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[jira] [Created] (ARROW-4076) [Python] schema validation and filters

George Sakkis created ARROW-4076:

             Summary: [Python] schema validation and filters
                 Key: ARROW-4076
                 URL: https://issues.apache.org/jira/browse/ARROW-4076
             Project: Apache Arrow
          Issue Type: Bug
          Components: Python
            Reporter: George Sakkis

Currently [schema validation|https://github.com/apache/arrow/blob/758bd557584107cb336cbc3422744dacd93978af/python/pyarrow/parquet.py#L900] of {{ParquetDataset}} takes place before filtering. This may raise a {{ValueError}}if the schema is different in some dataset pieces, even if these pieces would be subsequently filtered out. I think validation should happen after filtering to prevent such spurious errors:
--- a/pyarrow/parquet.py	
+++ b/pyarrow/parquet.py	
@@ -878,13 +878,13 @@
         if split_row_groups:
             raise NotImplementedError("split_row_groups not yet implemented")
-        if validate_schema:
-            self.validate_schemas()
         if filters is not None:
             filters = _check_filters(filters)
+        if validate_schema:
+            self.validate_schemas()
     def validate_schemas(self):
         open_file = self._get_open_file_func()

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