November 11, 2024 Everything you need to know about query planning can be understood from this query: SELECT * FROM xy WHERE y = 3 ORDER BY x Imagine we have two indexes, one ordered on y, and one ordered on x. Then two possible plans suggest themselves: Plan 1: scan the y index, restricting it to the values of y such that y = 3, then sort the result by x. Plan 2: scan the entire x index, selecting any rows having y = 3. The result will be "naturally" sorted by x, with no need for an extra sort....