George Sakkis created ARROW-4076:
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             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:
{noformat}
--- 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)
             self._filter(filters)
 
+        if validate_schema:
+            self.validate_schemas()
+
     def validate_schemas(self):
         open_file = self._get_open_file_func()
{noformat}



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