On 2016-05-18 2:19 AM, Stefan Evert wrote: >> On 18 May 2016, at 02:41, dandl <david at andl.org> wrote: >> >> Then you are mistaken. >> 1. Consider the following set S of integers: 1,3,5,7,42,99,83,11. >> 2. Divide it into two subsets such that S1 is of size 3 and all members of >> S1 are larger than those in S2. >> >> A sort is unnecessary -- there are many algorithms that can do that purely >> based on set logic, the ability to compare members and the ability to >> determine the cardinality of a set. > > I think the point is that your query may not have a solution if there are > ties, i.e. the ordering is not complete: > > Consider this set of integers: 1,3,5,7,42,99,83,11,83,83 > > In this case, there is no subset S1 of size 3 that satisfies your criterion. > In an SQL query, the set returned by LIMIT 3 would not be defined uniquely.
What you've both said is essentially the point I was trying to make. 1. If you want a deterministic portable result for all valid invocations of LIMIT, you need to either constrain it to use with a totally ordered set (it would be an error to use it on something with duplicates) in order to guarantee the number of rows specified in the LIMIT argument, or you need to possibly return a different number of rows than the LIMIT argument. 2. Otherwise, if exactly the number of specified rows must be returned without other restrictions, then the result is possibly indeterminate. The options with point 1 are not only deterministic but fully relational. -- Darren Duncan
