1. Lecture 15: Query Optimization

2. Very Big Picture Usually, there are many possible query execution plans. The optimizer is trying to chose a good one. Key: cost model, search space.

3. More Detail, but Still Big Picture Parse the SQL query into a logical tree: –identify distinct blocks (corresponding to nested sub- queries or views). Query rewrite phase: SQL-to-SQL transforms –apply algebraic transformations to yield a cheaper plan. –Merge blocks and move predicates between blocks. Optimize each block: join ordering. Complete the optimization: select scheduling (pipelining strategy).

4. Query Rewrite Summary The optimizer can use any semantically correct rule to transform one query to another. Rules are used for: –moving constraints between blocks (because each will be optimized separately) –Un-nesting blocks Especially important in automatically generated queries, which may be very complex

5. The Course in Perspective The relational data model, SQL –Views, updates, transactions Conceptual design –Expressing the constraints on the domain –Using them to get good schema designs Building a database system: –Storage and indexing –Query execution (join algorithms) –Query optimization

6. Perspective (continued) Data integration and sharing: –Databases were created independently –Schemas need to be matched –Need to access multiple databases in one query.