About method of PostgreSQL's Optimizer

Hello all!<br /><br /> On my master course, I'm studying the PostgreSQL's optimizer.<br /> I don't know if anyone in
thislist have been participated from the PostgreSQL's Optimizer development, but maybe someone can help me on this
question.<br/> PostgreSQL generates all possible plans of executing the query (using an almost exhaustive search), then
givesa cost to each plan and finally the cheapest one is selected for execution.<br /> There are other methods for
queryoptimization, one of them is based on plan transformations (for example, using A-Star algorithm) instead of plan
constructionsused by PostgreSQL. <br /> Does anyone know why this method was choosen? Are there any papers or
researchesabout it?<br /><br /> Thank's a lot,<br /><span class="sg"> Pryscila.<br /></span>