expression: handle empty input and improve compatibility for format

[eurekaka]

What problem does this PR solve?

Before this PR, in TiDB:

mysql> select format('', 1);
ERROR 2013 (HY000): Lost connection to MySQL server during query

mysql> select format(1, '');
ERROR 2013 (HY000): Lost connection to MySQL server during query

mysql> select length(format(1, 10240));
+--------------------------+
| length(format(1, 10240)) |
+--------------------------+
|                    10242 |
+--------------------------+
1 row in set (0.00 sec)

while in MySQL 5.7.10:

mysql> select format('', 1);
+---------------+
| format('', 1) |
+---------------+
| 0.0           |
+---------------+
1 row in set (0.00 sec)

mysql> select format(1, '');
+---------------+
| format(1, '') |
+---------------+
| 1             |
+---------------+
1 row in set, 1 warning (0.01 sec)

mysql> select format(1, 10240);
+----------------------------------+
| format(1, 10240)                 |
+----------------------------------+
| 1.000000000000000000000000000000 |
+----------------------------------+
1 row in set (0.00 sec)

mysql> select length(format(1, 10240));
+--------------------------+
| length(format(1, 10240)) |
+--------------------------+
|                       32 |
+--------------------------+
1 row in set (0.00 sec)

What is changed and how it works?

• handle empty input for function format properly to avoid panic;
• improve compatibility with MySQL to generate up to 30 decimals; number 30 is not documented in https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_format, we get it from MySQL source code:

const int FORMAT_MAX_DECIMALS= 30;

String *Item_func_format::val_str_ascii(String *str)
{
  /* Number of decimal digits */
  int dec;

  dec= (int) args[1]->val_int();
  if (args[1]->null_value)
  {
    null_value=1;
    return NULL;
  }

  dec= set_zone(dec, 0, FORMAT_MAX_DECIMALS);
  ...
}

• make result of select format(1, 4, null) compatible with MySQL.

Check List

Tests

• Unit test: new unit tests added, and existing unit tests are updated to be compatible with MySQL.

Code changes

N/A

Side effects

N/A

Related changes

• Need to cherry-pick to the release branch
• Some compatibility issues still exist for function format:
  • function format incompatible with MySQL for round up #8796
  • warning type returned is different from MySQL, but this should be a separate and general issue for a bunch of built-in functions;

---

This change is 

[winoros]

rest lgtm

[zz-jason]

The argument types are all ETString: https://github.com/pingcap/tidb/pull/8797/files#diff-314e997a9df9b116e8f0aad4149df468R2920. We should not change EvalString() to EvalReal().

[eurekaka]

This is the output of MySQL:

mysql> select format("1a","2a");
+-------------------+
| format("1a","2a") |
+-------------------+
| 1.00              |
+-------------------+
1 row in set, 2 warnings (0.00 sec)

mysql> show warnings;
+---------+------+-----------------------------------------+
| Level   | Code | Message                                 |
+---------+------+-----------------------------------------+
| Warning | 1292 | Truncated incorrect INTEGER value: '2a' |
| Warning | 1292 | Truncated incorrect DOUBLE value: '1a'  |
+---------+------+-----------------------------------------+
2 rows in set (0.00 sec)

Seems MySQL believes the type of parameters to be Double and Integer, but it is not reflected in https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_format.

[eurekaka]

In MySQL source code, for function format(X, D), X is evaluated as decimal or double type, while D is evaluated as integer type.

  dec= (int) args[1]->val_int();
  if (args[0]->result_type() == DECIMAL_RESULT ||
      args[0]->result_type() == INT_RESULT)
  {
    res= args[0]->val_decimal(&dec_val);
  }
  else
  {
    double nr= args[0]->val_real();
  }

[zz-jason]

seems we should modify https://github.com/pingcap/tidb/pull/8797/files#diff-314e997a9df9b116e8f0aad4149df468R2920 to change the argument type.

[eurekaka]

@zz-jason @winoros comments addressed, PTAL

[eurekaka]

@lamxTyler @XuHuaiyu PTAL

[alivxxx]

LGTM

[zz-jason]

LGTM

[zz-jason]

/run-all-tests

[eurekaka]

/run-common-test

[eurekaka]

/run-common-test