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23.3.5 Obtaining Information About Partitions
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:
- *************************** 1. row ***************************
- Table: trb3
- /*!50100 PARTITION BY RANGE (YEAR(purchased))
- (PARTITION p0 VALUES LESS THAN (1990) ENGINE = InnoDB,
- PARTITION p1 VALUES LESS THAN (1995) ENGINE = InnoDB,
- PARTITION p2 VALUES LESS THAN (2000) ENGINE = InnoDB,
- PARTITION p3 VALUES LESS THAN (2005) ENGINE = InnoDB) */
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, '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:
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:
EXPLAIN
also provides
information about keys used and possible keys:
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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Document créé le 26/06/2006, dernière modification le 26/10/2018
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