tustvold opened a new pull request, #2774:
URL: https://github.com/apache/arrow-rs/pull/2774
# Which issue does this PR close?
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Closes #1915
# Rationale for this change
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As described in
https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#backward-compatibility-rules
there are various backwards compatibility rules for converting parquet schema
to lists. This logic is applied by the schema inference logic added in
https://github.com/apache/arrow-rs/pull/1682, which "promotes" bare-repeated
fields to be ListArray in the generated arrow schema.
This works well, however, the mechanism by which array_reader is constructed
would perform this inference again, using the an inferred arrow schema, as
opposed to any schema embedded in the parquet metadata. Aside from being
potentially inefficient, this causes issues if the provided arrow schema no
longer matches the parquet schema, e.g. a field has been promoted to a
different arrow representation.
# What changes are included in this PR?
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This pushes projection into the array reader construction, in addition to
being more efficient, this avoids the double-inference issues. We infer the
schema once, and then project this.
# Are there any user-facing changes?
No
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