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分区裁剪

分区裁剪是只有当目标表为分区表时，才可以进行的一种优化方式。分区裁剪通过分析查询语句中的过滤条件，只选择可能满足条件的分区，不扫描匹配不上的分区，进而显著地减少计算的数据量。

例如：

CREATE TABLE t1 (
 id INT NOT NULL PRIMARY KEY,
 pad VARCHAR(100)
)
PARTITION BY RANGE COLUMNS(id) (
 PARTITION p0 VALUES LESS THAN (100),
 PARTITION p1 VALUES LESS THAN (200),
 PARTITION p2 VALUES LESS THAN (MAXVALUE)
);
INSERT INTO t1 VALUES (1, 'test1'),(101, 'test2'), (201, 'test3');
EXPLAIN SELECT * FROM t1 WHERE id BETWEEN 80 AND 120;
+----------------------------+---------+-----------+------------------------+------------------------------------------------+
| id                         | estRows | task      | access object          | operator info                                  |
+----------------------------+---------+-----------+------------------------+------------------------------------------------+
| PartitionUnion_8           | 80.00   | root      |                        |                                                |
| ├─TableReader_10           | 40.00   | root      |                        | data:TableRangeScan_9                          |
| │ └─TableRangeScan_9       | 40.00   | cop[tikv] | table:t1, partition:p0 | range:[80,120], keep order:false, stats:pseudo |
| └─TableReader_12           | 40.00   | root      |                        | data:TableRangeScan_11                         |
|   └─TableRangeScan_11      | 40.00   | cop[tikv] | table:t1, partition:p1 | range:[80,120], keep order:false, stats:pseudo |
+----------------------------+---------+-----------+------------------------+------------------------------------------------+
5 rows in set (0.00 sec)

分区裁剪的使用场景

分区表有 Range 分区和 hash 分区两种形式，分区裁剪对两种分区表也有不同的使用场景。

分区裁剪在 Hash 分区表上的应用

Hash 分区表上可以使用分区裁剪的场景

只有等值比较的查询条件能够支持 Hash 分区表的裁剪。

create table t (x int) partition by hash(x) partitions 4;
explain select * from t where x = 1;
+-------------------------+----------+-----------+-----------------------+--------------------------------+
| id                      | estRows  | task      | access object         | operator info                  |
+-------------------------+----------+-----------+-----------------------+--------------------------------+
| TableReader_8           | 10.00    | root      |                       | data:Selection_7               |
| └─Selection_7           | 10.00    | cop[tikv] |                       | eq(test.t.x, 1)                |
|   └─TableFullScan_6     | 10000.00 | cop[tikv] | table:t, partition:p1 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+-----------------------+--------------------------------+

在这条 SQL 中，由条件 x = 1 可以知道所有结果均在一个分区上。数值 1 在经过 Hash 后，可以确定其在分区 p1 中。因此只需要扫描分区 p1 ,而无需访问一定不会出现相关结果的 p2 、p3 、p4 分区。从执行计划来看，其中只出现了一个 TableFullScan 算子，且在 access object 中指定了 p1 分区，确认 partition pruning 生效了。

