PROJS

Specifies one or more projections to use for a queried table.

The PROJS hint can specify multiple projections; the optimizer determines which ones are valid and uses the one that is most cost-effective for the queried table. If no hinted projection is valid, the query returns a warning and ignores projection hints.

Syntax

FROM `*`table-name`*` /*+PROJS( [[`*`database`*`.]`*`schema.`*`]`*`projection`*`[,...] )*/

Arguments

[database.]schema

Database and schema. The default schema is public. If you specify a database, it must be the current database.

projection

The projection to use. You can specify a list of comma-delimited projections.

Examples

The employee_dimension table has two projections: segmented superprojection public.employee_dimension, which includes all table columns; and the unsegmented projection public.employee_dimension_rep, which includes a subset of the columns:

=> SELECT export_objects('','employee_dimension');
                                                      export_objects
--------------------------------------------------------------------------------------------------------------------------

CREATE TABLE public.employee_dimension
(
    employee_key int NOT NULL,
    employee_gender varchar(8),
    courtesy_title varchar(8),
    employee_first_name varchar(64),
    employee_middle_initial varchar(8),
    employee_last_name varchar(64),
    employee_age int,
    hire_date date,
    employee_street_address varchar(256),
    employee_city varchar(64),
    employee_state char(2),
    employee_region char(32),
    job_title varchar(64),
    reports_to int,
    salaried_flag int,
    annual_salary int,
    hourly_rate float,
    vacation_days int,
    CONSTRAINT C_PRIMARY PRIMARY KEY (employee_key) DISABLED
);

CREATE PROJECTION public.employee_dimension
...
AS
 SELECT employee_dimension.employee_key,
        employee_dimension.employee_gender,
        employee_dimension.courtesy_title,
        employee_dimension.employee_first_name,
        employee_dimension.employee_middle_initial,
        employee_dimension.employee_last_name,
        employee_dimension.employee_age,
        employee_dimension.hire_date,
        employee_dimension.employee_street_address,
        employee_dimension.employee_city,
        employee_dimension.employee_state,
        employee_dimension.employee_region,
        employee_dimension.job_title,
        employee_dimension.reports_to,
        employee_dimension.salaried_flag,
        employee_dimension.annual_salary,
        employee_dimension.hourly_rate,
        employee_dimension.vacation_days
 FROM public.employee_dimension
 ORDER BY employee_dimension.employee_key
SEGMENTED BY hash(employee_dimension.employee_key) ALL NODES KSAFE 1;

CREATE PROJECTION public.employee_dimension_rep
...
AS
 SELECT employee_dimension.employee_key,
        employee_dimension.employee_gender,
        employee_dimension.employee_first_name,
        employee_dimension.employee_middle_initial,
        employee_dimension.employee_last_name,
        employee_dimension.employee_age,
        employee_dimension.employee_street_address,
        employee_dimension.employee_city,
        employee_dimension.employee_state,
        employee_dimension.employee_region
 FROM public.employee_dimension
 ORDER BY employee_dimension.employee_key
UNSEGMENTED ALL NODES;

SELECT MARK_DESIGN_KSAFE(1);

(1 row)

The following query selects all table columns from employee_dimension and includes the PROJS hint, which specifies both projections. public.employee_dimension_rep does not include all columns in the queried table, so the optimizer cannot use it. The segmented projection includes all table columns so the optimizer uses it, as verified by the following query plan:

=> EXPLAIN SELECT * FROM employee_dimension /*+PROJS('public.employee_dimension_rep', 'public.employee_dimension')*/;

QUERY PLAN DESCRIPTION:
------------------------------
EXPLAIN SELECT * FROM employee_dimension /*+PROJS('public.employee_dimension_rep', 'public.employee_dimension')*/;

 Access Path:
 +-STORAGE ACCESS for employee_dimension [Cost: 177, Rows: 10K (NO STATISTICS)] (PATH ID: 1)
 |  Projection: public.employee_dimension_b0