haven't track the memory usage of PointGet/BatchPointGet

[zhaoxugang]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1.drop table if exists t1;
2.create table t1(a int primary key);
3.set tidb_mem_quota_query=1;
4.select * from t1 where a = 1;
5.select /*+ MEMORY_QUOTA(0 GB) / * from t1 where a = 1;
6.select /+ MEMORY_QUOTA(1 GB) */ * from t1 where a = 1;

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

after step 4
ERROR 1105 (HY000): Out Of Memory Quota![conn_id=2199023255555]
after step 5 or 6
+---+
| a |
+---+
| 1 |
+---+

3. What did you see instead (Required)

after step 4
+---+
| a |
+---+
| 1 |
+---+
after 5 or 6
+---+
| a |
+---+
| 1 |
+---+

4. What is your TiDB version? (Required)

master branch
| Release Version: None
Edition: Community
Git Commit Hash: None
Git Branch: None
UTC Build Time: None
GoVersion: go1.14.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306