Hash 分区表上不能使用分区裁剪的场景

场景一

不能确定查询结果只在一个分区上的条件：如 in, between, > < >= <= 等查询条件，不能使用分区裁剪的优化。

create table t (x int) partition by hash(x) partitions 4;
explain select * from t where x > 2;
+------------------------------+----------+-----------+-----------------------+--------------------------------+
| id                           | estRows  | task      | access object         | operator info                  |
+------------------------------+----------+-----------+-----------------------+--------------------------------+
| Union_10                     | 13333.33 | root      |                       |                                |
| ├─TableReader_13             | 3333.33  | root      |                       | data:Selection_12              |
| │ └─Selection_12             | 3333.33  | cop[tikv] |                       | gt(test.t.x, 2)                |
| │   └─TableFullScan_11       | 10000.00 | cop[tikv] | table:t, partition:p0 | keep order:false, stats:pseudo |
| ├─TableReader_16             | 3333.33  | root      |                       | data:Selection_15              |
| │ └─Selection_15             | 3333.33  | cop[tikv] |                       | gt(test.t.x, 2)                |
| │   └─TableFullScan_14       | 10000.00 | cop[tikv] | table:t, partition:p1 | keep order:false, stats:pseudo |
| ├─TableReader_19             | 3333.33  | root      |                       | data:Selection_18              |
| │ └─Selection_18             | 3333.33  | cop[tikv] |                       | gt(test.t.x, 2)                |
| │   └─TableFullScan_17       | 10000.00 | cop[tikv] | table:t, partition:p2 | keep order:false, stats:pseudo |
| └─TableReader_22             | 3333.33  | root      |                       | data:Selection_21              |
|   └─Selection_21             | 3333.33  | cop[tikv] |                       | gt(test.t.x, 2)                |
|     └─TableFullScan_20       | 10000.00 | cop[tikv] | table:t, partition:p3 | keep order:false, stats:pseudo |
+------------------------------+----------+-----------+-----------------------+--------------------------------+

在这条 SQL 中，x > 2 条件无法确定对应的 Hash Partition，所以不能使用分区裁剪。

场景二

由于分区裁剪的规则优化是在查询计划的生成阶段，对于执行阶段才能获取到过滤条件的场景，无法利用分区裁剪的优化。

create table t (x int) partition by hash(x) partitions 4;
explain select * from t2 where x = (select * from t1 where t2.x = t1.x and t2.x < 2);
+--------------------------------------+----------+-----------+------------------------+----------------------------------------------+
| id                                   | estRows  | task      | access object          | operator info                                |
+--------------------------------------+----------+-----------+------------------------+----------------------------------------------+
| Projection_13                        | 9990.00  | root      |                        | test.t2.x                                    |
| └─Apply_15                           | 9990.00  | root      |                        | inner join, equal:[eq(test.t2.x, test.t1.x)] |
|   ├─TableReader_18(Build)            | 9990.00  | root      |                        | data:Selection_17                            |
|   │ └─Selection_17                   | 9990.00  | cop[tikv] |                        | not(isnull(test.t2.x))                       |
|   │   └─TableFullScan_16             | 10000.00 | cop[tikv] | table:t2               | keep order:false, stats:pseudo               |
|   └─Selection_19(Probe)              | 0.80     | root      |                        | not(isnull(test.t1.x))                       |
|     └─MaxOneRow_20                   | 1.00     | root      |                        |                                              |
|       └─Union_21                     | 2.00     | root      |                        |                                              |
|         ├─TableReader_24             | 2.00     | root      |                        | data:Selection_23                            |
|         │ └─Selection_23             | 2.00     | cop[tikv] |                        | eq(test.t2.x, test.t1.x), lt(test.t2.x, 2)   |
|         │   └─TableFullScan_22       | 2500.00  | cop[tikv] | table:t1, partition:p0 | keep order:false, stats:pseudo               |
|         └─TableReader_27             | 2.00     | root      |                        | data:Selection_26                            |
|           └─Selection_26             | 2.00     | cop[tikv] |                        | eq(test.t2.x, test.t1.x), lt(test.t2.x, 2)   |
|             └─TableFullScan_25       | 2500.00  | cop[tikv] | table:t1, partition:p1 | keep order:false, stats:pseudo               |
+--------------------------------------+----------+-----------+------------------------+----------------------------------------------+

这个查询每从 t2 读取一行，都会去分区表 t1 上进行查询，理论上这时会满足 t1.x = val 的过滤条件，但实际上由于分区裁剪只作用于查询计划生成阶段，而不是执行阶段，因而不会做裁剪。

