Re: Hybrid Hash/Nested Loop joins and caching results from subplans

Hi,

On 2021-02-22 20:51:17 -0500, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > Except for the annoying issue that that we pervasively use Lists as
> > expressions, I'd argue that we should actually cache "subtree
> > volatility" in Expr nodes, similar to the way we use OpExpr.opfuncid
> > etc. That'd allow us to make contain_volatile_functions() very cheap
>
> ... and completely break changing volatility with ALTER FUNCTION.
> The case of OpExpr.opfuncid is okay only because we don't provide
> a way to switch an operator's underlying function. (See also
> 9f1255ac8.)

Hm. I was imagining we'd only set it within the planner. If so, I don't
think it'd change anything around ALTER FUNCTION.

But anyway, due to the List* issue, I don't think it's a viable approach
as-is anyway.

We could add a wrapper node around "planner expressions" that stores
metadata about them during planning, without those properties leaking
over expressions used at other times. E.g. having
preprocess_expression() return a PlannerExpr that that points to the
expression as preprocess_expression returns it today. That'd make it
easy to cache information like volatility. But it also seems
prohibitively invasive :(.

> It'd certainly be desirable to reduce the number of duplicated
> function property lookups in the planner, but I'm not convinced
> that that is a good way to go about it.

Do you have suggestions?

Greetings,

Andres Freund