Re: BUG #18959: Name collisions of expression indexes during parallel Index creations on a pratitioned table.

I wrote:
> This seems very closely related to commit 3db61db48 [1], which fixed
> a similar behavior for child foreign key constraints. Per that commit
> message, it's a good idea for the child objects to have names related
> to the parent objects, so we ought to change this behavior regardless
> of any concurrent-failure considerations.

I experimented with the attached, which borrows a couple of ideas
from 3db61db48 to produce names like "parent_index_2" when cloning
indexes. While it should help with the immediate problem, I'm not
sure if this is acceptable, because there are a *lot* of ensuing
changes in the regression tests, many more than 3db61db48 caused.
(Note that I didn't bother to fix places where the tests rely on
a generated name that has changed; the delta in the test outputs
is merely meant to give an idea of how much churn there is.
I didn't check non-core test suites, either.)

Also, looking at the error message changes, I'm less sure that
this is a UX improvement than I was about 3db61db48. Do people
care which partition a uniqueness constraint failed in? In
the current behavior, the index name will reflect that, but
with this behavior, not so much.

Anyway, maybe this is a good idea or maybe it isn't. Thoughts?

regards, tom lane