SET_AHM_EPOCH
Sets the Ancient History Mark (AHM) to the specified epoch. This function allows deleted data up to and including the AHM epoch to be purged from physical storage.
SET_AHM_EPOCH is normally used for testing purposes. Instead, consider using
SET_AHM_TIME which is easier to use.
This is a meta-function. You must call meta-functions in a top-level SELECT statement.
Behavior type
VolatileSyntax
SET_AHM_EPOCH ( epoch, [ true ] )
Parameters
epoch- Specifies one of the following:
-
The number of the epoch in which to set the AHM
-
Zero (0) (the default) disables PURGE
Important
The number of the specified epoch must be:
-
Greater than the current AHM epoch
-
Less than the current epoch
Query the SYSTEM table to view current epoch values relative to the AHM.
-
true- Allows the AHM to advance when nodes are down.
Caution
If you advance AHM beyond the last good epoch of the down nodes, those nodes must recover all data from scratch.
Privileges
Superuser
Setting AHM when nodes are down
If any node in the cluster is down, you must call SET_AHM_EPOCH with an argument of true; otherwise, the function returns an error.
Note
This requirement applies only to Enterprise mode; in Eon mode, it is ignored.Examples
The following command sets the AHM to a specified epoch of 12:
=> SELECT SET_AHM_EPOCH(12);
The following command sets the AHM to a specified epoch of 2 and allows the AHM to advance despite a failed node:
=> SELECT SET_AHM_EPOCH(2, true);