Add duplicate copy protection for the table or user-defined join index. Adding fallback creates and stores a duplicate copy of the table. The FALLBACK default for a base data table is determined by the CREATE TABLE request when the table is created or the default fallback specification for the database or user containing the table. You can override these defaults with an ALTER TABLE request. An ALTER TABLE request that changes a table with block-level compression and also adds FALLBACK does not pass the block-level compression characteristics of the primary table for the newly created fallback table by default. Teradata Database assigns block-level compression to a newly created fallback table, depending on the request. If the request changes the row definition, adds fallback, and also specifies the BlockCompression query band, then the fallback table:
- Has block-level compression if the value for BlockCompression is set to YES.
- Does not have block-level compression if the value for BlockCompression is set to NO.
See “SET QUERY_BAND" in SQL Data Definition Language - Detailed Topics, B035-1184.
If the request does not specify the BlockCompression query band, the fallback table defaults to the system-wide compression characteristics defined by the compression columns of the DBS Control record. See Utilities, B035-1102.
When a hardware read error occurs, the file system reads the fallback copy of the data and reconstructs the rows in memory on their home AMP. Read From Fallback applies to:
- Requests that do not attempt to modify data in the bad data block
- Primary subtable data blocks
- Reading the fallback data in place of the primary data. In some cases, Active Fallback can repair the damage to the primary data dynamically. In situations where the bad data block cannot be repaired, Read From Fallback substitutes an error-free fallback copy of the corrupt rows each time the read error occurs.
For a system-defined join index, the FALLBACK modification you make on a base table also applies to any system-defined join indexes defined on that table. You alter the FALLBACK for a system-defined join index directly.
- Optional default keyword.
- Fallback protection for the table is removed. Removing fallback deletes the existing duplicate copy.