Archives of the TeradataForum
Message Posted: Thu, 29 Jun 2006 @ 22:44:46 GMT
| Subj: || || Re: Archival of fallback tables and USI when an AMP is down |
| From: || || Fred W Pluebell |
Fallback NUSI copies are NOT stored on the same AMP as the primary.
The issue is NUSI "rows" can contain multiple table RowIDs but only for data rows on the same AMP; and if the same NUSI value occurs on
multiple AMPs, each AMP will have a NUSI "row" for that value. Add to that the fact that the fallback hash map distributes the fallback copies of
rows from each AMP across multiple other AMPS. If there were a NUSI "restore from fallback" it would require something like scanning the entire
fallback copy, building "normalized" work rows each with only one table RowID, redistributing those, and recombining them. Teradata chose not to
implement that, reasoning that the effort to do so may actually be more than scanning the table and rebuilding the NUSI locally on the AMPs.
I'd suggest you carefully consider whether it's worthwhile to archive indexes at all in your environment. An archived index can only be
RESTOREd to the same configuration (not COPY, not RESTORE to a different configuration) and even then a rebuild would sometimes be faster.