Re: hashjoins vs. Bloom filters (yet again)

On Mon Jul 13, 2026 at 8:55 PM -03, Tomas Vondra wrote:
> Which last patch you mean? The v4 creates the scan paths with filters at
> the beginning of the planning, which seems exactly the opposite to the
> post-processing approach. Which means it can change the join order, it
> can change which join algorithms we selected, etc.
>
> The approach in v1 is what I'd call post-processing, as it made the
> decisions in createplan, after everything else was already decided. It
> could not change join order or join algorithms, it was entirely
> opportunistic.
>

Sorry, that last paragraph was confusing — I didn't reread my reply
before sending it. I mixed up the v1 and v4 implementations while
writing it.

What I actually meant to say is that v4 (and v5, as an evolution of v4,
which I sent in my previous reply) seems like the way to go. Of course
there's a lot to improve and think about, but the overall idea —
creating paths with and without the pushed-down filters, accounting for
them when costing the scan path, and letting the cost-based path
selection decide which is better — makes sense to me. It consider the
fact that pushing down filters is not free and it did not make EXPLAIN
consufing with the expected x actual problem.

> Some engines may do that unconditionally in the post-planning phase. It
> can't change the plan anyway, so why bother with costing, right? But
> then they still do some cost/benefit decisions at execution time, either
> when deciding to build the filters, or to disable filters that turn out
> to be ineffective.
>
> As for the expected vs actual difference, I think this would be a big
> issue, because it'd make EXPLAIN output utterly confusing. I don't think
> I could convince myself to commit that into core ...
>

I definitely agree with this, it can lead to a lot of confusion, e.g.,
"is the expected-vs-actual difference because of filter pushdown, or
because the statistics are out of date?" So I'd also like to avoid that
kind of behavior.

--
Matheus Alcantara
EDB: https://www.enterprisedb.com