Chapter 22: Access Methods and Query Costing for Single Tables

This chapter introduces the methods that Adaptive Server uses to access rows in tables. It examines various types of queries on single tables, and describes the access methods that can be used, and the associated costs.

Chapter 19, “Adaptive Server Optimizer,” explains how the optimizer uses search arguments and join clauses to estimate the number of rows that a query will return. This chapter looks at how the optimizer uses row estimates and other statistics to estimate the number of pages that must be read for the query, and how many logical and physical I/Os are required.

This chapter looks at queries that affect a single table.

For queries that involve more than one table, see Chapter 23, “Accessing Methods and Costing for Joins and Subqueries.”

For parallel queries, see Chapter 25, “Parallel Query Optimization.”

This chapter contains information about query processing that you can use in several ways as it:

This chapter can also help you determine when to stop working to improve the performance of a particular query. If you know that it needs to read a certain number of index pages and data pages, and the number of I/Os cannot be reduced further by adding a covering index, you know that you have reached the optimum performance possible for query analysis and index selection. You might need to look at other issues, such as cache configuration, parallel query options, or object placement.