asolimando opened a new pull request, #19957:
URL: https://github.com/apache/datafusion/pull/19957
## Which issue does this PR close?
- Part of #15265
Related: #18628, #8227
(I am not sure if an new issue specifically for the scope of the PR is
needed, happy to create it if needed)
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## Rationale for this change
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This work originates from a discussion in datafusion-distributed about
improving the `TaskEstimator` API:
https://github.com/datafusion-contrib/datafusion-distributed/issues/296#issuecomment-3777726928
We agreed that improved statistics support in DataFusion would benefit both
projects. For distributed-datafusion, better cardinality estimation helps
decide how to split computation across network boundaries.
This also benefits DataFusion directly, as CBO is already in place, for
example, join cardinality estimation
([`joins/utils.rs:586-646`](https://github.com/apache/datafusion/blob/main/datafusion/physical-plan/src/joins/utils.rs#L586-L646))
uses `distinct_count` via `max_distinct_count` to compute join selectivity.
Currently this field is always `Absent` when reading from Parquet, so this
PR fills that gap.
## What changes are included in this PR?
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Commit 1 - Reading NDV from Parquet files:
- Extract `distinct_count` from Parquet row group column statistics
- Single row group with NDV -> `Precision::Exact(ndv)`
- Multiple row groups with NDV -> `Precision::Inexact(max)` as conservative
lower bound
- No NDV available -> `Precision::Absent`
Commit 2 - Statistics propagation (can be split to a separate PR, if
preferred):
- `Statistics::try_merge()`: use max as conservative lower bound instead of
discarding NDV
- `Projection`: preserve NDV for single-column expressions as upper bound
I'm including the second commit to showcase how I intend to use the
statistics, but these changes can be split to a follow-up PR to keep review
scope limited.
## Are these changes tested?
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Yes, 7 unit tests are added for NDV extraction:
- Single/multiple row groups with NDV
- Partial NDV availability across row groups
- Multiple columns with different NDV values
- Integration test reading a real Parquet file with distinct_count
statistics (following the pattern in
[`row_filter.rs:685-696`](https://github.com/apache/datafusion/blob/main/datafusion/datasource-parquet/src/row_filter.rs#L685-L696),
using `parquet_to_arrow_schema` to derive the schema from the file)
## Are there any user-facing changes?
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No breaking changes. Statistics consumers will now see populated
`distinct_count` values when available in Parquet metadata.
Disclaimer: I used AI (Claude Code) to assist translating my ideas into code
as I am still ramping up with the codebase and especially with Rust (guidance
on both aspects is highly appreciated). I have a good understanding of the core
concepts (statistics, CBO etc.) and have carefully double-checked that the PR
matches my intentions and understanding.
cc: @gabotechs @jayshrivastava @NGA-TRAN @gene-bordegaray
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