Jefffrey opened a new pull request, #4849:
URL: https://github.com/apache/arrow-datafusion/pull/4849
# Which issue does this PR close?
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Closes #4667
# Rationale for this change
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During PushDownProjection optimization, it does not properly account for
distinct, as it will attempt to push directly through the distinct, from input
plan:
```sql
Projection: data.id
Distinct:
Projection: data.id, data.data
TableScan: data
```
Will generate output plan:
```sql
Projection: data.id
Distinct:
TableScan: data projection=[id]
```
Where it has inadvertently removed the `data.data` projection during the
push down and hence leading to incorrect distinct behaviour.
# What changes are included in this PR?
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sometimes worth providing a summary of the individual changes in this PR.
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Properly handle distincts during recursion in PushDownProjection, to
'restart' the traversal with all required columns (derived from schema of the
distinct) rather than using the input required columns.
After fix, the above plan output will instead be:
```sql
Projection: data.id
Distinct:
TableScan: data projection=[id, data]
```
# Are these changes tested?
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New unit test in push_down_projection
# Are there any user-facing changes?
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