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doc-cn: add multi value index case from cnf and dnf combination (ping…
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…cap#15943)
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@AilinKid
AilinKid authored Jan 11, 2024
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Expand Up @@ -235,12 +235,16 @@ mysql> EXPLAIN SELECT /*+ use_index_merge(t2, idx) */ * FROM t2 WHERE a=1 AND JS
6 rows in set, 1 warning (0.00 sec)
```

对于由多个 `member of` 组成的 `OR` 条件，且这些条件可以被作用在同一个多值索引上，也可以使用 IndexMerge 进行访问：
对于由多个 `member of` 组成的 `OR`/`AND` 条件，且这些条件可以被作用在同一个或者多个多值索引上，由于 `member of` 的单路性质，也可以使用 IndexMerge `UNION` 或者 `INTERSECTION` 进行访问：

```sql
mysql> CREATE TABLE t3 (a INT, j JSON, INDEX idx(a, (CAST(j AS SIGNED ARRAY))));
mysql> CREATE TABLE t3 (a INT, j JSON, b INT, k JSON, INDEX idx(a, (CAST(j AS SIGNED ARRAY))), INDEX idx2(b,(CAST(k as SIGNED ARRAY))));
Query OK, 0 rows affected (0.04 sec)
```

使用 `UNION` 进行访问：

```sql
mysql> EXPLAIN SELECT /*+ use_index_merge(t3, idx) */ * FROM t3 WHERE ((a=1 AND (1 member of (j)))) OR ((a=2 AND (2 member of (j))));
+---------------------------------+---------+-----------+---------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
Expand All @@ -253,25 +257,45 @@ mysql> EXPLAIN SELECT /*+ use_index_merge(t3, idx) */ * FROM t3 WHERE ((a=1 AND
+---------------------------------+---------+-----------+---------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------+
```

使用 `INTERSECTION` 进行访问：

```sql
tidb> EXPLAIN SELECT /*+ use_index_merge(t3, idx, idx2) */ * FROM t3 WHERE ((a=1 AND (1 member of (j)))) AND ((b=1 AND (2 member of (k))));
+-------------------------------+---------+-----------+----------------------------------------------------+-------------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------------+---------+-----------+----------------------------------------------------+-------------------------------------------------+
| IndexMerge_8 | 0.10 | root | | type: intersection |
| ├─IndexRangeScan_5(Build) | 0.10 | cop[tikv] | table:t3, index:idx(a, cast(`j` as signed array)) | range:[1 1,1 1], keep order:false, stats:pseudo |
| ├─IndexRangeScan_6(Build) | 0.10 | cop[tikv] | table:t3, index:idx2(b, cast(`k` as signed array)) | range:[1 2,1 2], keep order:false, stats:pseudo |
| └─TableRowIDScan_7(Probe) | 0.10 | cop[tikv] | table:t3 | keep order:false, stats:pseudo |
+-------------------------------+---------+-----------+----------------------------------------------------+-------------------------------------------------+
4 rows in set (0.01 sec)
```

### 部分支持多值索引的场景

如果多个条件通过 `AND` 进行组合，且这些条件对应多个不同的索引，则最多只能使用一个索引来进行访问，如：
如果多个条件通过 `AND`/`OR` 进行组合，每个 `item` 条件对应多个不同的索引，则有以下情况：

* 如果单个 item 的路径也是 index merge，则只要其和外部的 `AND`/`OR` 的逻辑一致，就可以和外部 index merge 融合。
* 如果单个 item 的路径也是 index merge，且其 index partial path 只有一条，那么无论其自身 index merge 的逻辑是 `AND`/`OR`，都可以和外部 index merge 融合。

```sql
mysql> create table t(j1 json, j2 json, a int, INDEX k1((CAST(j1->'$.path' AS SIGNED ARRAY))), INDEX k2((CAST(j2->'$.path' AS SIGNED ARRAY))), INDEX ka(a));
mysql> CREATE TABLE t(j1 JSON, j2 JSON, a INT, INDEX k1((CAST(j1->'$.path' AS SIGNED ARRAY))), INDEX k2((CAST(j2->'$.path' AS SIGNED ARRAY))), INDEX ka(a));
Query OK, 0 rows affected (0.02 sec)
```

mysql> explain select /*+ use_index_merge(t, k1, k2, ka) */ * from t where (1 member of (j1->'$.path')) and (2 member of (j2->'$.path')) and (a = 3);
+---------------------------------+---------+-----------+----------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+---------------------------------+---------+-----------+----------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------+
| Selection_5 | 8.00 | root | | json_memberof(cast(1, json BINARY), json_extract(test.t.j1, "$.path")), json_memberof(cast(2, json BINARY), json_extract(test.t.j2, "$.path")) |
| └─IndexMerge_9 | 0.01 | root | | type: union |
| ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t, index:k1(cast(json_extract(`j1`, _utf8'$.path') as signed array)) | range:[1,1], keep order:false, stats:pseudo |
| └─Selection_8(Probe) | 0.01 | cop[tikv] | | eq(test.t.a, 3) |
| └─TableRowIDScan_7 | 10.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+---------------------------------+---------+-----------+----------------------------------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------+
5 rows in set, 6 warnings (0.01 sec)
```sql
tidb> EXPLAIN SELECT /*+ use_index_merge(t, k1, k2, ka) */ * FROM t WHERE (1 member of (j1->'$.path')) AND (2 member of (j2->'$.path')) AND (a = 3);
+-------------------------------+---------+-----------+----------------------------------------------------------------------------+---------------------------------------------+
| id | estRows | task | access object | operator info |
+-------------------------------+---------+-----------+----------------------------------------------------------------------------+---------------------------------------------+
| IndexMerge_9 | 10.00 | root | | type: intersection |
| ├─IndexRangeScan_5(Build) | 10.00 | cop[tikv] | table:t, index:ka(a) | range:[3,3], keep order:false, stats:pseudo |
| ├─IndexRangeScan_6(Build) | 10.00 | cop[tikv] | table:t, index:k1(cast(json_extract(`j1`, _utf8'$.path') as signed array)) | range:[1,1], keep order:false, stats:pseudo |
| ├─IndexRangeScan_7(Build) | 10.00 | cop[tikv] | table:t, index:k2(cast(json_extract(`j2`, _utf8'$.path') as signed array)) | range:[2,2], keep order:false, stats:pseudo |
| └─TableRowIDScan_8(Probe) | 10.00 | cop[tikv] | table:t | keep order:false, stats:pseudo |
+-------------------------------+---------+-----------+----------------------------------------------------------------------------+---------------------------------------------+
5 rows in set (0.00 sec)
```

目前只能使用 1 个索引进行访问，而无法产生如下面这样同时使用多个索引的计划：
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