ddl: fix add unique index on partitioned table (by RANGE COLUMNS) failed (#11946)

[sre-bot]

cherry-pick #11946 to release-3.0

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Signed-off-by: Lonng heng@lonng.org

What problem does this PR solve?

create table t(
    a int,
    b varchar(100),
    c int,
    INDEX idx_c(c))
PARTITION BY RANGE COLUMNS( a ) (
    PARTITION p0 VALUES LESS THAN (6),
    PARTITION p1 VALUES LESS THAN (11),
    PARTITION p2 VALUES LESS THAN (16),
    PARTITION p3 VALUES LESS THAN (21)
);

mysql> alter table t add unique index idx_a(a);
ERROR 1105 (HY000): line 1 column 7 near ""

What is changed and how it works?

The Expr field in PartitionInfo is not always be assigned a value (e.g: PARTITION BY RANGE COLUMNS(...)).
This PR use columns in PartitionInfo directly if the PartitionInfo.Expr is an empty string.

Check List

Tests

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

Related changes

• Need to cherry-pick to the release branch

Release note

• Fix add unique index on the partitioned table (by RANGE COLUMNS(...)) failed.

[sre-bot]

/run-all-tests

[tiancaiamao]

LGTM

[tangenta]

LGTM