MySQL分区表对NULL值的处理

fifalv
发布于 2023-1-18 11:23
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1.概述

MySQL的分区表没有禁止NULL值作为分区表达式的值,无论它是列值还是用户提供的表达式的值,需要记住NULL值不是数字。MySQL的分区实现中将NULL视为小于任何非NULL值,与order by类似。

2.range分区表处理NULL

1.创建range分区表

CREATE TABLE t_range (
c1 INT,
c2 VARCHAR(20)
)
PARTITION BY RANGE(c1) (
  PARTITION p0 VALUES LESS THAN (0),
  PARTITION p1 VALUES LESS THAN (10),
  PARTITION p2 VALUES LESS THAN MAXVALUE
);

2.插入2条分区列为null值的数据

insert into t_range values (NULL,'a'),(NULL,'b');

3.查看数据的分布情况

mysql> SELECT TABLE_NAME, PARTITION_NAME, TABLE_ROWS, AVG_ROW_LENGTH, DATA_LENGTH
FROM INFORMATION_SCHEMA.PARTITIONS
WHERE TABLE_SCHEMA = 'test1' AND TABLE_NAME = 't_range';
+------------+----------------+------------+----------------+-------------+
| TABLE_NAME | PARTITION_NAME | TABLE_ROWS | AVG_ROW_LENGTH | DATA_LENGTH |
+------------+----------------+------------+----------------+-------------+
| t_range    | p0             |          2 |           8192 |       16384 |
| t_range    | p1             |          0 |              0 |       16384 |
| t_range    | p2             |          0 |              0 |       16384 |
+------------+----------------+------------+----------------+-------------+
3 rows in set (0.01 sec)
mysql> select * from t_range partition(p0);
+------+------+
| c1   | c2   |
+------+------+
| NULL | a    |
| NULL | b    |
+------+------+
2 rows in set (0.00 sec)

可以看到分区列包含null值的2条数据都分布在p0分区上。

3.list分区表处理NULL

1.创建2张list分区表,t_list1分区列包含null值,t_list2分区列中不包含null值

CREATE TABLE t_list1 (
c1 INT,
c2 VARCHAR(20)
)
PARTITION BY LIST(c1) (
    PARTITION p0 VALUES IN (0, 3, 6),
    PARTITION p1 VALUES IN (1, 4, 7),
    PARTITION p2 VALUES IN (2, 5, 8),
    PARTITION p3 VALUES IN (NULL)
);

CREATE TABLE t_list2 (
c1 INT,
c2 VARCHAR(20)
)
PARTITION BY LIST(c1) (
    PARTITION p0 VALUES IN (0, 3, 6),
    PARTITION p1 VALUES IN (1, 4, 7),
    PARTITION p2 VALUES IN (2, 5, 8)
);

2.分别向2张表中插入2条分区列为null值的数据

mysql> insert into t_list1 values (NULL,'a'),(NULL,'b');
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0
mysql> insert into t_list2 values (NULL,'a'),(NULL,'b');
ERROR 1526 (HY000): Table has no partition for value NULL

可以看到 t_list2 表的分区列中不包含null值,所以数据插入失败。

3.查看数据的分布情况

mysql> SELECT TABLE_NAME, PARTITION_NAME, TABLE_ROWS, AVG_ROW_LENGTH, DATA_LENGTH
FROM INFORMATION_SCHEMA.PARTITIONS
WHERE TABLE_SCHEMA = 'test1' AND TABLE_NAME = 't_list1';
+------------+----------------+------------+----------------+-------------+
| TABLE_NAME | PARTITION_NAME | TABLE_ROWS | AVG_ROW_LENGTH | DATA_LENGTH |
+------------+----------------+------------+----------------+-------------+
| t_list1    | p0             |          0 |              0 |       16384 |
| t_list1    | p1             |          0 |              0 |       16384 |
| t_list1    | p2             |          0 |              0 |       16384 |
| t_list1    | p3             |          2 |           8192 |       16384 |
+------------+----------------+------------+----------------+-------------+
4 rows in set (0.00 sec)

可以看到 t_list1 表中插入的2条包含null值的数据,由于p3分区包含null值列,所以2条数据分布在p3分区中。

4.hash/key分区表处理NULL

1.创建2张测试表,一张hash分区表,一张key分区表

CREATE TABLE t_hash (
c1 INT,
c2 VARCHAR(20)
)
PARTITION BY HASH(c1)
PARTITIONS 2;

CREATE TABLE t_key (
c1 INT,
c2 VARCHAR(20)
)
PARTITION BY key(c1)
PARTITIONS 2;

