explain: add index merge (#3924)

Co-authored-by: TomShawn <[email protected]>

explain-index-merge.md

@@ -0,0 +1,45 @@
+---
+title: Explain Statements Using Index Merge
+summary: Learn about the execution plan information returned by the `EXPLAIN` statement in TiDB.
+---
+
+# Explain Statements Using Index Merge
+
+`IndexMerge` is a method introduced in TiDB v4.0 to access tables. Using this method, the TiDB optimizer can use multiple indexes per table and merge the results returned by each index. In some scenarios, this method makes the query more efficient by avoiding full table scans.
+
+```sql
+mysql> EXPLAIN SELECT * from t where a = 1 or b = 1;
++-------------------------+----------+-----------+---------------+--------------------------------------+
+| id                      | estRows  | task      | access object | operator info                        |
++-------------------------+----------+-----------+---------------+--------------------------------------+
+| TableReader_7           | 8000.00  | root      |               | data:Selection_6                     |
+| └─Selection_6           | 8000.00  | cop[tikv] |               | or(eq(test.t.a, 1), eq(test.t.b, 1)) |
+|   └─TableFullScan_5     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo       |
++-------------------------+----------+-----------+---------------+--------------------------------------+
+mysql> set @@tidb_enable_index_merge = 1;
+mysql> explain select * from t use index(idx_a, idx_b) where a > 1 or b > 1;
++--------------------------------+---------+-----------+-------------------------+------------------------------------------------+
+| id                             | estRows | task      | access object           | operator info                                  |
++--------------------------------+---------+-----------+-------------------------+------------------------------------------------+
+| IndexMerge_16                  | 6666.67 | root      |                         |                                                |
+| ├─IndexRangeScan_13(Build)     | 3333.33 | cop[tikv] | table:t, index:idx_a(a) | range:(1,+inf], keep order:false, stats:pseudo |
+| ├─IndexRangeScan_14(Build)     | 3333.33 | cop[tikv] | table:t, index:idx_b(b) | range:(1,+inf], keep order:false, stats:pseudo |
+| └─TableRowIDScan_15(Probe)     | 6666.67 | cop[tikv] | table:t                 | keep order:false, stats:pseudo                 |
++--------------------------------+---------+-----------+-------------------------+------------------------------------------------+
+```
+
+In the above query, the filter condition is a `WHERE` clause that uses `OR` as the connector. Without `IndexMerge`, you can use only one index per table. `a = 1` cannot be pushed down to the index `a`; neither can `b = 1` be pushed down to the index `b`. The full table scan is inefficient when a huge volume of data exists in `t`. To handle such a scenario, `IndexMerge` is introduced in TiDB to access tables.
+
+`IndexMerge` allows the optimizer to use multiple indexes per table, and merge the results returned by each index to generate the execution plan of the latter `IndexMerge` in the figure above. Here the `IndexMerge_16` operator has three child nodes, among which `IndexRangeScan_13` and `IndexRangeScan_14` get all the `RowID`s that meet the conditions based on the result of range scan, and then the `TableRowIDScan_15` operator accurately reads all the data that meets the conditions according to these `RowID`s.
+
+For the scan operation that is performed on a specific range of data, such as `IndexRangeScan`/`TableRangeScan`, the `operator info` column in the result has additional information about the scan range compared with other scan operations like `IndexFullScan`/`TableFullScan`. In the above example, the `range:(1,+inf]` in the `IndexRangeScan_13` operator indicates that the operator scans the data from 1 to positive infinity.
+
+> **Note:**
+>
+> At present, the `IndexMerge` feature is disabled by default in TiDB 4.0.0-rc.1. In addition, the currently supported scenarios of `IndexMerge` in TiDB 4.0 are limited to the disjunctive normal form (expressions connected by `or`). The conjunctive normal form (expressions connected by `and`) will be supported in later versions. Enable the `IndexMerge` in one of two ways:
+>
+> - Set `tidb_enable_index_merge=1`;
+>
+> - Use the SQL Hint [`USE_INDEX_MERGE`](/optimizer-hints.md#use_index_merget1_name-idx1_name--idx2_name-) in the query.
+>
+> SQL Hint has a higher priority than system variables.