIN Statement

{EXPLAIN | DESCRIBE | DESC}
    tbl_name [col_name | wild]

{EXPLAIN | DESCRIBE | DESC}
    [explain_type]
    {explainable_stmt | FOR CONNECTION connection_id}

{EXPLAIN | DESCRIBE | DESC} ANALYZE [explain_type] select_stmt

explain_type: {
    FORMAT = format_name
}

format_name: {
    TRADITIONAL
  | JSON
  | TREE
}

explainable_stmt: {
    select_stmt
  | TABLE ...
  | DELETE ...
  | INSERT ...
  | REPLACE ...
  | UPDATE ...
}

select_stmt:
    SELECT ...

The DESCRIBE and EXPLAIN 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 the DESCRIBE and EXPLAIN keywords in accordance with those uses, but the MySQL parser treats them as completely synonymous.

• Obtaining Table Structure Information

• Obtaining Execution Plan Information

• Obtaining Information with EXPLAIN ANALYZE

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 for SHOW COLUMNS. These statements also display information for views. The description for SHOW COLUMNS provides more information about the output columns. See Section 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.

The DESCRIBE statement is provided for compatibility with Oracle.

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

The explain_format system variable, added in MySQL 8.0.32, has no effect on the output of EXPLAIN when used to obtain information about table columns.

Obtaining Execution Plan Information

The EXPLAIN statement provides information about how MySQL executes statements:

• EXPLAIN works with SELECT, DELETE, INSERT, REPLACE, and UPDATE statements. In MySQL 8.0.19 and later, it also works with TABLE statements.

• When EXPLAIN is used with an explainable statement, MySQL displays information from the optimizer about the statement execution plan. That is, MySQL explains how it would process the statement, including information about how tables are joined and in which order. For information about using EXPLAIN to obtain execution plan information, see Section 10.8.2, “EXPLAIN Output Format”.

• When EXPLAIN is used with FOR CONNECTION connection_id rather than an explainable statement, it displays the execution plan for the statement executing in the named connection. See Section 10.8.4, “Obtaining Execution Plan Information for a Named Connection”.

• For explainable statements, EXPLAIN produces additional execution plan information that can be displayed using SHOW WARNINGS. See Section 10.8.3, “Extended EXPLAIN Output Format”.

• EXPLAIN is useful for examining queries involving partitioned tables. See Section 26.3.5, “Obtaining Information About Partitions”.

• The FORMAT option can be used to select the output format. TRADITIONAL presents the output in tabular format. This is the default if no FORMAT option is present. JSON format displays the information in JSON format. In MySQL 8.0.16 and later, TREE provides tree-like output with more precise descriptions of query handling than the TRADITIONAL format; it is the only format which shows hash join usage (see Section 10.2.1.4, “Hash Join Optimization”) and is always used for EXPLAIN ANALYZE.

  As of MySQL 8.0.32, the default output format used by EXPLAIN (that is, when it has no FORMAT option) is determined by the value of the explain_format system variable. The precise effects of this variable are described later in this section.

  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, set end_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.

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 the PROCESS privilege if the specified connection belongs to a different user.

The explain_format system variable introduced in MySQL 8.0.32 determines the format of the output from EXPLAIN when used to display a query execution plan. This variable can take any of the values used with the FORMAT option, with the addition of DEFAULT as a synonym for TRADITIONAL. The following example uses the country table from the world database which can be obtained from MySQL: 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 that EXPLAIN (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 to TREE, then rerun the same EXPLAIN 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 same EXPLAIN statement using FORMAT=JSON instead of FORMAT=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 to TRADITIONAL. Alternatively, you can set it to DEFAULT, 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 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 use EXPLAIN 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 a SELECT statement, begin the statement with SELECT STRAIGHT_JOIN rather than just SELECT. (See Section 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, see Section 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. See Section 15.7.3.1, “ANALYZE TABLE Statement”.

Note

MySQL Workbench has a Visual Explain capability that provides a visual representation of EXPLAIN output. See Tutorial: Using Explain to Improve Query Performance.

Obtaining Information with EXPLAIN ANALYZE

MySQL 8.0.18 introduces EXPLAIN ANALYZE, which runs a statement and produces EXPLAIN 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:

• Estimated execution cost

  (Some iterators are not accounted for by the cost model, and so are not included in the estimate.)

• Estimated number of returned rows

• Time to return first row

• Time spent executing this iterator (including child iterators, but not parent iterators), in milliseconds.

  (When there are multiple loops, this figure shows the average time per loop.)

• Number of rows returned by the iterator

• Number of loops

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

EXPLAIN ANALYZE can be used with SELECT statements, as well as with multi-table UPDATE and DELETE statements. Beginning with MySQL 8.0.19, it can also be used with TABLE statements.

Beginning with MySQL 8.0.20, you can terminate this statement using KILL QUERY or CTRL-C.

EXPLAIN ANALYZE cannot be used with FOR 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 for actual time in the output of this statement are expressed in milliseconds.

As of MySQL 8.0.32, the explain_format system variable has the following effects on EXPLAIN ANALYZE:

• If the value of this variable is TRADITIONAL or TREE (or the synonym DEFAULT), EXPLAIN ANALYZE uses the TREE format. This ensures that this statement continues to use the TREE format by default, as it did prior to the introduction of explain_format.

• If the value of explain_format is JSON, EXPLAIN ANALYZE returns an error unless FORMAT=TREE is specified as part of the statement. This is due to the fact that EXPLAIN ANALYZE supports only the TREE output format.

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

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

• Numbers in the range 0.001-999999.5 are printed as decimal numbers.

  Decimal numbers less than 1000 have three significant digits; the remainder have four, five, or six.

• Numbers outside the range 0.001-999999.5 are printed in engineering format. Examples of such values are 1.23e+9 and 934e-6.

• No trailing zeros are printed. For example, we print 2.3 rather than 2.30, and 1.2e+6 rather than 1.20e+6.

• Numbers less than 1e-12 are printed as 0.