Re: Self-join elimination discards differing row lock wait policies

Tender Wang <tndrwang(at)gmail(dot)com> writes:
> Self-join elimination discards one of the row lock wait policies when
> two aliases of the same relation use different policies.

Good catch.

> The attached patch requires the row mark type, locking strength, and
> wait policy to match before eliminating the self-join.
> Thoughts?

In some cases it might be possible to merge two "sufficiently similar"
rowmark requirements, eg keep only the stronger lock type. I'm not
sure that that wouldn't produce visible semantics changes, though.
On the other hand, the code is already willing to merge an RTE with
a RowMark with an RTE that has none, and I'm not convinced that that
has no visible semantics effects either --- for example, if the one
RowMark says "SKIP LOCKED", doesn't it represent a different set of
rows than an RTE with no RowMark? Maybe we should tighten this even
more and reject that case?

regards, tom lane