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23.3.5 Obtaining Information About Partitions

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This section discusses obtaining information about existing partitions, which can be done in a number of ways. Methods of obtaining such information include the following:

As discussed elsewhere in this chapter, SHOW CREATE TABLE includes in its output the PARTITION BY clause used to create a partitioned table. For example:

  1. mysql> SHOW CREATE TABLE trb3\G
  2. *************************** 1. row ***************************
  3.        Table: trb3
  4. Create Table: CREATE TABLE `trb3` (
  5.   `id` int(11) DEFAULT NULL,
  6.   `name` varchar(50) DEFAULT NULL,
  7.   `purchased` date DEFAULT NULL
  8. ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
  9. /*!50100 PARTITION BY RANGE (YEAR(purchased))
  10. (PARTITION p0 VALUES LESS THAN (1990) ENGINE = InnoDB,
  11.  PARTITION p1 VALUES LESS THAN (1995) ENGINE = InnoDB,
  12.  PARTITION p2 VALUES LESS THAN (2000) ENGINE = InnoDB,
  13.  PARTITION p3 VALUES LESS THAN (2005) ENGINE = InnoDB) */
  14. 0 row in set (0.00 sec)

The output from SHOW TABLE STATUS for partitioned tables is the same as that for nonpartitioned tables, except that the Create_options column contains the string partitioned. The Engine column contains the name of the storage engine used by all partitions of the table. (See Section 13.7.6.36, “SHOW TABLE STATUS Syntax”, for more information about this statement.)

You can also obtain information about partitions from INFORMATION_SCHEMA, which contains a PARTITIONS table. See Section 25.16, “The INFORMATION_SCHEMA PARTITIONS Table”.

It is possible to determine which partitions of a partitioned table are involved in a given SELECT query using EXPLAIN. The partitions column in the EXPLAIN output lists the partitions from which records would be matched by the query.

Suppose that a table trb1 is created and populated as follows:

  1. CREATE TABLE trb1 (id INT, name VARCHAR(50), purchased DATE)
  2.     PARTITION BY RANGE(id)
  3.     (
  4.         PARTITION p0 VALUES LESS THAN (3),
  5.         PARTITION p1 VALUES LESS THAN (7),
  6.         PARTITION p2 VALUES LESS THAN (9),
  7.         PARTITION p3 VALUES LESS THAN (11)
  8.     );
  9.  
  10. INSERT INTO trb1 VALUES
  11.     (1, 'desk organiser', '2003-10-15'),
  12.     (2, 'CD player', '1993-11-05'),
  13.     (3, 'TV set', '1996-03-10'),
  14.     (4, 'bookcase', '1982-01-10'),
  15.     (5, 'exercise bike', '2004-05-09'),
  16.     (6, 'sofa', '1987-06-05'),
  17.     (7, 'popcorn maker', '2001-11-22'),
  18.     (8, 'aquarium', '1992-08-04'),
  19.     (9, 'study desk', '1984-09-16'),
  20.     (10, 'lava lamp', '1998-12-25');

You can see which partitions are used in a query such as SELECT * FROM trb1;, as shown here:

  1. mysql> EXPLAIN SELECT * FROM trb1\G
  2. *************************** 1. row ***************************
  3.            id: 1
  4.   select_type: SIMPLE
  5.         table: trb1
  6.    partitions: p0,p1,p2,p3
  7.          type: ALL
  8. possible_keys: NULL
  9.           key: NULL
  10.       key_len: NULL
  11.           ref: NULL
  12.          rows: 10
  13.         Extra: Using filesort

In this case, all four partitions are searched. However, when a limiting condition making use of the partitioning key is added to the query, you can see that only those partitions containing matching values are scanned, as shown here:

  1. mysql> EXPLAIN SELECT * FROM trb1 WHERE id < 5\G
  2. *************************** 1. row ***************************
  3.            id: 1
  4.   select_type: SIMPLE
  5.         table: trb1
  6.    partitions: p0,p1
  7.          type: ALL
  8. possible_keys: NULL
  9.           key: NULL
  10.       key_len: NULL
  11.           ref: NULL
  12.          rows: 10
  13.         Extra: Using where

EXPLAIN also provides information about keys used and possible keys:

  1. mysql> ALTER TABLE trb1 ADD PRIMARY KEY (id);
  2. Query OK, 10 rows affected (0.03 sec)
  3. Records: 10  Duplicates: 0  Warnings: 0
  4.  
  5. mysql> EXPLAIN SELECT * FROM trb1 WHERE id < 5\G
  6. *************************** 1. row ***************************
  7.            id: 1
  8.   select_type: SIMPLE
  9.         table: trb1
  10.    partitions: p0,p1
  11.          type: range
  12. possible_keys: PRIMARY
  13.           key: PRIMARY
  14.       key_len: 4
  15.           ref: NULL
  16.          rows: 7
  17.         Extra: Using where

If EXPLAIN is used to examine a query against a nonpartitioned table, no error is produced, but the value of the partitions column is always NULL.

The rows column of EXPLAIN output displays the total number of rows in the table.

See also Section 13.8.2, “EXPLAIN Syntax”.


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