small cached table doesn't support index merge

[tiancaiamao]

Enhancement

While developing, I found this #29443 (comment), thanks to @lcwangchao for pointing it out.
and seaching the issues list, there is a known issue #28494 here

So small cached table doesn't support index merge

This limitation is imposed by the fact union scan doesn't support index merge...
small cached table implemetation depends on union scan, so it gets the same limitation.

[Defined2014]

I think the IndexMerge works well with small cached table now. And IndexMerge already supported UnionScan by #29875, so close this issue.

mysql> explain select /*+ use_index_merge(tcommon, idx_ac, idx_bc) */ * from tcommon where a = 1 or b = 1;
+---------------------------------+---------+-----------+-----------------------------------+--------------------------------------------------------------------------------+
| id                              | estRows | task      | access object                     | operator info                                                                  |
+---------------------------------+---------+-----------+-----------------------------------+--------------------------------------------------------------------------------+
| UnionScan_6                     | 1.00    | root      |                                   | or(eq(test.tcommon.a, 1), eq(test.tcommon.b, 1))                               |
| └─IndexMerge_10                 | 1.00    | root      |                                   | type: union                                                                    |
|   ├─IndexRangeScan_7(Build)     | 1.00    | cop[tikv] | table:tcommon, index:idx_ac(a, c) | range:[1,1], keep order:false, stats:partial[a:unInitialized, b:unInitialized] |
|   ├─IndexRangeScan_8(Build)     | 1.00    | cop[tikv] | table:tcommon, index:idx_bc(b, c) | range:[1,1], keep order:false, stats:partial[a:unInitialized, b:unInitialized] |
|   └─TableRowIDScan_9(Probe)     | 1.00    | cop[tikv] | table:tcommon                     | keep order:false, stats:partial[a:unInitialized, b:unInitialized]              |
+---------------------------------+---------+-----------+-----------------------------------+--------------------------------------------------------------------------------+
5 rows in set (0.01 sec)

mysql> select /*+ use_index_merge(tcommon, idx_ac, idx_bc) */ * from tcommon where a = 1 or b = 1;
+---+------+---+---+
| a | b    | c | d |
+---+------+---+---+
| 1 |    2 | 3 | 1 |
+---+------+---+---+
1 row in set (0.00 sec)