Adds tests for checking DECIMAL types

[yliuuuu]

Issue #71

Description of changes:

• Add IS operator test for decimal type.
• Add invalid decimal type parameter to fail/staticAnalysis

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[RCHowell]

Could you briefly explain the rules for DECIMAL type and IS operator? How to lower-scale and lower-precision values compare with high-scale and/or high-precision types and visa-versa. Thank you for adding these tests.

[RCHowell]

Could you please explain how 123.456 with three decimal places is considered a decimal with scale 2? Similarly, how is the precision only 2?

[yliuuuu]

The output is false.
So in general I believe that semantically decimal(p,s) means the underlying data value has upto p numeric digits, and exactly s digit after the decimal point.

I don't think the is operator is specified in the SQL spec, but based on the implementation, I believe x is T, where x is a value and T is a Type, is saying that x can be "hold" by T without truncate or rounding.

For example, we can store 123.456 as DECIMAL(6,3), we can also store it in DECIMAL(7,4) without truncate or rounding. (we append trailing zeros)

Hence both 123.456 is DECIMAL(6,3) and 123.456 is DECIMAL(7,4) should return true.

That said, it is based on our current kotlin-implementation and was not documented any where as far as i know.

I can open up an issue to track the decimal semantic documentation in PartiQL-docs if necessary.

But for IS operator, curious if you think we should track this in lang-kotlin or partiql-doc (i.e., spec issue or implementation issue)