15.10 - FALLBACK - Teradata Database

Teradata Database SQL Data Definition Language Syntax and Examples

Teradata Database
December 2015
Programming Reference

The join index uses fallback protection.
FALLBACK is the default. 
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. Support for Read From Fallback is limited to the following cases:

  • 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.
An optional keyword.
If a join index is not fallback protected, an AMP failure prevents the following events from occurring:
  • Processing queries that use the join index.
  • Updating the base table on which the join index is defined.