Limitations - Amazon Redshift
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Limitations

The following are limitations for using HyperLogLog in Amazon Redshift:

  • Amazon Redshift tables don't support an HLLSKETCH column as a sort key or a distribution key for an Amazon Redshift table.

  • Amazon Redshift doesn't support HLLSKETCH columns in ORDER BY, GROUP BY, or DISTINCT clauses.

  • You can only UNLOAD HLLSKETCH columns to text or CSV format. Amazon Redshift then writes the HLLSKETCH data in either a JSON format or a Base64 format. For more information about UNLOAD, see UNLOAD.

  • Amazon Redshift only supports HyperLogLog sketches with a precision (logm value) of 15.

  • JDBC and ODBC drivers don't support the HLLSKETCH data type. Therefore, the result set uses VARCHAR to represent the HLLSKETCH values.

  • Amazon Redshift Spectrum doesn't natively support the HLLSKETCH data. Therefore, you can't create or alter an external table with an HLLSKETCH column.

  • Data types for Python user-defined functions (UDFs) don't support the HLLSKETCH data type. For more information about Python UDFs, see Creating a scalar Python UDF.