Обсуждение: How to speed up min/max(id) in 50M rows table?

>>> On Fri, Oct 12, 2007 at  3:41 PM, in message
<BAY124-W503931258D0AE13AF37546F5A00@phx.gbl>, henk de wit
<henk53602@hotmail.com> wrote:
>
> I have a table with some 50 millions rows in PG 8.2. The table has indexes
> on relevant columns. My problem is that most everything I do with this table
> (which are actually very basic selects) is unbearable slow.

Do you have autovacuum turned on?  With what settings?

Do you run scheduled VACUUM ANALYZE?

What does the tail of the output from your last
VACUUM ANALYZE VERBOSE look like?

-Kevin

> This query takes a totally unrealistic amount of time for execution (I have it running for >30 minutes now on a machine with 8GB and 4 cores@2.66Ghz, and it still isn't finished).

To correct myself, I looked at the wrong window earlier, when I typed the email the query had in fact finished already and took about 18 minutes (which is still really long for such a query of course), but more than 30 minutes was wrong.

henk de wit <henk53602@hotmail.com> writes:
> The plan looks like this:

> "Result  (cost=3D0.37..0.38 rows=3D1 width=3D0) (actual time=3D184231.636..=
> 184231.638 rows=3D1 loops=3D1)"
> "  InitPlan"
> "    ->  Limit  (cost=3D0.00..0.37 rows=3D1 width=3D8) (actual time=3D18423=
> 1.620..184231.622 rows=3D1 loops=3D1)"
> "          ->  Index Scan Backward using trans_payment_id_index on transact=
> ions  (cost=3D0.00..19144690.58 rows=3D51122691 width=3D8) (actual time=3D1=
> 84231.613..184231.613 rows=3D1 loops=3D1)"
> "                Filter: (payment_id IS NOT NULL)"
> "Total runtime: 184231.755 ms"

The only way I can see for that to be so slow is if you have a very
large number of rows where payment_id is null --- is that the case?

There's not a lot you could do about that in existing releases :-(.
In 8.3 it'll be possible to declare the index as NULLS FIRST, which
moves the performance problem from the max end to the min end ...

> select min(time) from transactions where payment_id =3D 67

> There are indexes on both the time (a timestamp with time zone) and payment=
> _id (a bigint) columns.

Creating indexes at random with no thought about how the system could
use them is not a recipe for speeding up your queries.  What you'd need
to make this query fast is a double-column index on (payment_id, time)
so that a forward scan on the items with payment_id = 67 would
immediately find the minimum time entry.  Neither of the single-column
indexes offers any way to find the desired entry without scanning over
lots of unrelated entries.

            regards, tom lane

On Friday 12 October 2007, henk de wit <henk53602@hotmail.com> wrote:
> >   select payment_id from transactions order by payment_id desc limit 1;
>
> This one is indeed instant! Less than 50ms. In my case I can't use it for
> max though because of the fact that payment_id can be null (which is an
> unfortunate design choice). The other variant however didn't become
> instant. I.e. I tried:
>
> select time from transactions where payment_id = 67 order by time asc
> limit 1;
>
> But this one is still really slow.

If you had a compound index on payment_id,time (especially with a WHERE time
IS NOT NULL conditional) it would likely speed it up.


--
Ghawar is dying

I wrote:
> The only way I can see for that to be so slow is if you have a very
> large number of rows where payment_id is null --- is that the case?

> There's not a lot you could do about that in existing releases :-(.

Actually, there is a possibility if you are willing to change the query:
make a partial index that excludes nulls.  Toy example:

regression=# create table fooey(f1 int);
CREATE TABLE
regression=# create index fooeyi on fooey(f1) where f1 is not null;
CREATE INDEX
regression=# explain select max(f1) from fooey;
                          QUERY PLAN
---------------------------------------------------------------
 Aggregate  (cost=36.75..36.76 rows=1 width=4)
   ->  Seq Scan on fooey  (cost=0.00..31.40 rows=2140 width=4)
(2 rows)

regression=# explain select max(f1) from fooey where f1 is not null;
                                          QUERY PLAN
-----------------------------------------------------------------------------------------------
 Result  (cost=0.03..0.04 rows=1 width=0)
   InitPlan
     ->  Limit  (cost=0.00..0.03 rows=1 width=4)
           ->  Index Scan Backward using fooeyi on fooey  (cost=0.00..65.55 rows=2129 width=4)
                 Filter: (f1 IS NOT NULL)
(5 rows)

Probably the planner ought to be smart enough to figure this out without
the explicit WHERE in the query, but right now it isn't.

            regards, tom lane

henk de wit <henk53602@hotmail.com> writes:
> I see, that sounds very interesting too. As you might have noticed, I'm not=
>  an expert on this field but I'm trying to learn. I was under the impressio=
> n that the last few incarnations of postgresql automatically combined singl=
> e column indexes for cases where a multi-column index would be needed in ea=
> rlier releases.

It's possible to combine independent indexes for resolving AND-type
queries, but the combination process does not preserve ordering, so
it's useless for this type of situation.

            regards, tom lane

>select min(time) from transactions where payment_id is null
>So for that situation I tried whether a specific index helped, i.e. :
>create index transactions__time_payment_id__null__idx on transactions(time) where payment_id is null;
>But this does not really seem to help. It might be better to see if I can refactor the DB design though to not use nulls.

I was posting too fast again, the previous index -does- work, making the above query instant (<50ms) :) I actually mis-typed the test query before (it's rather late at my place):

select min(payment_id) from transactions where payment_id is null

Although not useful, it's an interesting case. One would say that it could return quickly, but it takes 26 minutes to execute this.