On 2018-05-01 14:35:46 -0700, Mark Dilger wrote: > > > On May 1, 2018, at 2:11 PM, Andres Freund <and...@anarazel.de> wrote: > > > > Hi, > > > > On 2018-05-01 14:09:39 -0700, Mark Dilger wrote: > >> I don't care which order the data is in, as long as x[i] and y[i] are > >> matched correctly. It sounds like this patch would force me to write > >> that as, for example: > >> > >> select array_agg(a order by a, b) AS x, array_agg(b order by a, b) AS y > >> from generate_a_b_func(foo); > >> > >> which I did not need to do before. > > > > Why would it require that? Rows are still processed row-by-row even if > > there's parallelism, no? > > I was responding in part to Tom's upthread statement: > > Your own example of assuming that separate aggregates are computed > in the same order reinforces my point, I think. In principle, anybody > who's doing that should write > > array_agg(e order by x), > array_agg(f order by x), > string_agg(g order by x) > > because otherwise they shouldn't assume that; > > It seems Tom is saying that you can't assume separate aggregates will be > computed in the same order. Hence my response. What am I missing here?
Afaict Tom was just making a theoretical argument, and one that seems largely independent of the form of parallelism we're discussing here. Greetings, Andres Freund
