system will read the fallback data if a node or diskarray is going down but you can't influence this with a SQL.
What would be the benifit if direct accessing it?
I am thinking from many directions, say that if my amp is down and in recovery stage online, then without wasting time, I can can redirect my query to read from fallback or primary, my choice.
There maybe critical transactions that I can diff the image of both, for amps in same cluster , when one amp is down...say
What you are describing is all automatic in Teradata. If an AMP is down, and fallback is in place, then the query automatically reads the fallback copy and reads all current data. If updates are performed, they are applied to the fallback copy automatically and queries raed those updates as tehy would in the primary copy. The updates are also saved to be reapplied to the primary when it is available. When the AMP comes back up, the data for the primary is recovered from fallback and from any saved updates until the primary is again equivalent to the fallback copy - all automatically. While this recovery is going on, any queries automatically continue to read the fallback copy. When the AMP is caught up with its recovery, a short system restart is performed to return the failed amp to the configuration and queries then automatically go back to reading the primary copy.
It's not your choice, it's already done by the system, that's why there's Fallback :-)
When an AMP looses it's vDisk, the system restarts and all queries accessing Fallback-protected tables on that down-AMP are automatically routed to the Fallback-AMP. When the AMP is online again,a Table Rebuild automatically recreates the lost data.
Of course only for Fallback tables, otherwise you need to restore from a backup.
I have faced a problem.
I have deleted some record from a fallback table using TPT but during deletion it is failing for no room space.
After that I select the table but is showing it show Mload is being loaded on the table.
After that I am trying the release mload "tableName" in apply but it shows that fallback recovers the data and it takes more time.
Now how can I release form fallback and mload ?