ddl: support modify decimal column precision in some cases.

[crazycs520]

background

PR #10433 prohibit modify decimal column precision, because change decimal column precision may cause data inconsistency when there is an index covering the decimal column.

What problem does this PR solve?

After think over, we can support modify a decimal column when satisfying the following conditions:

• No index covers the modified decimal column.
• Only support enlarges the decimal precision.
  • modify decimal(5,2) to decimal(10,5) is ok.
  • modify decimal(5,2) to decimal(4, 2) is not support.
  • modify decimal(5,2) to decimal(5, 3) is not support, because it may cause the column out range value error.

After TiDB supports all column type changes, there will be no such restrictions.

eg:

>create table t (a decimal(5,2),index(a));
Query OK
>alter table t change a a decimal(4,2);
(8200, u"Unsupported modify column: can't change decimal column precision with index covered now")
>alter table t drop index a;
Query OK
>alter table t change a a decimal(4,2);
(8200, u'Unsupported modify column: length 4 is less than origin 5')
>alter table t change a a decimal(5,3);
(8200, u'Unsupported modify column: modify original decimal(5,2) to decimal(5,3) may cause out of range value error')
>alter table t change a a decimal(10,5);
Query OK

What is changed and how it works?

Below is an example if only change ddl pkg to support modify decimal precision :

1. The original decimal precision is decimal(2,1), and write the data 1.1
2. modify the decimal precision to decimal(4,2), and get the data 1.1, but the expect data is 1.10.

Below is sql example:

test> create table t (a decimal(2,1));
test> insert into t set a=1.1;
test> select * from t;
+-----+
| a   |
+-----+
| 1.1 |
+-----+
test> alter table t change a a decimal(4,2);
test> select * from t;
+-----+
| a   |
+-----+
| 1.1 |   -- the value should be 1.10
+-----+

Since the decimal already encodes the precision info, So after modifying the decimal column precision, we must convert the decimal value to the new precision when reading the old decimal value.

So Also need to change the decode logical, both in TiDB and TiKV.

Related TiKV PR tikv/tikv#6532.

Check List

Tests

• Unit test

Code changes

• Has exported function/method change

Related changes

• Need to cherry-pick to the release branch?
• Need to update the documentation

Release note

• Support modify decimal column precision in the case like below:
  • No index covers the modified decimal column.
  • Only support enlarges the decimal precision.

[crazycs520]

/run-all-tests

[crazycs520]

@zimulala @bb7133 @coocood PTAL

[bb7133]

Support modify decimal column precision in some cases.

Please update the release note by adding descriptions to the cases.

[crazycs520]

/run-all-tests

[crazycs520]

/run-all-tests

[crazycs520]

/run-all-tests

[crazycs520]

/run-unit-test

[bb7133]

LGTM

[crazycs520]

/run-all-tests tikv=pr/6532

[crazycs520]

/run-all-tests tikv=pr/6532

[wjhuang2016]

Why we need this?

[devon-ye]

Which version started to support this feature？