planner: make the read storage hint not force plan

[lzmhhh123]

What problem does this PR solve?

mysql> set @@session.tidb_isolation_read_engines ="tiflash";
Query OK, 0 rows affected (0.00 sec)

Set TiFlash isolation.

mysql> explain select /*+ read_from_storage(tikv[test]) */ * from test;
ERROR 1815 (HY000): Internal : Can't find a proper physical plan for this query

It looks a good behavior.

mysql> set @@session.tidb_isolation_read_engines ="tikv";
Query OK, 0 rows affected (0.00 sec)

Then set TiKV isolation.

mysql> explain select /*+ read_from_storage(tiflash[test]) */ * from test
+-------------------+----------+-----------+------------------------------------------------------------+
| id                | count    | task      | operator info                                              |
+-------------------+----------+-----------+------------------------------------------------------------+
| TableReader_5     | 10000.00 | root      | data:TableScan_4                                           |
| └─TableScan_4     | 10000.00 | cop[tiflash] | table:t, range:[-inf,+inf], keep order:false, stats:pseudo |
+-------------------+----------+-----------+------------------------------------------------------------+
2 rows in set (0.00 sec)

That's not the same behavior of TiFlash isolation read.

What is changed and how it works?

Remove the TODO in set hint storage type and make the hint not force the possible path for the DataSource.

Check List

Tests

• Unit test
• Integration test

Code changes

• Has persistent data change

[alivxxx]

LGTM

[windtalker]

LGTM

[sre-bot]

/run-all-tests

[sre-bot]

@lzmhhh123 merge failed.

[lzmhhh123]

/run-unit-test

[lzmhhh123]

/run-unit-test