Useful links regarding query processing, optimization, etc.:
Conceptual Queries
ORM Conceptual Queries and the ConQuer query language
A Query Algebra for Object-Oriented Databases
Optimizing Object Queries Using an Effective Calculus
Generic Functions/CLOS DB stuff
Open Source Implementations
XXL - a Java query processing library
Lambda-DB (Monoid Comprehension Calculus -> Relational Algebra)
Lambda-DB includes a C++ implementation of the algorithms described in "Optimizing Object Queries Using an Effective Calculus", above.
Outer Join Processing
Misc. Optimization Techniques
Performing Group-By before Join
Guy with a 300 page draft on query compilation
Optimizing Queries across Diverse Data Sources
Python Stuff:
Other
Database System Concepts, 4th Edition
Proceedings of the 27th International Conference on Very Large Databases
Foundations of Object-Relational Mapping
Visual XXL - "Runtime 3D-Visualization of Relational Database-Queries"
Good info on ANSI SQL-92 syntax/semantics