Decimal type fields cannot be resized or precision improved within the partition table

[tonyhu214]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table test(
id int not null primary key auto_increment,
cost decimal(10,0) default 0 not null
)engine = innodb
partition by hash(id)
PARTITIONS 4 ;

alter table test modify column cost decimal(20,4) default 0 not null;

2. What did you expect to see? (Required)

sql error : Unsupported modify column: table is partition table

3. What did you see instead (Required)

Change fields to decimal(20,4)

4. What is your TiDB version? (Required)

6.5.0 +

[River2000i]

seems block from

tidb/pkg/ddl/ddl_api.go

Lines 6006 to 6014 in 7d5f1d7

	needChangeColData := needChangeColumnData(col.ColumnInfo, newCol.ColumnInfo)
	if needChangeColData {
	if err = isGeneratedRelatedColumn(t.Meta(), newCol.ColumnInfo, col.ColumnInfo); err != nil {
	return nil, errors.Trace(err)
	}
	if t.Meta().Partition != nil {
	return nil, dbterror.ErrUnsupportedModifyColumn.GenWithStackByArgs("table is partition table")
	}
	}

[tonyhu214]

TKS！Based on your information, I took a closer look at the code for the needChangeColumnData function.

tidb/pkg/ddl/column.go

Lines 386 to 402 in d8098b9

	// Deal with the same type.
	if oldCol.GetType() == newCol.GetType() {
	switch oldCol.GetType() {
	case mysql.TypeNewDecimal:
	// Since type decimal will encode the precision, frac, negative(signed) and wordBuf into storage together, there is no short
	// cut to eliminate data reorg change for column type change between decimal.
	return oldCol.GetFlen() != newCol.GetFlen() || oldCol.GetDecimal() != newCol.GetDecimal() || toUnsigned != originUnsigned
	case mysql.TypeEnum, mysql.TypeSet:
	return IsElemsChangedToModifyColumn(oldCol.GetElems(), newCol.GetElems())
	case mysql.TypeTiny, mysql.TypeShort, mysql.TypeInt24, mysql.TypeLong, mysql.TypeLonglong:
	return toUnsigned != originUnsigned
	case mysql.TypeString:
	// Due to the behavior of padding \x00 at binary type, always change column data when binary length changed
	if types.IsBinaryStr(&oldCol.FieldType) {
	return newCol.GetFlen() != oldCol.GetFlen()
	}
	}

How do I go about understanding how the decimal type is stored in TiDB？Is there any code that can be viewed？

[tonyhu214]

Of course. MySQL supports changes to the decimal type of partitioned tables. I hope Tidb can also support it.