Enable optimizer to generate index merge path for mv index + OR list nested in AND list

[time-and-fate]

Enhancement

Currently, tidb can't generate index merge access path for such case:

CREATE TABLE `t` (
  `a` int(11) DEFAULT NULL,
  `b` varchar(30) DEFAULT NULL,
  `c` float DEFAULT NULL,
  `j` json DEFAULT NULL,
  `pk` int(11) NOT NULL,
  PRIMARY KEY (`pk`) /*T![clustered_index] CLUSTERED */,
  KEY `mvi1` (`c`,(cast(json_extract(`j`, _utf8'$.a') as unsigned array)),`b`),
  KEY `mvi2` (`a`,(cast(json_extract(`j`, _utf8'$.c') as unsigned array))),
  KEY `mvi3` ((cast(json_extract(`j`, _utf8'$.d') as unsigned array)),`c`),
  KEY `idx` (`b`,`c`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

explain format=brief select /*+ use_index_merge(t, mvi1) */ * from t where ( 1 member of (j->'$.a') or  2 member of (j->'$.a') ) and a = 20 and c = 10;

> explain format=brief select /*+ use_index_merge(t, mvi1) */ * from t where ( 1 member of (j->'$.a') or  2 member of (j->'$.a') ) and a = 20 and c = 10;
+-----------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| id                    | estRows  | task      | access object | operator info                                                                                                                                                                  |
+-----------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| TableReader           | 0.01     | root      |               | data:Selection                                                                                                                                                                 |
| └─Selection           | 0.01     | cop[tikv] |               | eq(test.t.a, 20), eq(test.t.c, 10), or(json_memberof(cast(1, json BINARY), json_extract(test.t.j, "$.a")), json_memberof(cast(2, json BINARY), json_extract(test.t.j, "$.a"))) |
|   └─TableFullScan     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo                                                                                                                                                 |
+-----------------------+----------+-----------+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
3 rows in set, 1 warning (0.004 sec)