geoffreyclaude opened a new pull request, #19265:
URL: https://github.com/apache/datafusion/pull/19265
## Which issue does this PR close?
- Closes #19087.
## Rationale for this change
As noted in the issue, the `RelationPlanner` API (added in #17843) along
with `ExprPlanner` and `TypePlanner` allows DataFusion users to extend SQL in
powerful ways. However, these extension points are not well documented and may
be difficult for users to discover.
## What changes are included in this PR?
Adds a new Library User Guide page (`extending-sql.md`) that documents how
to extend DataFusion's SQL syntax:
1. **Introduction** explaining why SQL extensibility matters (different SQL
dialects, custom operators)
2. **Architecture overview** showing how SQL flows through the planner
extension points
3. **Extension points** with examples for each:
- `ExprPlanner`: Custom expressions and operators (e.g., `->` for JSON)
- `TypePlanner`: Custom SQL data types (e.g., `DATETIME` with precision)
- `RelationPlanner`: Custom FROM clause elements (TABLESAMPLE,
PIVOT/UNPIVOT)
4. **Implementation strategies** for RelationPlanner:
- Rewrite to standard SQL (PIVOT/UNPIVOT example)
- Custom logical and physical nodes (TABLESAMPLE example)
5. **Links to complete examples** in
`datafusion-examples/examples/relation_planner/`
Also adds a cross-reference from the existing `ExprPlanner` section in
`adding-udfs.md` to the new comprehensive guide.
## Are these changes tested?
This is documentation only. The code examples reference existing tested
examples in `datafusion-examples/`.
## Are there any user-facing changes?
Yes, new documentation is added to help users extend DataFusion's SQL syntax.
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