Empty string has different meanings in SELECT and UPDATE in a transaction

[TrocTool]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

/* init */ drop table if exists t;
/* init */ create table t(a int, b int);
/* init */ insert into t values(1, 1), (2, 2), (3, 3);

/* s1 */ begin; -- 1
/* s1 */ update t set a = 10 where ' '; -- 2
/* s1 */ select * from t where ' '; -- 3
/* s1 */commit; -- 4
/* s1 */ select * from t; -- 5

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

statement 3 gets empty and statement5 gets
+------+------+
| a    | b    |
+------+------+
|   1  |    1 |
|   2  |    2 |
|   3  |    3 |
+------+------+


or

statement 3 gets
+------+------+
| a    | b    |
+------+------+
|   10 |    1 |
|   10 |    2 |
|   10 |    3 |
+------+------+

and statement 5 gets
+------+------+
| a    | b    |
+------+------+
|   10 |    1 |
|   10 |    2 |
|   10 |    3 |
+------+------+

3. What did you see instead (Required)

statement 3 gets empty but statement5 gets
+------+------+
| a    | b    |
+------+------+
|   10 |    1 |
|   10 |    2 |
|   10 |    3 |
+------+------+

4. What is your TiDB version? (Required)

v5.2.0

[zyguan]

Transaction is not required for this issue.

/* init */ drop table if exists t;
/* init */ create table t(a int, b int);
/* init */ insert into t values(1, 1), (2, 2), (3, 3);

/* s1 */ update t set a = 10 where ' ';
/* s1 */ select * from t where ' ';

TableFullScan gets used (instead of TableDual) when executing update t set a = 10 where ' ';.

[yuqi1129]

/assign