Optimization outcome depends on the index order

On Fri, Dec 22, 2023 at 8:53 AM Andrei Lepikhov <a.lepikhov@postgrespro.ru>
wrote:

On 21/12/2023 12:10, Alexander Korotkov wrote:

I took a closer look at the patch in [9]. I should drop my argument
about breaking the model, because add_path() already considers other
aspects than just costs. But I have two more note about that patch:

1) It seems that you're determining the fact that the index path
should return strictly one row by checking path->rows <= 1.0 and
indexinfo->unique. Is it really guaranteed that in this case quals
are matching unique constraint? path->rows <= 1.0 could be just an
estimation error. Or one row could be correctly estimated, but it's
going to be selected by some quals matching unique constraint and
other quals in recheck. So, it seems there is a risk to select
suboptimal index due to this condition.

Operating inside the optimizer, we consider all estimations to be the
sooth. This patch modifies only one place: having two equal assumptions,
we just choose one that generally looks more stable.
Filtered tuples should be calculated and included in the cost of the
path. The decision on the equality of paths has been made in view of the
estimation of these filtered tuples.

Even if estimates are accurate the conditions in the patch doesn't
guarantee there is actually a unique condition.

# create table t as select i/1000 a, i % 1000 b, i % 1000 c from
generate_series(1,1000000) i;
# create unique index t_unique_idx on t(a,b);
# create index t_another_idx on t(a,c);
# \d t
Table "public.t"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
a | integer | | |
b | integer | | |
c | integer | | |
Indexes:
"t_another_idx" btree (a, c)
"t_unique_idx" UNIQUE, btree (a, b)
# set enable_bitmapscan = off; explain select * from t where a = 1 and c =
1;
SET
Time: 0.459 ms
QUERY PLAN
--------------------------------------------------------------------------
Index Scan using t_unique_idx on t (cost=0.42..1635.16 rows=1 width=12)
Index Cond: (a = 1)
Filter: (c = 1)
(3 rows)

2) Even for non-unique indexes this patch is putting new logic on top
of the subsequent code. How we can prove it's going to be a win?
That could lead, for instance, to dropping parallel-safe paths in
cases we didn't do so before.

Because we must trust all predictions made by the planner, we just
choose the most trustworthy path. According to the planner logic, it is
a path with a smaller selectivity. We can make mistakes anyway just
because of the nature of estimation.

Even if we need to take selectivity into account here, it's still not clear
why this should be on top of other logic later in add_path().

Anyway, please start a separate thread if you're willing to put more
work into this.

Done

Thanks.

------
Regards,
Alexander Korotkov

On 22/12/2023 11:48, Alexander Korotkov wrote:

Because we must trust all predictions made by the planner, we just
choose the most trustworthy path. According to the planner logic, it is
a path with a smaller selectivity. We can make mistakes anyway just
because of the nature of estimation.

Even if we need to take selectivity into account here, it's still not
clear why this should be on top of other logic later in add_path().

I got your point now, thanks for pointing it out. In the next version of
the patch selectivity is used as a criteria only in the case of COSTS_EQUAL.

--
regards,
Andrei Lepikhov
Postgres Professional

On Fri, Dec 22, 2023 at 7:24 PM Andrei Lepikhov
<a.lepikhov@postgrespro.ru> wrote:

On 22/12/2023 11:48, Alexander Korotkov wrote:

Because we must trust all predictions made by the planner, we just
choose the most trustworthy path. According to the planner logic, it is
a path with a smaller selectivity. We can make mistakes anyway just
because of the nature of estimation.

Even if we need to take selectivity into account here, it's still not
clear why this should be on top of other logic later in add_path().

I got your point now, thanks for pointing it out. In the next version of
the patch selectivity is used as a criteria only in the case of COSTS_EQUAL.

It looks better now. But it's hard for me to judge these heuristics
in add_path(). Tom, what do you think about this?

------
Regards,
Alexander Korotkov

On 25/12/2023 18:36, Alexander Korotkov wrote:

On Fri, Dec 22, 2023 at 7:24 PM Andrei Lepikhov
<a.lepikhov@postgrespro.ru> wrote:

On 22/12/2023 11:48, Alexander Korotkov wrote:

Because we must trust all predictions made by the planner, we just
choose the most trustworthy path. According to the planner logic, it is
a path with a smaller selectivity. We can make mistakes anyway just
because of the nature of estimation.

Even if we need to take selectivity into account here, it's still not
clear why this should be on top of other logic later in add_path().

I got your point now, thanks for pointing it out. In the next version of
the patch selectivity is used as a criteria only in the case of COSTS_EQUAL.

It looks better now. But it's hard for me to judge these heuristics
in add_path(). Tom, what do you think about this?

Just food for thought:
Another approach I have considered was to initialize the relation index
list according to some consistent rule: place unique indexes at the head
of the list, arrange indexes according to the number of columns involved
and sort in some lexical order.
But IMO, the implemented approach looks better.

--
regards,
Andrei Lepikhov
Postgres Professional