OLAP_TABLE Overview

Limit Maps

OLAP_TABLE uses a limit map to map dimensions and measures defined in an analytic workspace to columns in a logical table. The limit map combines with the WHERE clause of a SQL SELECT statement to generate a series of OLAP DML LIMIT commands that are executed in the analytic workspace.

OLAP_TABLE can use a limit map in conjunction with a predefined logical table, or it can use the information in a limit map to dynamically generate a logical table at runtime.

Logical Tables

The logical table populated by OLAP_TABLE is actually a table type whose rows are user-defined object types, also known as Abstract Data Types or ADTs.

A user-defined object type is composed of attributes, which are equivalent to the columns of a table. The basic syntax for defining a row is as follows.

CREATE TYPE object_name AS OBJECT (
   attribute1      datatype,
   attribute2      datatype,
   attributen      datatype);

A table type is a collection of object types; this collection is equivalent to the rows of a table. The basic syntax for creating a table type is as follows.

CREATE TYPE table_name AS TABLE OF object_name;

Using OLAP_TABLE With Predefined ADTs

You can predefine the table of objects or generate it dynamically. When you create the table type in advance, it is available in the database for use by any invocation of OLAP_TABLE. Queries that use predefined objects typically perform better than queries that dynamically generate the objects.

Example 11-1 shows how to create a view of an analytic workspace using predefined ADTs.

SET ECHO ON
SET SERVEROUT ON

DROP TYPE table_obj;
DROP TYPE row_obj;

CREATE TYPE row_obj AS OBJECT (
            column_first     datatype,
            column_next      datatype,
            column_n         datatype);
/   
CREATE TYPE table_obj AS TABLE OF row_obj;
/
CREATE OR REPLACE VIEW view_name AS
   SELECT column_first, column_next, column_n
      FROM TABLE(OLAP_TABLE(
         'analytic_workspace', 
         'table_obj', 
         'olap_command',
         'limit_map'));
/
COMMIT;
/ 
GRANT SELECT ON view_name TO PUBLIC;        

Example 11-2 uses OLAP_TABLE with a predefined table type to create a relational view of the TIME dimension in the GLOBAL analytic workspace of the GLOBAL_AW schema. The first parameter in the OLAP_TABLE call is the name of the analytic workspace. The second is the name of the predefined table type. The forth is the limit map that specifies how to map the workspace dimension to the columns of the predefined table type. The third parameter is not specified.

CREATE TYPE time_cal_row AS OBJECT (
            time_id           varchar2(32),
            cal_short_label   varchar2(32),
            cal_end_date      date,
            cal_timespan      number(6));

CREATE TYPE time_cal_table AS TABLE OF time_cal_row;

CREATE OR REPLACE VIEW time_cal_view AS
   SELECT time_id, cal_short_label, cal_end_date, cal_timespan
      FROM TABLE(OLAP_TABLE(
         'global_aw.global duration session',
         'time_cal_table',
          '',
         'DIMENSION time_id from time with
            HIERARCHY time_parentrel
               INHIERARCHY time_inhier
            ATTRIBUTE cal_short_label from time_short_description
            ATTRIBUTE cal_end_date    from time_end_date
            ATTRIBUTE cal_timespan    from time_time_span'));

Using OLAP_TABLE With Automatic ADTs

If you do not supply the name of a table type as an argument, OLAP_TABLE uses information in the limit map to generate the logical table automatically. In this case, the table type is only available at runtime within the context of the calling SQL SELECT statement.

Example 11-3 shows how to create a view of an analytic workspace using automatic ADTs.

SET ECHO ON
SET SERVEROUT ON

CREATE OR REPLACE VIEW view_name AS
   SELECT column_first, column_next, column_n
      FROM TABLE(OLAP_TABLE(
         'analytic_workspace',
         '', 
         'olap_command',
         'limit_map'));
/
COMMIT;
/ 
GRANT SELECT ON view_name TO PUBLIC;        

Example 11-4 creates the same view produced by Example 11-2, but it automatically generates the ADTs instead of using a predefined table type. It uses AS clauses in the limit map to specify the data types of the target columns.

CREATE OR REPLACE VIEW time_cal_view AS
   SELECT time_id, cal_short_label, cal_end_date, cal_timespan
      FROM TABLE(OLAP_TABLE(
        'global_aw.global duration session',
        null,
        null,
        'DIMENSION time_id AS varchar2(32) FROM time WITH
           HIERARCHY time_parentrel
              INHIERARCHY time_inhier
           ATTRIBUTE cal_short_label AS VARCHAR2(32) from time_short_description
           ATTRIBUTE cal_end_date AS DATE            from time_end_date
           ATTRIBUTE cal_timespan AS NUMBER(6)       from time_time_span'));

When automatically generating ADTs, OLAP_TABLE uses default relational data types for the target columns unless you override them with AS clauses in the limit map. The default data type conversions used by OLAP_TABLE are described in Table 11-1.

Using a MODEL Clause

You can specify a MODEL clause in a SELECT FROM OLAP_TABLE statement to significantly improve query performance. The MODEL clause causes OLAP_TABLE to use an internal optimization.

You can use the following syntax to maximize the performance advantage of the MODEL clause with OLAP_TABLE. This is the recommended syntax for views of analytic workspaces.

SELECT column_first, column_next, column_n
      FROM TABLE(OLAP_TABLE(
         'analytic_workspace', 
         'table_obj', 
         'olap_command',
         'limit_map'))
      MODEL 
         DIMENSION BY(dimensions, gids)
         MEASURES(measures, attributes, rowtocell)
         RULES UPDATE SEQUENTIAL ORDER();

The MODEL clause must include DIMENSION BY and MEASURES subclauses that specify the columns in the table object. DIMENSION BY should list all the dimensions, as defined in the limit map. The list should include the GID columns for applications that use the OLAP API or BI Beans. MEASURES should list all the measures, attributes, ROW2CELL columns, and any other columns excluded from the DIMENSION BY list.

A MODEL clause lets you view the results of a query as a multidimensional array and specify calculations (rules) to perform on individual cells and ranges of cells within the array. You can specify calculation rules in the MODEL clause with OLAP_TABLE, but they will affect response time. If you wish to obtain the full benefit of the performance optimization, you should specify UPDATE and SEQUENTIAL ORDER in the RULES clause.

The UPDATE keyword indicates that you are not adding any custom members in the DIMENSION BY clause. If you do not include this keyword, the SQL WHERE clauses for measures will be discarded, which can significantly degrade performance.

The SEQUENTIAL ORDER keyword prevents Oracle from evaluating the rules to ascertain their dependencies.