分区裁剪在 Range 分区表上的应用

Range 分区表上可以使用分区裁剪的场景

场景一

等值比较的查询条件可以使用分区裁剪。

create table t (x int) partition by range (x) (
    partition p0 values less than (5),
    partition p1 values less than (10),
    partition p2 values less than (15)
    );
explain select * from t where x = 3;
+-------------------------+----------+-----------+-----------------------+--------------------------------+
| id                      | estRows  | task      | access object         | operator info                  |
+-------------------------+----------+-----------+-----------------------+--------------------------------+
| TableReader_8           | 10.00    | root      |                       | data:Selection_7               |
| └─Selection_7           | 10.00    | cop[tikv] |                       | eq(test.t.x, 3)                |
|   └─TableFullScan_6     | 10000.00 | cop[tikv] | table:t, partition:p0 | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+-----------------------+--------------------------------+

使用 in 条件的等值比较查询条件也可以使用分区裁剪。

create table t (x int) partition by range (x) (
    partition p0 values less than (5),
    partition p1 values less than (10),
    partition p2 values less than (15)
    );
explain select * from t where x in(1,13);
+-----------------------------+----------+-----------+-----------------------+--------------------------------+
| id                          | estRows  | task      | access object         | operator info                  |
+-----------------------------+----------+-----------+-----------------------+--------------------------------+
| Union_8                     | 40.00    | root      |                       |                                |
| ├─TableReader_11            | 20.00    | root      |                       | data:Selection_10              |
| │ └─Selection_10            | 20.00    | cop[tikv] |                       | in(test.t.x, 1, 13)            |
| │   └─TableFullScan_9       | 10000.00 | cop[tikv] | table:t, partition:p0 | keep order:false, stats:pseudo |
| └─TableReader_14            | 20.00    | root      |                       | data:Selection_13              |
|   └─Selection_13            | 20.00    | cop[tikv] |                       | in(test.t.x, 1, 13)            |
|     └─TableFullScan_12      | 10000.00 | cop[tikv] | table:t, partition:p2 | keep order:false, stats:pseudo |
+-----------------------------+----------+-----------+-----------------------+--------------------------------+

在这条 SQL 中，由条件 x in(1,13) 可以知道所有结果只会分布在几个分区上。经过分析，发现所有 x = 1 的记录都在分区 p0 上, 所有 x = 13 的记录都在分区 p2 上，因此只需要访问 p0、p2 这两个分区，

场景二

区间比较的查询条件如 between, > < = >= <= 可以使用分区裁剪。

create table t (x int) partition by range (x) (
    partition p0 values less than (5),
    partition p1 values less than (10),
    partition p2 values less than (15)
    );
explain select * from t where x between 7 and 14;
+-----------------------------+----------+-----------+-----------------------+-----------------------------------+
| id                          | estRows  | task      | access object         | operator info                     |
+-----------------------------+----------+-----------+-----------------------+-----------------------------------+
| Union_8                     | 500.00   | root      |                       |                                   |
| ├─TableReader_11            | 250.00   | root      |                       | data:Selection_10                 |
| │ └─Selection_10            | 250.00   | cop[tikv] |                       | ge(test.t.x, 7), le(test.t.x, 14) |
| │   └─TableFullScan_9       | 10000.00 | cop[tikv] | table:t, partition:p1 | keep order:false, stats:pseudo    |
| └─TableReader_14            | 250.00   | root      |                       | data:Selection_13                 |
|   └─Selection_13            | 250.00   | cop[tikv] |                       | ge(test.t.x, 7), le(test.t.x, 14) |
|     └─TableFullScan_12      | 10000.00 | cop[tikv] | table:t, partition:p2 | keep order:false, stats:pseudo    |
+-----------------------------+----------+-----------+-----------------------+-----------------------------------+
场景三

