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HeatWave User Guide  /  ...  /  Dictionary Encoding

11.4.2 Dictionary Encoding

Dictionary encoding (SORTED) has the following characteristics:

  • Best suited to string columns with a low number of distinct values relative to the cardinality of the table. Dictionary encoding reduces the space required for column values on the HeatWave nodes but requires space on the MySQL DB System node for dictionaries.

  • Supports GROUP BY and ORDER BY operations on string columns.

  • Supports only a subset of the operations supported by variable-length encoding such as LIKE with prefix expressions, and comparison with the exact same column. Dictionary-encoded columns cannot be compared in any way with other columns or constants, or with other dictionary-encoded columns.

  • Does not support JOIN operations.

  • Does not support operations that use string operators. Queries that use string operators on dictionary-encoded string columns are not offloaded.

  • Does not support LIKE predicates.

  • Does not support comparison with variable-length encoded columns.

  • The dictionaries required to decode dictionary-encoded string columns must fit in MySQL DB System node memory. Dictionary size depends on the size of the column and the number of distinct values. Load operations for tables with dictionary-encoded string columns that have a high number of distinct values can fail if there is not enough available memory on the MySQL DB System node.