2.分别向2张表中插入3条数据

mysql> insert into t_hash values (NULL,'a'),(0,'b'),(1,'c');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0
mysql> insert into t_key values (NULL,'a'),(0,'b'),(1,'c');
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

3.查看数据的分布情况

mysql> SELECT TABLE_NAME,PARTITION_NAME,TABLE_ROWS,AVG_ROW_LENGTH,DATA_LENGTH 
FROM INFORMATION_SCHEMA.PARTITIONS 
WHERE TABLE_SCHEMA = 'test1' AND TABLE_NAME in ('t_hash','t_key');
+------------+----------------+------------+----------------+-------------+
| TABLE_NAME | PARTITION_NAME | TABLE_ROWS | AVG_ROW_LENGTH | DATA_LENGTH |
+------------+----------------+------------+----------------+-------------+
| t_hash     | p0             |          2 |           8192 |       16384 |
| t_hash     | p1             |          1 |          16384 |       16384 |
| t_key      | p0             |          2 |           8192 |       16384 |
| t_key      | p1             |          1 |          16384 |       16384 |
+------------+----------------+------------+----------------+-------------+
4 rows in set (0.00 sec)
mysql> select * from t_hash partition(p0);
+------+------+
| c1   | c2   |
+------+------+
| NULL | a    |
|    0 | b    |
+------+------+
2 rows in set (0.00 sec)
mysql> select * from t_key partition(p0);
+------+------+
| c1   | c2   |
+------+------+
| NULL | a    |
|    1 | c    |
+------+------+
2 rows in set (0.00 sec)

可以看到分区列中包含null值的记录都在p0分区。

4.如果我们增加hash/key分区表的分区数,分区列为null值的记录会分布到其他分区

# 创建hash/key分区表,分区数为3
CREATE TABLE t_hash1 (
c1 INT,
c2 VARCHAR(20)
)
PARTITION BY HASH(c1)
PARTITIONS 3;

CREATE TABLE t_key1 (
c1 INT,
c2 VARCHAR(20)
)
PARTITION BY key(c1)
PARTITIONS 3;

# 插入数据
insert into t_hash1 values (NULL,'a'),(0,'b'),(1,'c');
insert into t_key1 values (NULL,'a'),(0,'b'),(1,'c');

# 查看数据的分布情况
mysql> SELECT TABLE_NAME,PARTITION_NAME,TABLE_ROWS,AVG_ROW_LENGTH,DATA_LENGTH 
FROM INFORMATION_SCHEMA.PARTITIONS 
WHERE TABLE_SCHEMA = 'test1' AND TABLE_NAME in ('t_hash1','t_key1');
+------------+----------------+------------+----------------+-------------+
| TABLE_NAME | PARTITION_NAME | TABLE_ROWS | AVG_ROW_LENGTH | DATA_LENGTH |
+------------+----------------+------------+----------------+-------------+
| t_hash1    | p0             |          1 |          16384 |       16384 |
| t_hash1    | p1             |          1 |          16384 |       16384 |
| t_hash1    | p2             |          1 |          16384 |       16384 |
| t_key1     | p0             |          0 |              0 |       16384 |
| t_key1     | p1             |          2 |           8192 |       16384 |
| t_key1     | p2             |          1 |          16384 |       16384 |
+------------+----------------+------------+----------------+-------------+
6 rows in set (0.00 sec)
mysql> select * from t_hash1 partition(p2);
+------+------+
| c1   | c2   |
+------+------+
| NULL | a    |
+------+------+
1 row in set (0.00 sec)
mysql> select * from t_key1 partition(p2);
+------+------+
| c1   | c2   |
+------+------+
| NULL | a    |
+------+------+
1 row in set (0.00 sec)

可以看到,当hash/key分区表的分区数为3时,分区列为null值的记录分布在了p2分区。

5.总结

range分区表:如果插入记录的分区列值为NULL,则将该行记录插入到最小的分区中。

list分区表:对NULL值的处理有2种方式:

(1)当且仅当只有一个分区使用包含NULL的值做分区表达式时(例如:PARTITION p3 VALUES IN (NULL)),允许插入分区列为NULL的值。

(2)当表中没有显示使用包含NULL的值做分区表达式时,会拒绝插入分区列为NULL的值。

hash/key分区表:对NULL的处理略有不同,不同的分区数,会导致分区列为NULL值的记录分布到不同的分区。


本文转载自公众号:GreatSQL社区

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已于2023-1-18 11:23:29修改
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