Refreshing projections

When you create a projection for a table that already contains data, Vertica does not automatically load that data into the new projection.

When you create a projection for a table that already contains data, Vertica does not automatically load that data into the new projection. Instead, you must explicitly refresh that projection. Until you do so, the projection cannot participate in executing queries on its anchor table.

You can refresh a projection with one of the following functions:

  • START_REFRESH refreshes projections in the current schema with the latest data of their respective anchor tables. START_REFRESH runs asynchronously in the background.

  • REFRESH synchronously refreshes one or more table projections in the foreground.

Both functions update system tables that maintain information about a projection's refresh status: PROJECTION_REFRESHES, PROJECTIONS, and PROJECTION_CHECKPOINT_EPOCHS.

If a refresh would violate a table or schema disk quota, the operation fails. For more information, see Disk quotas.

Getting projection refresh information

You can query the PROJECTION_REFRESHES and PROJECTIONS system tables to view the progress of the refresh operation. You can also call the GET_PROJECTIONS function to view the final status of projection refreshes for a given table:

=> SELECT GET_PROJECTIONS('customer_dimension');
 Current system K is 1.
# of Nodes: 3.
Table public.customer_dimension has 2 projections.

Projection Name: [Segmented] [Seg Cols] [# of Buddies] [Buddy Projections] [Safe] [UptoDate] [Stats]
public.customer_dimension_b1 [Segmented: Yes] [Seg Cols: "public.customer_dimension.customer_key"] [K: 1]
       [public.customer_dimension_b0] [Safe: Yes] [UptoDate: Yes] [Stats: RowCounts]
public.customer_dimension_b0 [Segmented: Yes] [Seg Cols: "public.customer_dimension.customer_key"] [K: 1]
       [public.customer_dimension_b1] [Safe: Yes] [UptoDate: Yes] [Stats: RowCounts]

(1 row)

Refresh methods

Vertica can refresh a projection from one of its buddies, if one is available. In this case, the target projection gets the source buddy's historical data. Otherwise, the projection is refreshed from scratch with data of the latest epoch at the time of the refresh operation. In this case, the projection cannot participate in historical queries on any epoch that precedes the refresh operation.

Vertica can perform incremental refreshes when the following conditions are met:

  • The table being refreshed is partitioned.

  • The table does not contain any unpartitioned data.

  • The operation is a full projection refresh (not a partition range projection refresh).

In an incremental refresh, the refresh operation first loads data from the partition with the highest range of keys. After refreshing this partition, Vertica begins to refresh the partition with next highest partition range. This process continues until all projection partitions are refreshed. While the refresh operation is in progress, projection partitions that have completed the refresh process become available to process query requests.

The method used to refresh a given projection is recorded in the REFRESH_METHOD column of the PROJECTION_REFRESHES system table.