"Jim C. Nasby" <jnasby@xxxxxxxxxxxxx> writes: > Is the issue that the optimizer won't combine two function calls (ie: > SELECT foo(..) ... WHERE foo(..)), or is it that sometimes it won't make > the optimization (maybe depending on the query plan, for example)? What the STABLE category actually does is give the planner permission to use the function within an indexscan qualification, eg, WHERE indexed_column = f(42) Since an indexscan involves evaluating the comparison expression just once and using its value to search the index, this would be incorrect if the expression's value might change from row to row. (For VOLATILE functions, we assume that the correct behavior is the naive SQL semantics of actually computing the WHERE clause at each candidate row.) There is no function cache and no checking for duplicate expressions. I think we do check for duplicate aggregate expressions, but not anything else. regards, tom lane
