Jefffrey opened a new pull request, #4661:
URL: https://github.com/apache/arrow-datafusion/pull/4661
# Which issue does this PR close?
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Closes #3658.
# Rationale for this change
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# What changes are included in this PR?
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During CSV schema inference, instead of reading entire object/file into
memory before taking as many rows as configured to infer the schema, will
instead stream in the CSV (using helper method from object_store) which feeds
into newline_delimited_stream to produce the stream of CSV rows, which can be
read as needed instead of needing entire file in memory.
Certain adjustments needed for this:
- Only read header for first chunk (if header present)
- As inferring chunks of the file separately, must merge the inferred
datatypes of these chunks together akin to what
`arrow::csv::reader::infer_reader_schema(...)` already does
- If has header row, and in first chunk only the header row was read (i.e.
no data rows) then must ignore the datatype (as will default to Utf8 which can
cause issues with merging the schemas of the chunks)
# Are these changes tested?
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CSV schema inference logic covered by existing tests.
(Could add benchmark since this is meant to be performance improvement?)
# Are there any user-facing changes?
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