[VL] Incorrect result type of nested decimal arithmetic expressions

[jiangjiangtian]

Backend

VL (Velox)

Bug description

The SQL as follows will produce wrong result:

select col0 / (col1 + 0.00001) from table;

where col0 and col1 is decimal(20, 0).
The values of col0 and col1 are 25 and 72. Gluten returns 3.4722217399692027821 and spark returns 0.3472221739969202782.
The result type needs to be decimal(38, 19). But during calculation, the actual result type is decimal(38, 20):

The reason is that the type of the subexpression col1 + 0.00001 will be rescaled to decimal(27, 5):

But we don't notice it when we rescale the topmost expression:

Spark version

None

Spark configurations

No response

System information

No response

Relevant logs

No response

[jiangjiangtian]

@kecookier

[jiangjiangtian]

Maybe we should not remove the PromotePrecision(Cast(...)). Otherwise, we will always have the bug.
Besides, maybe we can also override DecimalPrecision in gluten and make it consistent with high version Spark.

[jiangjiangtian]

After some investigations, I find that if we have some literals in the decimal arithmetic expressions , we may have wrong result type. The reason is that the existence of constant folding makes we can't get the original type of the literal.

[rui-mo]

Hi @jiangjiangtian, would you also post the expected result and incorrect result in this issue? Thanks.

[jiangjiangtian]

Hi @jiangjiangtian, would you also post the expected result and incorrect result in this issue? Thanks.

Thanks for reminding me. I add the result in the issue.

[jiangjiangtian]

For now, we pass the result type when we create the decimal function, but it is not an elegant solution, and we will come up with a better solution.