asolimando commented on code in PR #19957:
URL: https://github.com/apache/datafusion/pull/19957#discussion_r2728830062
##########
datafusion/physical-expr/src/projection.rs:
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@@ -660,9 +660,25 @@ impl ProjectionExprs {
}
}
} else {
- // TODO stats: estimate more statistics from expressions
- // (expressions should compute their statistics themselves)
- ColumnStatistics::new_unknown()
+ // TODO: expressions should compute their own statistics
Review Comment:
Good question! I have an idea of how this could evolve, based on my
experience with Apache Calcite.
The idea is to make statistics propagation pluggable, with each relational
operator having a default but configurable logic for how statistics propagate
through it.
The default implementation would follow classic Selinger-style estimation
(selectivity factors, independence assumptions), as seen in this PR. A nice
intro to this can be found
[here](https://15799.courses.cs.cmu.edu/spring2025/slides/13-cardinalities1.pdf).
That's what most OSS databases implement by default.
Following DataFusion's philosophy as a customizable framework, users should
be able to override and complement this logic when needed.
Proposed architecture:
- `StatisticsProvider`: chain element that computes statistics for specific
operators (returns `Computed` or `Delegate`)
- `StatisticsRegistry`: chains providers, would live in `SessionState`
- `CardinalityEstimator`: unified interface for metadata queries (row count,
selectivity, NDV, ...) - similar to Calcite's
[RelMetadataProvider](https://calcite.apache.org/javadocAggregate/org/apache/calcite/rel/metadata/RelMetadataProvider.html)/[RelMetadataQuery](https://calcite.apache.org/javadocAggregate/org/apache/calcite/rel/metadata/RelMetadataQuery.html)
- `ExtendedStatistics`: `Statistics` with type-safe custom extensions for
histograms, sketches, etc. (I am looking at type-erased maps for that but I am
not sure that's the best way to implement it)
- `ExpressionAnalyzerRegistry`+`ExpressionAnalyzer`: similar concept of
chain for expression analyzers, equivalent to what detailed above for the
operators, so that built-in and UDF can be covered
This follows the same chain-of-responsibility pattern that the
https://datafusion.apache.org/blog/2026/01/12/extending-sql/ solved for custom
syntax/relations. Built-in operators get default handling, custom
`ExecutionPlan` nodes can plug in their own logic, and unknown cases delegate
down the chain. To override the default estimation (e.g., with histogram-based
approaches), you register your provider before the default one.
StatisticsV2 and Distribution-based are very advanced and interesting
statistics, but I see them more as an extension via `ExtendedStatistics` than
taking over the default implementation.
If this sounds interesting and aligns with community interest, I can provide
a more detailed design doc and an epic to break down the work.
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