tobixdev opened a new pull request, #15106:
URL: https://github.com/apache/datafusion/pull/15106
Related to: #14828, #12644, #14247
This PR is a *rough* proposal for an implementation of user-defined sorting.
The main goal is to get a discussion starting whether this is a direction we
want to go.
Some design considerations that help understand the PR:
- `PhysicalSortExpr` allows defining a custom sort order, but does not know
anything about extension types.
- "Resolving" logical types happens during the creation of the initial
physical plan.
- `LogicalTypePlanningInformation` allows parameterizing this creation
process
- Obtaining the logical type is currently done by inspecting the `Field`
whether there is an entry in the metadata with the key
`EXTENSION_TYPE_NAME_KEY`.
- An extension type registry is required for making use of this
information.
- Currently, this works only for direct access to columns. See #14247
for some discussions on this issue.
- In the future, maybe we can move `DFSchema` towards including a
`LogicalType` and a `DataType` (at least as a first step). The remaining
approach still works with such an assumption.
This is similar to "Option 1: User defined operators" from the discussion in
#14247. Here are some thoughts on that:
- From a logical standpoint, the expressions themselves do not change. It's
still a `SortExpr` just on a special type.
- Rewriting all `Expr` to UDFs via analyzers can be also tricky as we would
then, for example, need to parameterize `SortExpr` with a custom ordering to
allow such a rewriting.
- This makes the physical plan creation more involved.
- I think this is a great avenue to making it easy to work with custom types
in DataFusion. Currently, AFAIK, if you want to do custom sorting or use
`AggregateExec` (which assumes your data has a natural order) for your
extension types you basically have to re implement a custom physical node or
resort to work-arounds that cause problems (e.g., projecting out columns that
are sortable).
There are many changes in tests etc. so here is a list of "highlights" you
should check out when taking a look at this PR.
Highlights:
- `datafusion/common/src/sort.rs`: new sort data structures and adapted
procedures from arrow-rs.
- `datafusion/common/src/types/logical.rs`: Extension to logical type to
provide a `LogicalTypePlanningInformation`.
- `datafusion/core/tests/dataframe/test_types.rs`: A logical type
(`IntOrFloatType`) that implements a custom order.
- `datafusion/core/tests/dataframe/mod.rs`: A test for sorting a on union
type.
- `datafusion/core/src/datasource/mod.rs`: Applying custom order during
physical plan creation.
- `datafusion/core/src/physical_planner.rs`: Applying custom order during
physical plan creation.
To get (a similar) approach upstream I'd suggest the following steps
(splitting up the PR, adding tests etc.):
1. Add `SortOrdering` to `PhysicalSortExpr` and add support for user-defined
sorting operations (`DynComparator` for now)
2. Add `ExtensionTypeRegistry` to support registering extension types.
3.1 Use said registry to look up whether we have planning information for
`SortExpr` during planning and construct the correct `PhysicalSortExpr`.
3.X Adapt existing code that relies on the fact that types have a natural
ordering (e.g., `AggregateExec`). This step will be done interatively.
Note that this procedure leads to a state where some parts of DF already
make use of user defined sorting, while other do not yet support it. However, I
don't think this is a deal breaker.
I'm eager to hear your thoughts!
# Further Notes
I think this would also require users to return a `Field` from a UDF instead
of just a data type (#14247) to allow UDFs to return extension types. See TODO
note in `datafusion/core/src/physical_planner.rs`.
I think supported the row-based sorting from arrow-rs can also be supported
in a similar fashion by extending `CustomOrdering`.
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