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
.
From MySQL 8.0.16, when insertions, deletions, or updates are made to partitioned tables, the binary log records information about the partition and (if any) the subpartition in which the row event took place. A new row event is created for a modification that takes place in a different partition or subpartition, even if the table involved is the same. So if a transaction involves three partitions or subpartitions, three row events are generated. For an update event, the partition information is recorded for both the“before”image and the“after”image. The partition information is displayed if you specify the-v
or--verbose
option when viewing the binary log usingmysqlbinlog. Partition information is only recorded when row-based logging is in use (binlog_format=ROW
).
As discussed elsewhere in this chapter,SHOW CREATE TABLE
includes in its output thePARTITION BY
clause used to create a partitioned table. For example:
mysql> 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=InnoDB DEFAULT CHARSET=utf8mb4 /*!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) */ 0 row in set (0.00 sec)
The output fromSHOW TABLE STATUS
for partitioned tables is the same as that for nonpartitioned tables, except that theCreate_options
column contains the stringpartitioned
. TheEngine
column contains the name of the storage engine used by all partitions of the table. (SeeSection 13.7.7.38, “SHOW TABLE STATUS Statement”, for more information about this statement.)
You can also obtain information about partitions fromINFORMATION_SCHEMA
, which contains aPARTITIONS
table. SeeSection 26.3.21, “The INFORMATION_SCHEMA PARTITIONS Table”.
It is possible to determine which partitions of a partitioned table are involved in a givenSELECT
query usingEXPLAIN
. Thepartitions
column in theEXPLAIN
output lists the partitions from which records would be matched by the query.
Suppose that a tabletrb1
is created and populated as follows:
CREATE 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 asSELECT * FROM trb1;
, as shown here:
mysql >解释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:
mysql> 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:
mysql> 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
IfEXPLAIN
is used to examine a query against a nonpartitioned table, no error is produced, but the value of thepartitions
column is alwaysNULL
.
Therows
column ofEXPLAIN
output displays the total number of rows in the table.
See alsoSection 13.8.2, “EXPLAIN Statement”.