yukkit opened a new pull request, #8143:
URL: https://github.com/apache/arrow-datafusion/pull/8143
## Which issue does this PR close?
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Closes #7923 .
Current Pull Request is an Experimental Demo for Validating the Feasibility
of Logical Types
## Rationale for this change
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## What changes are included in this PR?
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## Features
- Create User-Defined Types (UDTs) through SQL, specifying the field types
as UDTs during table creation.
- Support the use of `UDT` as a function signature in `udf/udaf`.
- Register extension types through the `register_data_type` function in the
`SessionContext`.
## New Additions
- `LogicalType` struct.
- `ExtensionType` trait. Abstraction for extension types.
- `TypeSignature` struct. Uniquely identifies a data type.
## Major Changes
- Added `get_data_type(&self, _name: &TypeSignature) -> Option<LogicalType>`
function to the `ContextProvider` trait.
- In `DFSchema`, `DFField` now uses `LogicalType`, removing arrow `Field`
and retaining only `data_type`, `nullable`, `metadata` since `dict_id`,
`dict_is_ordered` are not necessary at the logical stage.
- `ExprSchemable` and `ExprSchema` now use `LogicalType`.
- `ast` to logical plan conversion now uses `LogicalType`.
## To Be Implemented
- `TypeCoercionRewriter` in the analyze stage uses logical types. For
example, functions like `comparison_coercion`, `get_input_types`,
`get_valid_types`, etc.
- Functions signatures for `udf/udaf` use `TypeSignature` instead of the
existing `DataType` for ease of use in `udf/udaf`.
## To Be Determined
- Should `ScalarValue` use `LogicalType` or arrow `DataType`?
- Should `TableSource` return `DFSchema` or arrow `Schema`?
* Still using `Schema`.
- Conversion between physical types and logical types (in Datafusion, type
conversion is achieved through the conversion of `DFSchema` to `Schema`;
logical plans use `DFSchema`, physical plans use `Schema`).
- Conversion between `Schema` and `DFSchema`
* When to convert `Schema` to `DFSchema`? During the construction of the
logical `TableScan` node, obtain arrow `Schema` through `TableSource` and then
convert it to `DFSchema`.
* When to convert `DFSchema` to `Schema`? Directly obtain arrow `Schema`
from `TableSource` in physical planner, no need for conversion.
## Some Thoughts
- In [this
comment](https://github.com/apache/arrow-rs/issues/4472#issuecomment-1749805039),
the use case of converting from `dyn Array` to `LineStringArray` or
`MultiPointArray` was raised. In my perspective, assuming there is a function
specifically designed for handling `LineString` data, the function signature
can be defined as `LineString`, ensuring that the input data must be of a type
acceptable by `LineStringArray`.
## Are these changes tested?
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## Are there any user-facing changes?
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