executor: group_concat aggr panic when session.group_concat_max_len is small

[guo-shaoge]

Signed-off-by: guo-shaoge shaoge1994@163.com

What problem does this PR solve?

Issue Number: close #23129

Problem Summary:

drop table if exists t1;
create table t1(cid int, sname varchar(100));
insert into t1 values(1, 'Bob'), (1, 'Alice');
insert into t1 values(3, 'Ace');
set @@group_concat_max_len=5;
select group_concat(sname order by sname) from t1 group by cid;

What is changed and how it works?

What's Changed: when concat all strings from a group(AppendFinalResult2Chunk), we add the last separator to result, and then truncate to sesssion.group_concat_max_len. But we forgot to check if the concated string is shorter than sesssion.group_concat_max_len. So program panic.

PS: Another compatible bug found when fixing this bug, but it's not critical. Will fix in another issue.

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Integration test: Add test in aggregate_test.go::TestGroupConcatAggr()

Side effects: no

Release note

• group_concat aggr panic when session.group_concat_max_len is small.

[guo-shaoge]

Another compatible bug found(Same case as this PR.)

mysql 8.0:

Server version: 8.0.23 MySQL Community Server - GPL

Copyright (c) 2000, 2021, Oracle and/or its affiliates.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> set @@group_concat_max_len=5;
Query OK, 0 rows affected (0.00 sec)

mysql> select group_concat(sname order by sname) from t1 group by cid;
+------------------------------------+
| group_concat(sname order by sname) |
+------------------------------------+
| Alice                              |
| Ace                                |
+------------------------------------+
2 rows in set, 1 warning (0.00 sec)

TiDB master:

mysql> set @@group_concat_max_len=5;
Query OK, 0 rows affected (0.00 sec)

mysql> select group_concat(sname order by sname) from t1 group by cid;
+------------------------------------+
| group_concat(sname order by sname) |
+------------------------------------+
| Alice                              |
| Ace,                               |
+------------------------------------+
2 rows in set, 1 warning (0.01 sec)

The result of TiDB has an extra comma.Not critical, will fix in another issue.

Actually, we should add "truncated" flag for each group instead of use one flag for all groups. For each group, if got truncated flag, we add the last separator. Otherwise, donot add.

[guo-shaoge]

@XuHuaiyu

[XuHuaiyu]

Have you tested the case in #23129?

[guo-shaoge]

Have you tested the case in #23129?

Yes. The case is OK.

[XuHuaiyu]

Please refine the release note to describe what you fixed in this commit, not the phenomenon of the bug.

[XuHuaiyu]

1. sep1 --> sep
2. sep2 --> sep
3. what's the group_concat_length of this example

[XuHuaiyu]

(sep2, str3) --> (sep, str3)
partial sep2 --> part of the second sep

[guo-shaoge]

@lzmhhh123 @XuHuaiyu PTAL

[XuHuaiyu]

/LGTM

[wshwsh12]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• XuHuaiyu
• wshwsh12

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[wshwsh12]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: b10f031

[wshwsh12]

/rebuild

[wshwsh12]

/run-tics-test

[ti-srebot]

cherry pick to release-4.0 in PR #23257