planner: show the reason why cannot hit plan cache for EXECUTE statements accessing partition tables

[qw4990]

Enhancement

See the case below, when executing PREPARE statement, it returns the exact reason why this query cannot hit the plan cache.
But later when executing EXECUTE statement, no warning to explain why it cannot hit the plan cache.

mysql> create table t (a int) partition by hash(a) partitions 4;
Query OK, 0 rows affected (0.01 sec)

mysql> analyze table t;
Query OK, 0 rows affected, 4 warnings (0.03 sec)

mysql> prepare st from 'select * from t';
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+-----------------------------------------------------------------------------+
| Level   | Code | Message                                                                     |
+---------+------+-----------------------------------------------------------------------------+
| Warning | 1105 | skip prepared plan-cache: query accesses partitioned tables is un-cacheable |
+---------+------+-----------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> execute st;
Empty set (0.00 sec)

mysql> show warnings;
Empty set (0.00 sec)

mysql> execute st;
Empty set (0.00 sec)

mysql> select @@last_plan_from_cache;
+------------------------+
| @@last_plan_from_cache |
+------------------------+
|                      0 |
+------------------------+
1 row in set (0.00 sec)

[qw4990]

@hawkingrei Please take a look at this issue, THX!