expression: handle max_allowed_packet warnings for space function.

[hhu-cc]

What have you changed? (mandatory)

Return NULL and a warning when the result of space exceeds max_allowed_packetbs.

Before this PR:

tidb> select space(18446744073709551617);
+-----------------------------+
| space(18446744073709551617) |
+-----------------------------+
| NULL                        |
+-----------------------------+
1 row in set, 1 warning (0.00 sec)

tidb> show warnings;
+---------+------+-----------------------------------------------------------+
| Level   | Code | Message                                                   |
+---------+------+-----------------------------------------------------------+
| Warning | 1292 | Truncated incorrect DECIMAL value: '18446744073709551617' |
+---------+------+-----------------------------------------------------------+
1 row in set (0.00 sec)

After this PR:

tidb> select space(18446744073709551617);
+-----------------------------+
| space(18446744073709551617) |
+-----------------------------+
| NULL                        |
+-----------------------------+
1 row in set, 2 warnings (0.00 sec)

tidb> show warnings;
+---------+------+-----------------------------------------------------------------------------+
| Level   | Code | Message                                                                     |
+---------+------+-----------------------------------------------------------------------------+
| Warning | 1292 | Truncated incorrect DECIMAL value: '18446744073709551617'                   |
| Warning | 1301 | Result of space() was larger than max_allowed_packet (67108864) - truncated |
+---------+------+-----------------------------------------------------------------------------+
2 rows in set (0.00 sec)

What is the type of the changes? (mandatory)

Bug fix (non-breaking change which fixes an issue)

How has this PR been tested? (mandatory)

• unit test
• explain test

Does this PR affect documentation (docs/docs-cn) update? (mandatory)

no

Does this PR affect tidb-ansible update? (mandatory)

no

Does this PR need to be added to the release notes? (mandatory)

Yes

release note:

Return `NULL` when the result of function `SPACE` is larger than `max_allowed_packet`

Refer to a related PR or issue link (optional)

to #7153

Benchmark result if necessary (optional)

Add a few positive/negative examples (optional)

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[hhu-cc]

/run-all-tests

[XuHuaiyu]

LGTM

[zz-jason]

LGTM

[zz-jason]

@XuHuaiyu Please give this PR an approve.