1. Query Optimization

2. Very Big Picture A query execution plan is a program.
There are many of them.
The optimizer is trying to chose a good one.
Hence, the optimizer is reasoning about programs.
Key: cost model, search space.
Compilers don’t have cost models. Why?

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 try to:
move constraints between blocks (because each will be optimized separately)
Unnest blocks
Especially important in decision support applications where queries are very complex.
Rules can be very tricky!

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
XML
A format for data exchange. Has its own query language.
Basis for web services.

6. Perspective (continued)
Building a database system:
Storage and indexing
Query execution (join algorithms)
Query optimization
Data integration and sharing:
Databases were created independently
Schemas need to be matched
Need to access multiple databases in one query.