分区表达式为 fn(col) 的简单形式，查询条件是 > < = >= <= 之一，且 fn 是单调函数，可以使用分区裁剪。

关于 fn 函数，对于任意 x y，如果 x > y，则 fn(x) > fn(y)，那么这种是严格递增的单调函数。非严格递增的单调函数也可以符合分区裁剪要求，只要函数 fn 满足：对于任意 x y，如果 x > y，则 fn(x) >= fn(y)。理论上，所有满足单调条件（严格或者非严格）的函数都支持分区裁剪。目前，TiDB 支持的单调函数如下：

unix_timestamp
to_days

例如，分区表达式是 fn(col) 形式，fn 为我们支持的单调函数 to_days，就可以使用分区裁剪：

create table t (id datetime) partition by range (to_days(id)) (
    partition p0 values less than (to_days('2020-04-01')),
    partition p1 values less than (to_days('2020-05-01')));
explain select * from t where id > '2020-04-18';
+-------------------------+----------+-----------+-----------------------+-------------------------------------------+
| id                      | estRows  | task      | access object         | operator info                             |
+-------------------------+----------+-----------+-----------------------+-------------------------------------------+
| TableReader_8           | 3333.33  | root      |                       | data:Selection_7                          |
| └─Selection_7           | 3333.33  | cop[tikv] |                       | gt(test.t.id, 2020-04-18 00:00:00.000000) |
|   └─TableFullScan_6     | 10000.00 | cop[tikv] | table:t, partition:p1 | keep order:false, stats:pseudo            |
+-------------------------+----------+-----------+-----------------------+-------------------------------------------+

Range 分区表上不能使用分区裁剪的场景

由于分区裁剪的规则优化是在查询计划的生成阶段，对于执行阶段才能获取到过滤条件的场景，无法利用分区裁剪的优化。

create table t1 (x int) partition by range (x) (
    partition p0 values less than (5),
    partition p1 values less than (10));
create table t2 (x int);
explain select * from t2 where x < (select * from t1 where t2.x < t1.x and t2.x < 2);
+--------------------------------------+----------+-----------+------------------------+-----------------------------------------------------------+
| id                                   | estRows  | task      | access object          | operator info                                             |
+--------------------------------------+----------+-----------+------------------------+-----------------------------------------------------------+
| Projection_13                        | 9990.00  | root      |                        | test.t2.x                                                 |
| └─Apply_15                           | 9990.00  | root      |                        | CARTESIAN inner join, other cond:lt(test.t2.x, test.t1.x) |
|   ├─TableReader_18(Build)            | 9990.00  | root      |                        | data:Selection_17                                         |
|   │ └─Selection_17                   | 9990.00  | cop[tikv] |                        | not(isnull(test.t2.x))                                    |
|   │   └─TableFullScan_16             | 10000.00 | cop[tikv] | table:t2               | keep order:false, stats:pseudo                            |
|   └─Selection_19(Probe)              | 0.80     | root      |                        | not(isnull(test.t1.x))                                    |
|     └─MaxOneRow_20                   | 1.00     | root      |                        |                                                           |
|       └─Union_21                     | 2.00     | root      |                        |                                                           |
|         ├─TableReader_24             | 2.00     | root      |                        | data:Selection_23                                         |
|         │ └─Selection_23             | 2.00     | cop[tikv] |                        | lt(test.t2.x, 2), lt(test.t2.x, test.t1.x)                |
|         │   └─TableFullScan_22       | 2.50     | cop[tikv] | table:t1, partition:p0 | keep order:false, stats:pseudo                            |
|         └─TableReader_27             | 2.00     | root      |                        | data:Selection_26                                         |
|           └─Selection_26             | 2.00     | cop[tikv] |                        | lt(test.t2.x, 2), lt(test.t2.x, test.t1.x)                |
|             └─TableFullScan_25       | 2.50     | cop[tikv] | table:t1, partition:p1 | keep order:false, stats:pseudo                            |
+--------------------------------------+----------+-----------+------------------------+-----------------------------------------------------------+
14 rows in set (0.00 sec)

这个查询每从 t2 读取一行，都会去分区表 t1 上进行查询，理论上这时会满足 t1.x > val 的过滤条件，但实际上由于分区裁剪只作用于查询计划生成阶段，而不是执行阶段，因而不会做裁剪。

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