Segmented Projections

You typically create segmented projections for large fact tables. Vertica splits segmented projections into chunks (segments) of similar size and distributes these segments evenly across the cluster. System K-safety determines how many duplicates (buddies) of each segment are created and maintained on different nodes.

You create segmented projections with a CREATE PROJECTION statement that includes a SEGMENTED BY clause.

The following CREATE PROJECTION statement creates projection public.employee_dimension_super. It specifies to include all columns in table public.employee_dimension. The hash segmentation clause invokes the Vertica HASH function to segment projection data on the column employee_key; it also includes the ALL NODES clause, which specifies to distribute projection data evenly across all nodes in the cluster:

=> CREATE PROJECTION public.employee_dimension_super 
    AS SELECT * FROM public.employee_dimension
    ORDER BY employee_key
    SEGMENTED BY hash(employee_key) ALL NODES;

If the database is K-safe, Vertica creates multiple buddies for this projection and distributes them on different nodes across the cluster. In this case, database K-safety is set to 1, so Vertica creates two buddies for this projection. It uses the projection name employee_dimension_super as the basename for the two buddy identifiers it creates—in this example, employee_dimension_super_b0 and employee_dimension_super_b1:

=> SELECT projection_name FROM projections WHERE projection_basename='employee_dimension_super';
(2 rows)