felix@xxxxxxxxxxx writes: > Suppose I have an index on 5 columns (A, B, C, D, E). > If my WHERE clause is not in that order, will the optimizer reorder > them as necessary and possible? Yes, the optimizer understands about commutativity/associativity of AND and OR ;-) > If I don't specify columns in the WHERE clause, how much can it use > the index? Before (if memory serves) 8.1, the planner would only consider leading index columns as potential indexscan qualifiers. So given where a = 5 and c = 4; only the a = 5 clause would be used with the index. As of 8.1 it will consider using nonconsecutive index columns, but if you think for a bit about the storage order of a btree, you'll realize that you really need leading columns to keep down the amount of the index that gets scanned. A lot of the time, such a plan will be rejected as apparently more expensive than a seqscan. (This is for btrees, I don't recall the state of play for GIST indexes exactly.) regards, tom lane
