- You cannot use ALC to compress columns that have a data type of structured UDT.
The TD_LZ_COMPRESS and TD_LZ_DECOMPRESS system functions compress all large UDTs including UDT-based system types such as Geospatial, XML, and JSON. However, if you write your own compression functions, the following restrictions apply:
- Custom compression functions cannot be used to compress UDT-based system types (except for ARRAY and Period types).
- Custom compression functions cannot be used to compress distinct UDTs that are based on UDT-based system types (except for ARRAY and Period types).
- You cannot write your own compression functions to perform algorithmic compression on JSON type columns. However, Teradata provides the JSON_COMPRESS and JSON_DECOMPRESS functions that you can use to perform ALC on JSON type columns.
- You cannot use ALC to compress temporal columns:
- A column defined as SYSTEM_TIME, VALIDTIME, or TRANSACTIONTIME.
- The DateTime columns that define the beginning and ending bounds of a temporal derived period column (SYSTEM_TIME, VALIDTIME, or TRANSACTIONTIME).
You can use ALC to compress period data types in columns that are nontemporal; however, you cannot use ALC to compress derived period columns.
For details about temporal tables, see Temporal Table Support and ANSI Temporal Table Support.
- You cannot specify multi-value or algorithmic compression for a row-level security constraint column.