codewarnab opened a new pull request, #27011:
URL: https://github.com/apache/datafusion/pull/27011
## Which issue does this PR close?
- Part of #15774.
## Rationale for this change
The TopK example in `user_defined_plan.rs` shows how to add a custom
operator to DataFusion. Users who read the Extending Operators guide cannot
find it there. This PR adds a short walkthrough of the example to that guide.
## What changes are included in this PR?
- A new "A custom TopK operator" section in `extending-operators.md`. It
covers:
- the custom logical node, `TopKPlanNode`
- the optimizer rule that adds the node to the plan
- the physical operator that runs it
- the limits of the example
- A small update to the intro, so that it covers both examples.
The µWheel section, the example code, and its invariant tests are unchanged.
A follow-up PR will move the invariant tests into focused unit tests. That PR
will also update the link in this section if the example moves.
## What is the testing strategy for this PR?
This PR changes only documentation. The snippets and descriptions match the
current code in `user_defined_plan.rs`.
## Are there any user-facing changes?
Yes. The library user guide has a new section. There are no API changes.
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