Karel Zak <[EMAIL PROTECTED]> writes: > I'm surprise with query plan that PostgreSQL planner prepare for > selects with ORDER BY if all data are from sub-select that is already > sorted.
This isn't simply a matter of "omitting the sort". Even if the inputs are sorted, their concatenation (Append result) isn't sorted: "1 2 3 4" and "1 3 7 9" are sorted, but "1 2 3 4 1 3 7 9" isn't. To do what you're thinking about, we'd have to build a variant implementation of Unique that merges two presorted inputs --- and it wouldn't work for more than two inputs (at least not without a lot of pain ... Append is a messy special case in many ways, and we'd have to duplicate most of that cruft to make an N-input version of Unique). This is possible, without doubt, but I'm not excited about expending that much time on it. You haven't shown any evidence that this would be an important optimization in practice. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]