expression: fix type infer when wrap cast decimal as string

[XuHuaiyu]

What problem does this PR solve?

MySQL [test]> create table ee(a decimal(11,8),b decimal(11,8));
Query OK, 0 rows affected (1.16 sec)

MySQL [test]> insert into ee values("114.57011441","38.04620115");
Query OK, 1 row affected (0.28 sec)

Before this PR:

MySQL [test]> select a,b,concat_ws(",",a,b) from ee;
+--------------+-------------+-------------------------+
| a            | b           | concat_ws(",",a,b)      |
+--------------+-------------+-------------------------+
| 114.57011441 | 38.04620115 | 114.5701144,38.04620115 |
+--------------+-------------+-------------------------+
1 row in set, 1 warning (0.00 sec)

After this PR:

MySQL [test]> select a,b,concat_ws(",",a,b) from ee;
+--------------+-------------+-------------------------+
| a            | b           | concat_ws(",",a,b)      |
+--------------+-------------+-------------------------+
| 114.57011441 | 38.04620115 | 114.57011441,38.04620115 |
+--------------+-------------+-------------------------+
1 row in set, 1 warning (0.00 sec)

What is changed and how it works?

When do type inferring for concat_ws, the argument will be wrapped a
CastAsString if the type of the arg is not type string. Especially when the
argument is of type decimal, the result length will be set to the length of the
decimal which do not take the decimal point and the negative sign into
consideration. During evaluation, the result of CastDecimalAsString will be
truncated to the wrong length, and cause the unexpected result of concat_ws.

Check List

Tests

• Integration test

Code changes

• Has exported function/method change

Side effects

• Increased code complexity

Related changes

• Need to cherry-pick to the release branch

[XuHuaiyu]

/run-all-tests

[zz-jason]

LGTM

[winoros]

lgtm