Rewriting a query in terms of a summary based on functional...

Details Rewriting a query in terms of a summary based on functional... Rewriting a query in terms of a summary based on functional...

1998-12-28

2002-12-17

Jung, David (Department: 2175)

Data processing: database and file management or data structures

Database design

Data structure types

C707S793000, C707S793000

Reexamination Certificate

active

06496819

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to database systems, and in particular, to rewriting queries to access data sources that are not specifically referenced in the queries.
BACKGROUND OF THE INVENTION
In a database management system (DBMS), data is stored in one or more data containers, each container contains records, and the data within each record is organized into one or more fields. In relational database systems, the data containers are referred to as tables, the records are referred to as rows, and the fields are referred to as columns. In object oriented databases, the data containers are referred to as object classes, the records are referred to as objects, and the fields are referred to as attributes. Other database architectures may use other terminology.
The present invention is not limited to any particular type of data container or database architecture. However, for the purpose of explanation, the examples and the terminology used herein shall be that typically associated with relational databases. Thus, the terms “table”, “row” and “column” shall be used herein to refer respectively to the data container, record, and field.
For various reasons, it may not be desirable for certain users to have access to all of the columns of a table. For example, one column of an employee table may hold the salaries for the employees. Under these circumstances, it may be desirable to limit access to the salary column to management, and allow all employees to have access to the other columns. To address this situation, the employees may be restricted from directly accessing the table. Instead, they may be allowed to indirectly access the appropriate columns in the table through a “view”.
A view is a logical table. As logical tables, views may be queried by users as if they were a table. However, views actually present data that is extracted or derived from existing tables. Thus, the problem described above may be solved by (1) creating a view that extracts data from all columns of the employee table except the salary column, and (2) allowing all employees to access the view.
A view is defined by metadata referred to as a view definition. The view definition contains mappings to one or more columns in the one or more tables containing the data. Typically, the view definition is in the form of a database query. Columns and tables that are mapped to a view are referred to herein as base columns and base tables of the view, respectively. The data maintained in the base columns is referred to herein as base data.
The data presented by conventional views is gathered and derived on-the-fly from the base tables in response to queries that access the views. That data gathered for the view is not persistently stored after the query accessing the view has been processed. Because the data provided by conventional views is gathered from the base tables at the time the views are accessed, the data from the views will reflect the current state of the base tables. However, the overhead associated with gathering the data from the base tables for a view every time the view is accessed may be prohibitive.
A materialized view, on the other hand, is a view for which a copy of the view data is stored separate from the base tables from which the data was originally gathered and derived. The data contained in a materialized view is referred to herein as (“materialized data”). Materialized views eliminate the overhead associated with gathering and deriving the view data every time a query accesses the view. As new data is periodically added to the base tables, the materialized view needs to be updated (i.e., refreshed) to reflect the new base data.
Materialized views that are derived from more than one base table are created by performing a join between the base tables. A join is a query that combines rows from two or more tables, views, or materialized views. A join is performed whenever multiple tables appear in a query's FROM clause. The query's select list can select any columns from any of the base tables listed in the FROM clause.
Most join queries contain WHERE clause conditions that compare two columns, each from a different table. Such a condition is called a join condition. To execute a join, the DBMS combines pairs of rows for which the join condition evaluates to TRUE, where each pair contains one row from each table.
To execute a join of three or more tables, the DBMS first joins two of the tables based on the join conditions comparing their columns and then joins the result to another table based on join conditions containing columns of the joined tables and the new table. The DBMS continues this process until all tables are joined into the result.
In addition to join conditions, the WHERE clause of a join query can also contain other conditions that refer to columns of only one table. These conditions can further restrict the rows returned by the join query.
An equijoin is a join with a join condition containing an equality operator. An equijoin combines rows that have equivalent values for the specified columns. Query
1
is an equijoin that combines the rows of tables R and S where the value in column r.a is the same as the value in column s.a:
QUERY
1
SELECT *
FROM R, S
WHERE r.a=s.a;
In this example, table R is the “left” or “child” side table of the join, and table S is the “right” or “parent” table of the join. The join illustrated by Query
1
is a “simple” or “inner” join. With an inner join, rows from the child table that do not satisfy the join condition are not reflected in the join result. In contrast, an outer join returns all child rows that satisfy the join condition and those rows from the child table for which no rows from the parent satisfy the join condition.
Computer database systems that are used for data warehousing frequently maintain materialized views that contain pre-computed summary information in order to speed up query processing. Such summary information is created by applying an aggregate function, such as SUM, COUNT, or AVERAGE, to values contained in the base tables. Materialized views that contain pre-computed summary information are referred to herein as “summary tables” or more simply, “summaries”.
Summary tables typically store aggregated information, such as “sum of PRODUCT_SALES, by region, by month.” Other examples of aggregated information include counts of tally totals, minimum values, maximum values, and average calculations. Summary tables are used to reduce the overhead associated with processing queries that request summary information. Because the summary information is already pre-computed, it need not be re-computed during the execution of each query that requests summary information. Instead, the pre-computed summary values can be directly retrieved from the summary columns of the summary table.
When a database user knows that a particular materialized view contains the data desired by the user, the user can formulate a query that extracts the desired data directly from that materialized view. However, there are numerous circumstances in which a user will design a query that does not reference a materialized view to retrieve a set of data that can be most efficiently retrieved by accessing the materialized view. For example, the user may not be aware of all of the materialized views available in the database, or may not be sure how a particular materialized view could be used to retrieve the desired information.
Even when the user is aware that materialized views are available that have data desired by the user, the user may prefer to have the server transparently rewrite the query in terms of the materialized views, rather than writing the query to directly reference the materialized views. By allowing the server to rewrite the query, the database administrator may easily add and drop materialized views without affecting the application code that imbeds the user query. For example, if a user query is rewritten by the server to use a particular materialized view MV
1
, then the application will not break if

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