[PostgreSQL] Retrieve NUMERIC and DECIMAL as a more compatible Kotlin type

[veyndan]

We currently retrieve NUMERIC and DECIMAL as kotlin.Long, but this kt type doesn't correctly encapsulate NUMERIC nor DECIMAL, as both these types:

• can be floating point numbers.
• can far exceed the kotlin.Long bounds ("up to 131072 digits before the decimal point; up to 16383 digits after the decimal point").
• can be Infinity, -Infinity, or NaN.

I can't think of a data type that abides by all three constraints. Exposing the type as BigDecimal would be a step in the right direction, but it doesn't support Infinity, -Infinity, or NaN. The only valid solution that I can currently think of is exposing the type as a String, and put the burden on the user to have a custom adapter that meets their constraints (for example, they can use BigDecimal if they don't care about the third constraint).

[AlecKazakova]

I think ideal case is we have a postgres-specific runtime which has some sort of sealed class that encapsulates all the numeric types

[tropikoder]

The key feature of the NUMERIC datatype in postgres isn't so much that it can handle huge numbers (though some folks probably use that feature), but that it is exact.

Real and double-precision types are inexact which means numbers after the decimal may change at times in unpleasantly surprising ways.

If you could add support for Numeric types that fit in a Double that would probably cover 90% of use cases. Folks who need larger numbers could have another way to adapt the value to java.

[veyndan]

@tropikoder I agree that Double would probably cover most people's use cases, so I've made a PR (#2118) as an interim solution.

In the long term, I'd probably go with something like @AlecStrong's suggestion:

sealed class Numeric {
  object PositiveInfinity : Numeric()
  object NegativeInfinity : Numeric()
  object NaN : Numeric()
  data class Finite(val value: BigDecimal) : Numeric()
}