Manually Purging Data

You manually purge deleted data as follows:

1. Set the cut-off date for purging deleted data. First, call one of the following functions to verify the current ancient history mark (AHM):
   • GET_AHM_TIME returns a TIMESTAMP value of the AHM.
   • GET_AHM_EPOCH returns the number of the epoch in which the AHM is located.
2. Set the AHM to the desired cut-off date with one of the following functions:
   • SET_AHM_TIME sets the AHM to the epoch that includes the specified TIMESTAMP value on the initiator node.
   • SET_AHM_EPOCH sets the AHM to the specified epoch.
   • MAKE_AHM_NOW sets the AHM to the greatest allowable value. This lets you purge all deleted data.

   If you call SET_AHM_TIME, keep in mind that the timestamp you specify is mapped to an epoch, which by default has a three-minute granularity. Thus, if you specify an AHM time of 2008-01-01 00:00:00.00, Vertica might purge data from the first three minutes of 2008, or retain data from last three minutes of 2007.

   You cannot advance the AHM beyond a point where Vertica is unable to recover data for a down node.

3. Purge deleted data from the desired projections with one of the following functions:
   • PURGE purges all projections in the physical schema.
   • PURGE_TABLE purges all projections anchored to the specified table.
   • PURGE_PROJECTION purges the specified projection.
   • PURGE_PARTITION purges a specified partition.

   The tuple mover performs a mergeout operation to purge the data. Vertica periodically invokes the tuple mover to perform mergeout operations, as configured by tuple mover parameters. You can manually invoke the tuple mover by calling the function DO_TM_TASK.

See Epochs for additional information about how Vertica uses epochs.