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:
Using the
SHOW CREATE TABLE
statement to view the partitioning clauses used in creating a partitioned table.Using the
SHOW TABLE STATUS
statement to determine whether a table is partitioned.Querying the Information Schema
PARTITIONS
table.Using the statement
EXPLAIN SELECT
to see which partitions are used by a givenSELECT
.
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:
Press CTRL+C to copymysql> SHOW CREATE TABLE trb3\G *************************** 1. row *************************** Table: trb3 Create Table: CREATE TABLE `trb3` ( `id` int(11) default NULL, `name` varchar(50) default NULL, `purchased` date default NULL ) ENGINE=MyISAM DEFAULT CHARSET=latin1 PARTITION BY RANGE (YEAR(purchased)) ( PARTITION p0 VALUES LESS THAN (1990) ENGINE = MyISAM, PARTITION p1 VALUES LESS THAN (1995) ENGINE = MyISAM, PARTITION p2 VALUES LESS THAN (2000) ENGINE = MyISAM, PARTITION p3 VALUES LESS THAN (2005) ENGINE = MyISAM ) 1 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 13.7.5.36, “SHOW TABLE STATUS Statement”, for more information about
this statement.)
You can also obtain information about partitions from
INFORMATION_SCHEMA
, which contains a
PARTITIONS
table. See
Section 24.3.16, “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 you have a table trb1
created
and populated as follows:
Press CTRL+C to copyCREATE TABLE trb1 (id INT, name VARCHAR(50), purchased DATE) PARTITION BY RANGE(id) ( PARTITION p0 VALUES LESS THAN (3), PARTITION p1 VALUES LESS THAN (7), PARTITION p2 VALUES LESS THAN (9), PARTITION p3 VALUES LESS THAN (11) ); INSERT INTO trb1 VALUES (1, 'desk organiser', '2003-10-15'), (2, 'CD player', '1993-11-05'), (3, 'TV set', '1996-03-10'), (4, 'bookcase', '1982-01-10'), (5, 'exercise bike', '2004-05-09'), (6, 'sofa', '1987-06-05'), (7, 'popcorn maker', '2001-11-22'), (8, 'aquarium', '1992-08-04'), (9, 'study desk', '1984-09-16'), (10, 'lava lamp', '1998-12-25');
You can see which partitions are used in a query such as
SELECT * FROM trb1;
, as shown here:
Press CTRL+C to copymysql> EXPLAIN SELECT * FROM trb1\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: trb1 partitions: p0,p1,p2,p3 type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 10 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:
Press CTRL+C to copymysql> EXPLAIN SELECT * FROM trb1 WHERE id < 5\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: trb1 partitions: p0,p1 type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 10 Extra: Using where
EXPLAIN
also provides
information about keys used and possible keys:
Press CTRL+C to copymysql> ALTER TABLE trb1 ADD PRIMARY KEY (id); Query OK, 10 rows affected (0.03 sec) Records: 10 Duplicates: 0 Warnings: 0 mysql> EXPLAIN SELECT * FROM trb1 WHERE id < 5\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: trb1 partitions: p0,p1 type: range possible_keys: PRIMARY key: PRIMARY key_len: 4 ref: NULL rows: 7 Extra: Using where
If EXPLAIN
PARTITIONS
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 Statement”.