expression, executor: length(binary(m)) should return 'm' instead of the concrete string length(#3644)

[zz-jason]

No description provided.

[XuHuaiyu]

need varbinary be treated as this?

[zz-jason]

no, just binary

DROP TABLE IF EXISTS table_binary;
CREATE TABLE table_binary (
    a BINARY(20),
    b VARBINARY(20),
    c TINYBLOB,
    d BLOB(20),
    e MEDIUMBLOB,
    f LONGBLOB,
    g BIGINT,
    UNIQUE KEY(a),
    UNIQUE KEY(g)
);
INSERT INTO table_binary (a, b, c, d, e, f, g) VALUES ("a ", "b ", "c ", "d ", "e ", "f ", 2);

select length(a), length(b), length(c), length(d), length(e), length(f), length(g) from table_binary;
+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
| length(a) | length(b) | length(c) | length(d) | length(e) | length(f) | length(g) |
+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
|        20 |         2 |         2 |         2 |         2 |         2 |         1 |
+-----------+-----------+-----------+-----------+-----------+-----------+-----------+

[hanfei1991]

LGTM

[XuHuaiyu]

This PR may not really fix #3644 .
In MySQL:

create table t (a binary(10));
select length(concat(a, "123")) from t;
--> 13

alter table t modify a binary(20);
select length(concat(a, "123")) from t;
--> 23

In TiDB, we may still get 13 even after merging this PR.