> On Thu, Sep 17, 2020 at 02:47:54PM +0200, Pavel Stehule wrote: > > I have my concerns about the performance side of this implementation as > > well as how surprising this would be for users, but at the same time the > > patch already does something similar and the code change should not be > > that big, so why not - I can include this change into the next rebased > > version. But it still can cause some confusion as it's not going to work > > for negative indices, so > > > > update foo set a[-10] = 1; > > > > and > > > > select a[-10] from foo; > > > > can return different value from what was assigned. Otherwise, if we will > > try to fix a[-10] assignment in the same way, it will prepend the array > > and a[10] will not return the same value. > > What is semantic of negative index? It has clean semantic in C, but in > PLpgSQL?
It's just a common pattern for jsonb when a negative index count from the end of an array. I believe it was like that from the very earlier implementations, although can't comment on that from the semantic point of view.
ok, then I think we can design some workable behaviour
My first rule - there should not be any implicit action that shifts positions in the array. It can be explicit, but not implicit. It is true for positive indexes, and it should be true for negative indexes too.
