ddl: fix drop index failed when index contain auto_increment column and the auto_increment column is primary key (#15861)

[sre-bot]

cherry-pick #15861 to release-3.1

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Signed-off-by: crazycs520 crazycs520@gmail.com

What problem does this PR solve?

Before this PR:

mysql>create table t3 (a int auto_increment key, b int, index idx(b,a))
Query OK, 0 rows affected
Time: 0.074s
mysql>alter table t3 drop index idx;   -- should sucessful.
(1075, u'Incorrect table definition; there can be only one auto column and it must be defined as a key')

What is changed and how it works?

Fix DDL drop index with auto_increament check function bug.

Related changes

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

Side effects

Release note

Fix issue of drop index failed when index contain auto_increment column and the auto_increment column is the primary key.

[sre-bot]

/run-all-tests

[zimulala]

LGTM

Please fix CI @crazycs520

[AilinKid]

LGTM

[crazycs520]

/run-all-tests

[sre-bot]

Your auto merge job has been accepted, waiting for 16117, 16326, 16310, 16008

[sre-bot]

/run-all-tests

[sre-bot]

@sre-bot merge failed.