unexpected dropping predicate for memtable

[zhongzc]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql root@127.0.0.1:information_schema> explain select * from `CLUSTER_HARDWARE` where `TYPE`;
+----------------+----------+------+------------------------+---------------+
| id             | estRows  | task | access object          | operator info |
+----------------+----------+------+------------------------+---------------+
| MemTableScan_5 | 10000.00 | root | table:CLUSTER_HARDWARE |               |
+----------------+----------+------+------------------------+---------------+

2. What did you expect to see? (Required)

mysql root@127.0.0.1:information_schema> explain select * from `CLUSTER_HARDWARE` where `TYPE`;
+------------------+----------+------+------------------------+---------------+
| id               | estRows  | task | access object          | operator info |
+------------------+----------+------+------------------------+---------------+
| Selection_5      | 8000.00  | root |                        | Column#1      |
| └─MemTableScan_6 | 10000.00 | root | table:CLUSTER_HARDWARE |               |
+------------------+----------+------+------------------------+---------------+

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master