add system variable tidb_opt_range_max_size

system-variables.md

@@ -1807,6 +1807,136 @@ explain select * from t where age=5;
 - 默认值：`OFF`
 - 指定是否允许优化器将 `Projection` 算子下推到 TiKV 或者 TiFlash。
 
+### `tidb_opt_range_max_size` <span class="version-mark">从 v6.4.0 版本开始引入</span>
+
+- 作用域：SESSION | GLOBAL
+- 是否持久化到集群：是
+- 默认值：`67108864` (64 MiB)
+- 取值范围：`[0, 9223372036854775807]`
+- 单位：字节
+- 该变量用于指定优化器构造扫描范围的内存用量上限。当该变量为 `0` 时，表示对扫描范围没有内存限制。如果构造精确的扫描范围会超出内存用量限制，优化器会使用更宽松的扫描范围（例如 `[[NULL,+inf]]`）。如果执行计划中未使用精确的扫描范围，可以调大该变量的值让优化器构造精确的扫描范围。
+
+该变量的使用示例如下：
+
+<details>
+<summary><code>tidb_opt_range_max_size</code> 使用示例</summary>
+
+查看该变量的默认值，即优化器构造扫描范围最多使用 64 MiB 内存。
+
+```sql
+SELECT @@tidb_opt_range_max_size;
+```
+
+```sql
++----------------------------+
+| @@tidb_opt_range_max_size |
++----------------------------+
+| 67108864                   |
++----------------------------+
+1 row in set (0.01 sec)
+```
+
+```sql
+EXPLAIN SELECT * FROM t use index (idx) WHERE a IN (10,20,30) AND b IN (40,50,60);
+```
+
+在 64 MiB 的内存最大限制约束下，优化器构造出精确的扫描范围 `[10 40,10 40], [10 50,10 50], [10 60,10 60], [20 40,20 40], [20 50,20 50], [20 60,20 60], [30 40,30 40], [30 50,30 50], [30 60,30 60]`，见如下执行计划返回结果。 
+
+```sql
++-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| id                            | estRows | task      | access object            | operator info                                                                                                                                                               |
++-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| IndexLookUp_7                 | 0.90    | root      |                          |                                                                                                                                                                             |
+| ├─IndexRangeScan_5(Build)     | 0.90    | cop[tikv] | table:t, index:idx(a, b) | range:[10 40,10 40], [10 50,10 50], [10 60,10 60], [20 40,20 40], [20 50,20 50], [20 60,20 60], [30 40,30 40], [30 50,30 50], [30 60,30 60], keep order:false, stats:pseudo |
+| └─TableRowIDScan_6(Probe)     | 0.90    | cop[tikv] | table:t                  | keep order:false, stats:pseudo                                                                                                                                              |
++-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+3 rows in set (0.00 sec)
+```
+
+现将优化器构造扫描范围的内存用量上限设为 1500 字节。
+
+```sql
+SET @@tidb_opt_range_max_size = 1500;
+```
+
+```sql
+Query OK, 0 rows affected (0.00 sec)
+```
+
+```sql
+EXPLAIN SELECT * FROM t USE INDEX (idx) WHERE a IN (10,20,30) AND b IN (40,50,60);
+```
+
+在 1500 字节内存的最大限制约束下，优化器构造出了更宽松的扫描范围 `[10,10], [20,20], [30,30]`，并用 warning 提示用户构造精确的扫描范围所需的内存用量超出了 `tidb_opt_range_max_size` 的限制。
+
+```sql
++-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------+
+| id                            | estRows | task      | access object            | operator info                                                   |
++-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------+
+| IndexLookUp_8                 | 0.09    | root      |                          |                                                                 |
+| ├─Selection_7(Build)          | 0.09    | cop[tikv] |                          | in(test.t.b, 40, 50, 60)                                        |
+| │ └─IndexRangeScan_5          | 30.00   | cop[tikv] | table:t, index:idx(a, b) | range:[10,10], [20,20], [30,30], keep order:false, stats:pseudo |
+| └─TableRowIDScan_6(Probe)     | 0.09    | cop[tikv] | table:t                  | keep order:false, stats:pseudo                                  |
++-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------+
+4 rows in set, 1 warning (0.00 sec)
+```
+
+```sql
+SHOW WARNINGS;
+```
+
+```sql
++---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
+| Level   | Code | Message                                                                                                                                     |
++---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
+| Warning | 1105 | Memory capacity of 1500 bytes for 'tidb_opt_range_max_size' exceeded when building ranges. Less accurate ranges such as full range are chosen |
++---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
+1 row in set (0.00 sec)
+```
+
+再将优化器构造扫描范围的内存用量上限设为 100 字节。
+
+```sql
+set @@tidb_opt_range_max_size = 100;
+```
+
+```sql
+Query OK, 0 rows affected (0.00 sec)
+```
+
+```sql
+EXPLAIN SELECT * FROM t USE INDEX (idx) WHERE a IN (10,20,30) AND b IN (40,50,60);
+```
+
+在 100 字节的内存最大限制约束下，优化器选择了 `IndexFullScan`，并用 warning 提示用户构造精确的扫描范围所需的内存超出了 `tidb_opt_range_max_size` 的限制。
+
+```sql
++-------------------------------+----------+-----------+--------------------------+----------------------------------------------------+
+| id                            | estRows  | task      | access object            | operator info                                      |
++-------------------------------+----------+-----------+--------------------------+----------------------------------------------------+
+| IndexLookUp_8                 | 8000.00  | root      |                          |                                                    |
+| ├─Selection_7(Build)          | 8000.00  | cop[tikv] |                          | in(test.t.a, 10, 20, 30), in(test.t.b, 40, 50, 60) |
+| │ └─IndexFullScan_5           | 10000.00 | cop[tikv] | table:t, index:idx(a, b) | keep order:false, stats:pseudo                     |
+| └─TableRowIDScan_6(Probe)     | 8000.00  | cop[tikv] | table:t                  | keep order:false, stats:pseudo                     |
++-------------------------------+----------+-----------+--------------------------+----------------------------------------------------+
+4 rows in set, 1 warning (0.00 sec)
+```
+
+```sql
+SHOW WARNINGS;
+```
+
+```sql
++---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
+| Level   | Code | Message                                                                                                                                     |
++---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
+| Warning | 1105 | Memory capacity of 100 bytes for 'tidb_opt_range_max_size' exceeded when building ranges. Less accurate ranges such as full range are chosen |
++---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
+1 row in set (0.00 sec)
+```
+
+</details>
+
 ### `tidb_opt_scan_factor`
 
 - 作用域：SESSION | GLOBAL