[GitHub] [arrow-datafusion] tustvold commented on issue #3658: CSV inference reads in the whole file to memory, regardless of row limit

[GitBox <gi...@apache.org>]

tustvold commented on issue #3658:
URL: https://github.com/apache/arrow-datafusion/issues/3658#issuecomment-1270008874

   You will want to do something vaguely like (not properly tested)
   
   ```
   async fn infer_schema(
           &self,
           store: &Arc<dyn ObjectStore>,
           objects: &[ObjectMeta],
       ) -> Result<SchemaRef> {
           let mut schemas = vec![];
   
           let mut records_to_read = self.schema_infer_max_rec.unwrap_or(usize::MAX);
   
           for object in objects {
               match store.get(&object.location).await? {
                   GetResult::File(f, _) => {
                       let (schema, records_read) = arrow::csv::reader::infer_reader_schema(
                           f,
                           self.delimiter,
                           Some(records_to_read),
                           self.has_header,
                       )?;
                       schemas.push(schema);
                       records_to_read -= records_read;
                   }
                   GetResult::Stream(s) => {
                       let mut has_header = self.has_header;
                       let s = newline_delimited_stream(s.map_err(Into::into));
                       pin_mut!(s);
                       while let Some(bytes) = s.next().await.transpose()? {
                           let (schema, records_read) =
                               arrow::csv::reader::infer_reader_schema(
                                   bytes.as_ref(),
                                   self.delimiter,
                                   Some(records_to_read),
                                   has_header,
                               )?;
                           has_header = false;
   
                           schemas.push(schema);
                           records_to_read -= records_read;
                           if records_to_read == 0 {
                               break;
                           }
                       }
                   }
               }
           }
   
           let merged_schema = Schema::try_merge(schemas)?;
           Ok(Arc::new(merged_schema))
       }
   ```
   
   
   
   
   


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