On my master course, I'm studying the PostgreSQL's optimizer. I don't know if anyone in this list have been participated from the PostgreSQL's Optimizer development, but maybe someone can help me on this question. PostgreSQL generates all possible plans of executing the query (using an almost exhaustive search), then gives a cost to each plan and finally the cheapest one is selected for execution. There are other methods for query optimization, one of them is based on plan transformations (for example, using A-Star algorithm) instead of plan constructions used by PostgreSQL. Does anyone know why this method was choosen? Are there any papers or researches about it?