[jira] [Created] (ARROW-5747) [C++] Better column name and header support in CSV reader

["Neal Richardson (JIRA)" <ji...@apache.org>]

Neal Richardson created ARROW-5747:
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             Summary: [C++] Better column name and header support in CSV reader
                 Key: ARROW-5747
                 URL: https://issues.apache.org/jira/browse/ARROW-5747
             Project: Apache Arrow
          Issue Type: Improvement
            Reporter: Neal Richardson


While working on ARROW-5500, I found a number of issues around the CSV parse options {{header_rows}}:
 * If header_rows is 0, [the reader errors|https://github.com/apache/arrow/blob/8b0318a11bba2aa2cf39bff245ff916a3283d372/cpp/src/arrow/csv/reader.cc#L150]
 * It's not possible to supply your own column names, as [this TODO|https://github.com/apache/arrow/blob/8b0318a11bba2aa2cf39bff245ff916a3283d372/cpp/src/arrow/csv/reader.cc#L149] notes. ARROW-4912 allows renaming columns after reading in, which _maybe_ is enough as long as header_rows == 0 doesn't error, but then you can't naturally specify column types in the convert options because that takes a map of column name to type.
 * If header_rows is > 1, every cell gets turned into a column name, so if header_rows == 2, you get twice the number of column names as columns. This doesn't error, but it leads to unexpected results.

IMO a better interface would be to have a {{skip_rows}} argument to let you ignore a large header, and a {{column_names}} argument that, if provided, gives the column names. If not provided, the first row after {{skip_rows}} is taken to be the column names. I don't think there's value in trying to be clever about multirow headers and converting those to column names; if there's meaningful information in a tall header, let the user parse it themselves.



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