Business Transactions In many business environments there are a subset of transaction that are time critical. The amount of time that it takes to perform the transaction radically impacts other business processes downstream. An example of this may be a marketing report that allows the marketing division of an organization to judge the effectiveness of an advertising campaign in various cities across the country. The ability to judge the effectiveness of the campaign in a timely manner can be critical in making decisions concerning the purchase of ad space in newspapers or magazines. The inability to make that decision because a report is performing poorly can potentially lead to inaccurate and costly decisions. If the DBA is concerned about making certain that business critical queries are insulated from factors that may change their performance characteristics then stored outlines can provide some measure of performance stability.
Query Processing Each query that is delivered to the Oracle server goes through a number of processes before it is executed. Some of these basic steps are: Parse - this step does an analysis of the query to determine a number of basic attributes associated with the query such as if the table the query is referencing exists in the database. Another function of this step is to hash the query. The result of the hash is a value that can be used to search the library buffer cache to determine if the query has been executed before. If the query has been executed before then an execution plan has already been created for the query.
If the query has never been executed since the instance has started then an execution plan needs to be created for the query. Execute - this step actually executes the query once it has bee parsed. Fetch - this step returns rows to the process executing the query assuming that the command was a select. Execution Plan During the parse phase of query processing an execution plan is created for a query.
The execution plan defines how the data is to be accessed from the database. For instance it defines if a table will be accessed with a full table scan or an index retrieval, it will also define how an index will be accessed assuming an index is available for data retrieval. Design Objective The design objective for stored outlines was to provide a mechanism that would allow a business critical transaction to remain stable regardless of some types of database changes. As an example if the statistics used by the cost based optimizer are accidentally deleted then the cost based optimizer may create a different execution plan for a query. This could radically alter the performance characteristics of the query. Functionality The implementation of stored outlines relies on the ability to provide hints to the cost based optimizer.
Consequently, the scope of stored outlines is limited by the capabilities of hints to alter the execution plan of a query. Data Structures The following section will define the views representing the data structures used to implement stored outlines. DBA OUTLINES