Support IndexMerge with UnionScan

[ichn-hu]

Enhancement

currently TiDB's IndexMerge has a restriction that if it is in a transaction that has data changed before, it can not be applied, because we haven't yet supported index merge with union scan,

tidb/planner/core/stats.go

Line 414 in e52dbd6

// TODO: implement UnionScan + IndexMerge

This fixes the todo here.

[ichn-hu]

for some context, see pingcap/docs-cn#7237

[Defined2014]

I think it already supportted in v7.1, so close it.

mysql> explain select * from t use index(idx, idx1) where a = 1 or b = 2;
+------------------------------------+---------+-----------+---------------------------+--------------------------------------------------------------------------------+
| id                                 | estRows | task      | access object             | operator info                                                                  |
+------------------------------------+---------+-----------+---------------------------+--------------------------------------------------------------------------------+
| Projection_5                       | 0.00    | root      |                           | test.t.a, test.t.b, test.t.c                                                   |
| └─UnionScan_6                      | 1.00    | root      |                           | or(eq(test.t.a, 1), eq(test.t.b, 2))                                           |
|   └─IndexMerge_18                  | 1.00    | root      |                           | type: union                                                                    |
|     ├─IndexRangeScan_15(Build)     | 1.00    | cop[tikv] | table:t, index:idx(a, c)  | range:[1,1], keep order:false, stats:partial[a:unInitialized, b:unInitialized] |
|     ├─IndexRangeScan_16(Build)     | 1.00    | cop[tikv] | table:t, index:idx1(b, c) | range:[2,2], keep order:false, stats:partial[a:unInitialized, b:unInitialized] |
|     └─TableRowIDScan_17(Probe)     | 1.00    | cop[tikv] | table:t                   | keep order:false, stats:partial[a:unInitialized, b:unInitialized]              |
+------------------------------------+---------+-----------+---------------------------+--------------------------------------------------------------------------------+
6 rows in set (0.00 sec)

[Defined2014]

related PR #29875