Thank you very much for confirming my query results and taking the time to iterate out a test environment and document in such detail. It would be really awesome if the Postgres query plans could infer check constraints from the where clause's foreign key usage in addition to the native key usage. Hopefully the postgres team will take an interest in this capability.
----- Original Message -----
From: "Ondrej Ivanič" <ondrej.ivanic@xxxxxxxxx>
Sent: Sun, 9/30/2012 8:35pm
To: "Tom Lane" <tgl@xxxxxxxxxxxxx>
Cc: "Waldo, Ethan" <ewaldo@xxxxxxxxxxxxxxxx> ; pgsql-general@xxxxxxxxxxxxxx
Subject: Re: transitive pruning optimization on the right side of a join for partition tables
Hi,
On 1 October 2012 01:14, Tom Lane <tgl@xxxxxxxxxxxxx> wrote:
> "Waldo, Ethan" <ewaldo@xxxxxxxxxxxxxxxx> writes:
>> This query does a sequence scan and append across all the partition tables:
>> select "dates"."date_description" FROM "myfact" as "myfact", "dates" as "dates" where "myfact"."recorded_on_id" = "dates"."recorded_on_id" and "dates"."recorded_on_id" IN ('4617', '4618', '4619', '4620', '4621', '4622', '4623', '4624', '4625', '4626', '4627', '4628', '4629', '4630', '4631', '4632', '4633', '4634', '4635', '4636', '4637', '4638', '4639', '4640', '4641', '4642', '4643', '4644', '4645', '4646', '4647');
>
> When I try that in 9.1, I get a plan with inner indexscans for each
> child table; which, while not exactly what you're asking for, should
> perform well enough when the fact table is large enough that
> partitioning is actually a useful activity.
I do not have 9.1 handy but this is from 9.0.4:
db=# EXPLAIN select dates.datetime, myfact.recorded_on_id from myfact,
dates where myfact.recorded_on_id = dates.recorded_on_id and
dates.recorded_on_id in (1813, 1814);
QUERY PLAN
--------------------------------------------------------------------------------------------
Hash Join (cost=33.46..86124.15 rows=2858831 width=16)
Hash Cond: (public.myfact.recorded_on_id = public.dates.recorded_on_id)
-> Append (cost=0.00..46245.73 rows=3001773 width=8)
-> Seq Scan on myfact (cost=0.00..27.70 rows=1770 width=8)
-> Seq Scan on myfact_y2004w51 myfact (cost=0.00..15406.01
rows=1000001 width=8)
-> Seq Scan on myfact_y2004w52 myfact (cost=0.00..15406.01
rows=1000001 width=8)
-> Seq Scan on myfact_y2004w53 myfact (cost=0.00..15406.01
rows=1000001 width=8)
-> Hash (cost=33.21..33.21 rows=20 width=16)
-> Append (cost=0.00..33.21 rows=20 width=16)
-> Seq Scan on dates (cost=0.00..32.12 rows=18 width=16)
Filter: (recorded_on_id = ANY ('{1813,1814}'::bigint[]))
-> Seq Scan on dates_y2004w51 dates (cost=0.00..1.09
rows=2 width=16)
Filter: (recorded_on_id = ANY ('{1813,1814}'::bigint[]))
(13 rows)
Postgres can't infer that myfact.recorded_on_id is in (1813, 1814)
from the join condition (myfact.recorded_on_id = dates.recorded_on_id)
hence all partitons are included (myfact_y2004w51, myfact_y2004w53,
myfact_y2004w53). Adding "myfact.recorded_on_id in (1813, 1814)"
creates much better plan:
db=# EXPLAIN select dates.datetime, myfact.recorded_on_id from myfact,
dates where myfact.recorded_on_id = dates.recorded_on_id and
dates.recorded_on_id in (1813, 1814) and myfact.recorded_on_id in
(1813, 1814);
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
Hash Join (cost=33.46..18878.72 rows=296673 width=16)
Hash Cond: (public.myfact.recorded_on_id = public.dates.recorded_on_id)
-> Append (cost=0.00..14710.38 rows=311507 width=8)
