add support for casting string to decimal

[kmitchener]

Is your feature request related to a problem or challenge? Please describe what you are trying to do. A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] (This section helps Arrow developers understand the context and why for this feature, in addition to the what)

Describe the solution you'd like A clear and concise description of what you want to happen. Would like to be able to cast string to decimal as is supported by float types.

❯ select arrow_typeof('1.1'::decimal);
Internal("Unsupported CAST from Utf8 to Decimal128(38, 10)")

but should work like it does for float:

❯ select arrow_typeof('1.1'::double);
+--------------------------+
| arrowtypeof(Utf8("1.1")) |
+--------------------------+
| Float64                  |
+--------------------------+
1 row in set. Query took 0.010 seconds.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Additional context Add any other context or screenshots about the feature request here.

[liukun4515]

about the cast, you can refer the https://github.com/apache/arrow-rs/issues/1043 https://github.com/apache/arrow-datafusion/issues/1443

we don't support the cast/try_cast from decimal to utf8 or utf8 to decimal

you can implement this in the arrow-rs kernel about the cast. @kmitchener

[alamb]

Here is a PR to add the appropriate support in arrow-rs: https://github.com/apache/arrow-rs/pull/3106 👍

[Dandandan]

❯ select arrow_typeof('1.1'::decimal);
+--------------------------+
| arrowtypeof(Utf8("1.1")) |
+--------------------------+
| Decimal128(38, 10)       |
+--------------------------+
1 row in set. Query took 0.030 seconds.

[finlaydotb]

❯ select arrow_typeof('1.1'::decimal);
+--------------------------+
| arrowtypeof(Utf8("1.1")) |
+--------------------------+
| Decimal128(38, 10)       |
+--------------------------+
1 row in set. Query took 0.030 seconds.

Maybe someone can help clarify...Why does this come out as Decimal128(38, 10)? Where is the 38 and 10 coming from

[alamb]

The 38 is the maximum precision of Decimal128 in arrow

https://docs.rs/arrow-schema/51.0.0/src/arrow_schema/datatype.rs.html#676

The 10 is the default scale: https://docs.rs/arrow-schema/51.0.0/src/arrow_schema/datatype.rs.html#689