DB2 Access Path Selection

Enhanced Definition

DB2 Access Path Selection is the process by which the DB2 optimizer, a component of the DB2 database manager, determines the most efficient method to retrieve or modify data for a given SQL statement. Its primary purpose is to minimize resource consumption (CPU, I/O, memory) and execution time, ensuring optimal performance for database operations.

Key Characteristics

- Cost-Based Optimization: The DB2 optimizer is a cost-based optimizer, evaluating various potential access paths and estimating the cost (in terms of CPU cycles, I/O operations, and memory usage) for each before selecting the path with the lowest estimated cost.
- Reliance on Statistics: Access path selection heavily depends on current and accurate statistics about tables, indexes, and columns, which are gathered by the RUNSTATS utility. Outdated or missing statistics can lead to suboptimal path choices.
- Index Utilization: A key decision involves whether to use an available index, which specific index to use, or to perform a table space scan (reading all data pages).
- Join Method Selection: For SQL statements involving multiple tables, the optimizer determines the most efficient join method (e.g., Nested Loop Join, Merge Scan Join, Hash Join) and the join order.
- Dynamic vs. Static SQL: Access paths for static SQL are typically chosen at bind time, while for dynamic SQL, they are chosen at run time when the statement is prepared.
- Predictive Modeling: The optimizer uses complex algorithms and models to predict the number of rows that will satisfy predicates and the cost associated with different operations.

Use Cases

- SQL Statement Execution: Every time an SQL statement is executed (whether bound or dynamically prepared), the DB2 optimizer performs access path selection to determine the most efficient way to fulfill the request.
- Performance Tuning: DBAs and application developers analyze the chosen access paths (using the EXPLAIN facility) to identify and resolve performance bottlenecks in slow-running queries.
- Index Design and Maintenance: Understanding how DB2 selects access paths is crucial for designing effective indexes that support common query patterns and for deciding when to rebuild or reorganize indexes.
- RUNSTATS Utility Planning: The need for accurate statistics drives the scheduling and scope of RUNSTATS utility executions, particularly after significant data changes or database reorganizations.

Related Concepts

DB2 Access Path Selection is the core function of the DB2 Optimizer, which is responsible for query optimization. Its effectiveness is directly tied to the accuracy of statistics collected by the RUNSTATS utility; outdated statistics can lead to poor access path choices. The EXPLAIN facility is the primary tool used by DBAs and developers to view and analyze the access path chosen by the optimizer, making it indispensable for performance tuning. The design and existence of Indexes significantly influence the available access paths and the optimizer's decisions, as does the way SQL statements are written.

Best Practices:

Maintain Current Statistics: Regularly run RUNSTATS on tables, indexes, and column groups, especially after significant data loads, updates, or reorganizations, to provide the optimizer with accurate information.
Utilize EXPLAIN Routinely: Always use the EXPLAIN facility to understand and validate the chosen access paths for critical or performance-sensitive SQL statements, both before and after deployment.
Design Effective Indexes: Create indexes that directly support common query predicates, join conditions, and sorting requirements, but avoid over-indexing, which can degrade DML performance.
Write Optimizer-Friendly SQL: Structure SQL statements to be easily optimized. Avoid anti-patterns like applying functions to indexed columns in predicates (e.g., WHERE SUBSTR(COL,1,1) = 'A'), using SELECT * unnecessarily, or complex correlated subqueries where alternatives exist.
Monitor and Re-evaluate: Continuously monitor SQL performance and periodically re-evaluate access paths, particularly after DB2 version upgrades, schema changes, or significant data distribution shifts.