-> Seq Scan on myfact (cost=0.00..32.12 rows=18 width=8)
Filter: (recorded_on_id = ANY ('{1813,1814}'::bigint[]))
-> Bitmap Heap Scan on myfact_y2004w51 myfact
(cost=5378.64..14678.25 rows=311489 width=8)
Recheck Cond: (recorded_on_id = ANY ('{1813,1814}'::bigint[]))
-> Bitmap Index Scan on myfact_y2004w51_recorded_on_id
(cost=0.00..5300.77 rows=311489 width=0)
Index Cond: (recorded_on_id = ANY
('{1813,1814}'::bigint[]))
-> Hash (cost=33.21..33.21 rows=20 width=16)
-> Append (cost=0.00..33.21 rows=20 width=16)
-> Seq Scan on dates (cost=0.00..32.12 rows=18 width=16)
Filter: (recorded_on_id = ANY ('{1813,1814}'::bigint[]))
-> Seq Scan on dates_y2004w51 dates (cost=0.00..1.09
rows=2 width=16)
Filter: (recorded_on_id = ANY ('{1813,1814}'::bigint[]))
(15 rows)
--------------
create table myfact (
id bigint not null,
recorded_on_id bigint not null
);
create table myfact_y2004w51 (
check (recorded_on_id >= 1812 and recorded_on_id <= 1818),
primary key (id)
) inherits (myfact);
create table myfact_y2004w52 (
check (recorded_on_id >= 1819 and recorded_on_id <= 1825),
primary key (id)
) inherits (myfact);
create table myfact_y2004w53 (
check (recorded_on_id >= 1826 and recorded_on_id <= 1832),
primary key (id)
) inherits (myfact);
create table dates (
datetime timestamp without time zone not null,
recorded_on_id bigint not null
);
create table dates_y2004w51 (
check (recorded_on_id >= 1812 and recorded_on_id <= 1818),
primary key (datetime)
) inherits (dates);
create table dates_y2004w52 (
check (recorded_on_id >= 1819 and recorded_on_id <= 1825),
primary key (datetime)
) inherits (dates);
create table dates_y2004w53 (
check (recorded_on_id >= 1826 and recorded_on_id <= 1832),
primary key (datetime)
) inherits (dates);
insert into myfact_y2004w51 select s.i, (random() * 6 + 1812)::integer
from generate_series(0, 1000000, 1) as s(i);
insert into myfact_y2004w52 select s.i, (random() * 6 + 1819)::integer
from generate_series(0, 1000000, 1) as s(i);
insert into myfact_y2004w53 select s.i, (random() * 6 + 1826)::integer
from generate_series(0, 1000000, 1) as s(i);
insert into dates_y2004w51 select distinct on (recorded_on_id)
to_timestamp(recorded_on_id), recorded_on_id from myfact_y2004w51;
insert into dates_y2004w52 select distinct on (recorded_on_id)
to_timestamp(recorded_on_id), recorded_on_id from myfact_y2004w52;
insert into dates_y2004w53 select distinct on (recorded_on_id)
to_timestamp(recorded_on_id), recorded_on_id from myfact_y2004w53;
create index myfact_y2004w51_recorded_on_id on myfact_y2004w51(recorded_on_id);
create index myfact_y2004w52_recorded_on_id on myfact_y2004w52(recorded_on_id);
create index myfact_y2004w53_recorded_on_id on myfact_y2004w53(recorded_on_id);
create index dates_y2004w51_recorded_on_id on dates_y2004w51(recorded_on_id);
create index dates_y2004w52_recorded_on_id on dates_y2004w52(recorded_on_id);
create index dates_y2004w53_recorded_on_id on dates_y2004w53(recorded_on_id);
ANALYZE;ANALYZE;ANALYZE;ANALYZE;ANALYZE;
EXPLAIN select dates.datetime, myfact.recorded_on_id from myfact,
dates where myfact.recorded_on_id = dates.recorded_on_id and
dates.recorded_on_id in (1813, 1814);
EXPLAIN select dates.datetime, myfact.recorded_on_id from myfact,
dates where myfact.recorded_on_id = dates.recorded_on_id and
dates.recorded_on_id in (1813, 1814) and myfact.recorded_on_id in
(1813, 1814);
----------------------
--
Ondrej Ivanic
(ondrej.ivanic@xxxxxxxxx)
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