Fokko commented on issue #159:
URL: https://github.com/apache/iceberg-rust/issues/159#issuecomment-1887912820
First of all, thanks for the explanation of the Literal.
> This sounds a good idea to me, which doesn't introduce extra maintain
effort. The only concern is that it maybe not quite user friendly, but it seems
that we can add some api to make it easier to use.
I think the user should not care about the type of the field. We have the
information once we bind to the schema. In extend to that, it can also be that
a field has evolved from an int to a long, so you have to convert it anyway
based om the file schema.
> Regarding @Fokko's example: Doesn't initially storing the Decimal as a
LiteralFloat loose accuracy because the 3.25 is stored as something like
3.24999999987. If you then convert it to Decimal, it's inaccurate. Maybe you
could use PrimitiveLiteral::String here.
This should not happen in practice, because you would only do safe
conversions. For example, the expression `a > 3.25` should parse to an
expression that has enough precision to capture the original value. If `a`
would be of type `float`, then you would convert the literal to a float one
once you bind to the schema.
> Error message is just an example, not all use cases. For example when we
convert unbound expression to bound expression, how do we know its original
scale?
Also in the case of evolution, the precision can be widened. When we bind
the decimal to the actual schema, we'll [set the scale using
`set_scale`](https://docs.rs/rust_decimal/latest/rust_decimal/struct.Decimal.html#method.set_scale).
> String is enough for storing decimal, or everything, but it maybe weird in
api, since with only a string we don't know its original type, e.g. user may
write an unbound expression like a < "3.23", where a is a decimal and it's
legal to compare it with string.
SQL is a perfect example of that API I would say. Once you resolve the type
of `a < "3.23"`, [you'll call
`i128::from_str("3.23")`](https://docs.rs/rust_decimal/latest/rust_decimal/prelude/trait.FromStr.html#tymethod.from_str).
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