When an ALTER TABLE request adds FALLBACK, the operation generates a full-table scan. To add FALLBACK to a table, all AMPs must be online and operational.
When a table or join index is partitioned, its fallback rows are partitioned the same as its primary data rows. If the primary data rows have a primary index, the fallback data rows have the same primary index. If the primary data rows do not have a primary index, the fallback data rows do not have a primary index.
You can change the FALLBACK properties of a global temporary table definition only if there are no materialized instances of that table anywhere in the system. However, when an instance of a global temporary table is materialized, Vantage creates the table without fallback irrespective of its definition unless there is a down AMP in the system. In that case only, the table is materialized with fallback defined to do so.
On the Block File System, enable the file system to detect all hardware read errors for tables and join indexes by setting CHECKSUM to ON. On the Object File System, CHECKSUM has a default value that you cannot change.
ALTER TABLE, FALLBACK, and Read from Fallback
A system must reconstruct data from fallback copies if a hardware read error occurs when the system tries to read the primary copy of the data. When a read error occurs, the file system reads the fallback copy of the rows and reconstructs a memory-resident image of them on their home AMP. This is called Read From Fallback. See Reading or Repairing Data from Fallback.
- Requests that do not try to modify data in the bad data block
- Primary subtable data blocks
- Reading the fallback data in place of the primary data.
Active Fallback may be able to repair the damage to the primary data dynamically. If not, Read From Fallback substitutes an error-free fallback copy of the corrupt rows each time the read error occurs.