Optimize lxml iterparse memory consumption

eurostat_to_dbnomics.py

@@ -240,6 +240,14 @@ def iter_git_objects_in_sdmx_file(sdmx_file_path, category_tree, validate_json):
                 yield series_tree
                 dataset_tree.add(series_code.encode('utf-8'), git_tree_filemode, series_tree.id)
 
+                # From https://stackoverflow.com/questions/12160418/why-is-lxml-etree-iterparse-eating-up-all-my-memory
+                # It's safe to call clear() here because no descendants will be accessed
+                series_element.clear()
+                # Also eliminate now-empty references from the root node to elem
+                for ancestor in series_element.xpath('ancestor-or-self::*'):
+                    while ancestor.getprevious() is not None:
+                        del ancestor.getparent()[0]
+
     # Write dataset.json
 
     if validate_json: