Re: pg_plan_advice

On Tue, Jan 27, 2026 at 2:49 AM Ajay Pal <ajay(dot)pal(dot)k(at)gmail(dot)com> wrote:
> #1 Grouped Hash Join, This forces the join of dim1 and dim2 to happen
> first, and then places that resulting set on the inner side of a Hash
> Join against fact.
> but the planner partially matches the generated advice.
>
> -- We want (dim1 JOIN dim2) to be the inner side of a Hash Join
> SET LOCAL pg_plan_advice.advice = 'HASH_JOIN((dim1 dim2))';
>
> postgres=*# EXPLAIN (COSTS OFF, PLAN_ADVICE)
> SELECT * FROM fact
> JOIN dim1 ON fact.d1_id = dim1.id
> JOIN dim2 ON fact.d2_id = dim2.id;
> QUERY PLAN
> -----------------------------------------------------------
> Nested Loop
> Disabled: true
> -> Nested Loop
> Disabled: true
> -> Seq Scan on fact
> -> Index Scan using dim1_pkey on dim1
> Index Cond: (id = fact.d1_id)
> -> Index Scan using dim2_pkey on dim2
> Index Cond: (id = fact.d2_id)
> Supplied Plan Advice:
> HASH_JOIN((dim1 dim2)) /* partially matched */
> Generated Plan Advice:
> JOIN_ORDER(fact dim1 dim2)
> NESTED_LOOP_PLAIN(dim1 dim2)
> SEQ_SCAN(fact)
> INDEX_SCAN(dim1 public.dim1_pkey dim2 public.dim2_pkey)
> NO_GATHER(fact dim1 dim2)
> (17 rows)

Thanks for the report, but this is actually correct behavior. There's
no join clause between dim1 and dim2, so the planner doesn't consider
a dim1-dim2 join. This is a good example of the phenomenon described
in the documentation: you can't force the planner to create an
arbitrary plan that it wouldn't otherwise have considered. I might
tweak the documentation wording a little to try to mention that this
is another way "partially matched" can happen, but there's no bug
here.

> #2 Multiple Instances of Same Table in Subqueries, here target the
> second instance of dim1 inside the subquery 'sq'. both seq_scan and
> index_scan advices are not matching.
>
> SET LOCAL pg_plan_advice.advice = 'SEQ_SCAN(dim1#2(at)sq)
> INDEX_SCAN(dim1(at)sq dim1_pkey)';
>
> postgres=*# EXPLAIN (COSTS OFF, PLAN_ADVICE)
> SELECT * FROM fact
> JOIN (
> SELECT a.id FROM dim1 a
> JOIN dim1 b ON a.id = b.id
> OFFSET 0
> ) sq ON fact.d1_id = sq.id;
> QUERY PLAN
> ---------------------------------------------------
> Hash Join
> Hash Cond: (fact.d1_id = b.id)
> -> Seq Scan on fact
> -> Hash
> -> Seq Scan on dim1 b
> Supplied Plan Advice:
> SEQ_SCAN(dim1#2(at)sq) /* not matched */
> INDEX_SCAN(dim1(at)sq dim1_pkey) /* not matched */
> Generated Plan Advice:
> JOIN_ORDER(fact sq)
> HASH_JOIN(sq)
> SEQ_SCAN(b(at)sq fact)
> NO_GATHER(fact b(at)sq)
> (13 rows)

I'm not sure what why you expected this to work. You can see what the
correct relation identifiers are from the generated plan advice, and
you've used something else, so it doesn't match. It's documented in
both the SGML documentation and the README that relation identifiers
are based on the relation alias, not the relation name.

In general, this seems like a good to reiterate that this is first and
foremost a plan stability feature. More than anything, these examples
show that if you try to write your own plan advice from scratch to
force a novel plan that the planner has never produced itself, you may
not have much luck. If you do want to try to produce a novel plan, you
should at least look at the generated plan advice and adapt it instead
of starting from scratch. And if you find, when trying to produce a
novel plan, that it doesn't work, you need to consider the possibility
that this is because the optimizer did not ever consider that plan,
and that is why pg_plan_advice is unable to induce the planner to
prefer it. That's not to say there can't be any remaining bugs in
pg_plan_advice; there probably are. But it also is absolutely not a
"write your own plan and do anything you like" feature.

--
Robert Haas
EDB: http://www.enterprisedb.com