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The cardinality aggregation estimates the number of distinct values in a field. 
SELECT pdb.agg('{"cardinality": {"field": "rating"}}') FROM mock_items
WHERE id @@@ pdb.all();
Expected Response
      agg
----------------
 {"value": 5.0}
(1 row)
Unlike SQL’s DISTINCT clause, which returns an exact value but is very computationally expensive, the cardinality aggregation uses the HyperLogLog++ algorithm to closely approximate the number of distinct values. See the Tantivy documentation for all available options.