Re: pg_plan_advice

On Mon Nov 24, 2025 at 11:14 AM EST, Robert Haas wrote:
> On Sat, Nov 22, 2025 at 7:43 PM Dian Fay <di(at)nmfay(dot)com> wrote:
>> Since the policies don't contain any execution boundaries, all the quals
>> should be going into a single bucket for planning if I understand the
>> process correctly. The bitmap heap scan should be a candidate given the
>> `tags &&` predicate (and indeed if I switch to a privileged role, the
>> advice matches successfully without any policies in the mix), but gdb
>> shows the walker bouncing out of pgpa_walker_contains_scan without any
>> candidate scans for the BITMAP_HEAP_SCAN strategy.
>
> In this particular case, I think the problem is that the user-supplied
> qual item.tags @> ARRAY[id] is not leakproof and therefore must be
> tested after the security qual. There's no way to use a Bitmap Heap
> Scan without reversing the order of those tests.

Right, I keep forgetting the functions underneath those array operators
aren't leakproof. Thanks for digging.

> And honestly, this is one of the things I'm worried about if we go
> forward with this, that we'll get a ton of people who think it doesn't
> work because it doesn't force the planner to do things which the
> planner rejects on non-cost considerations. We're going to need really
> good documentation to explain to people that if you use this to try to
> force a plan and you can't, that's not a bug, that's the planner
> telling you that that plan shape is not able to be considered for some
> reason.

Once we're closer to consensus on pg_plan_advice or something like it
landing, I'm interested in helping out on this end of things!