CREATE DIMENSION

Purpose

Use the CREATE DIMENSION statement to create a dimension. A dimension defines a parent-child relationship between pairs of column sets, where all the columns of a column set must come from the same table. However, columns in one column set (or "level") can come from a different table than columns in another set. The optimizer uses these relationships with materialized views to perform query rewrite. The Summary Advisor uses these relationships to recommend creation of specific materialized views.

Prerequisites

To create a dimension in your own schema, you must have the CREATE DIMENSION system privilege. To create a dimension in another user's schema, you must have the CREATE ANY DIMENSION system privilege. In either case, you must have the SELECT object privilege on any objects referenced in the dimension.

Syntax

create_dimension::=

Text description of create_dimension

level_clause::=

Text description of level_clause

hierarchy_clause::=

Text description of hierarchy_clause

join_clause::=

Text description of join_clause

attribute_clause::=

Text description of attribute_clause

Semantics

schema

Specify the schema in which the dimension will be created. If you do not specify schema, Oracle creates the dimension in your own schema.

dimension

Specify the name of the dimension. The name must be unique within its schema.

level_clause

The level_clause defines a level in the dimension. A level defines dimension hierarchies and attributes.

level

Specify the name of the level

level_table . level_column

Specify the columns in the level. You can specify up to 32 columns. The tables you specify in this clause must already exist.

Restrictions on Level Columns

• All of the columns in a level must come from the same table.
• If columns in different levels come from different tables, then you must specify the join_clause.
• The set of columns you specify must be unique to this level.
• The columns you specify cannot be specified in any other dimension.
• Each level_column must be non-null. (However, these columns need not have NOT NULL constraints.)

hierarchy_clause

The hierarchy_clause defines a linear hierarchy of levels in the dimension. Each hierarchy forms a chain of parent-child relationships among the levels in the dimension. Hierarchies in a dimension are independent of each other. They may (but need not) have columns in common.

Each level in the dimension should be specified at most once in this clause, and each level must already have been named in the level_clause.

hierarchy

Specify the name of the hierarchy. This name must be unique in the dimension.

child_level

Specify the name of a level that has an n:1 relationship with a parent level: the level_columns of child_level cannot be null, and each child_level value uniquely determines the value of the next named parent_level.

If the child level_table is different from the parent level_table, you must specify a join relationship between them in the join_clause.

parent_level

Specify the name of a level.

join_clause

The join_clause lets you specify an inner equijoin relationship for a dimension whose columns are contained in multiple tables. This clause is required and permitted only when the columns specified in the hierarchy are not all in the same table.

child_key_column

Specify one or more columns that are join-compatible with columns in the parent level.

If you do not specify the schema and table of each child_column, the schema and table are inferred from the CHILD OF relationship in the hierarchy_clause. If you do specify the schema and column of a child_key_column, the schema and table must match the schema and table of columns in the child of parent_level in the hierarchy_clause.

parent_level

Specify the name of a level.

Restrictions on Join Dimensions

• You can specify only one join_clause for a given pair of levels in the same hierarchy.
• The child_key_columns must be non-null and the parent key must be unique and non-null. You need not define constraints to enforce these conditions, but queries may return incorrect results if these conditions are not true.
• Each child key must join with a key in the parent_level table.
• Self-joins are not permitted. That is, the child_key_columns cannot be in the same table as parent_level.
• All of the child-key columns must come from the same table.
• The number of child-key columns must match the number of columns in parent_level, and the columns must be joinable.
• Do not specify multiple child key columns unless the parent level consists of multiple columns.

attribute_clause

The attribute_clause lets you specify the columns that are uniquely determined by a hierarchy level. The columns in level must all come from the same table as the dependent_columns. The dependent_columns need not have been specified in the level_clause.

For example, if the hierarchy levels are city, state, and country, then city might determine mayor, state might determine governor, and country might determine president.

Examples

Creating a Dimension: Example

This statement was used to create the customers_dim dimension in the sample schema sh:

CREATE DIMENSION customers_dim 
   LEVEL customer   IS (customers.cust_id)
   LEVEL city       IS (customers.cust_city) 
   LEVEL state      IS (customers.cust_state_province) 
   LEVEL country    IS (countries.country_id) 
   LEVEL subregion  IS (countries.country_subregion) 
   LEVEL region     IS (countries.country_region) 
   HIERARCHY geog_rollup (
      customer      CHILD OF
      city          CHILD OF 
      state         CHILD OF 
      country       CHILD OF 
      subregion     CHILD OF 
      region 
   JOIN KEY (customers.country_id) REFERENCES country
   )
   ATTRIBUTE customer DETERMINES
   (cust_first_name, cust_last_name, cust_gender, 
    cust_marital_status, cust_year_of_birth, 
    cust_income_level, cust_credit_limit) 
   ATTRIBUTE country DETERMINES (countries.country_name)
;