RE: Problem with default partition pruning

Hosoya-san

On Wed, Feb 27, 2019 at 6:51 AM, Yuzuko Hosoya wrote:
> > From: Amit Langote [mailto:Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp]
> > Sent: Wednesday, February 27, 2019 11:22 AM
> >
> > Hosoya-san,
> >
> > On 2019/02/22 17:14, Yuzuko Hosoya wrote:
> > > Hi,
> > >
> > > I found the bug of default partition pruning when executing a range
> query.
> > >
> > > -----
> > > postgres=# create table test1(id int, val text) partition by range
> > > (id); postgres=# create table test1_1 partition of test1 for values
> > > from (0) to (100); postgres=# create table test1_2 partition of
> > > test1 for values from (150) to (200); postgres=# create table
> > > test1_def partition of test1 default;
> > >
> > > postgres=# explain select * from test1 where id > 0 and id < 30;
> > > QUERY PLAN
> > > ----------------------------------------------------------------
> > > Append (cost=0.00..11.83 rows=59 width=11)
> > > -> Seq Scan on test1_1 (cost=0.00..5.00 rows=58 width=11)
> > > Filter: ((id > 0) AND (id < 30))
> > > -> Seq Scan on test1_def (cost=0.00..6.53 rows=1 width=12)
> > > Filter: ((id > 0) AND (id < 30))
> > > (5 rows)
> > >
> > > There is no need to scan the default partition, but it's scanned.
> > > -----
> > >
> > > In the current implement, whether the default partition is scanned
> > > or not is determined according to each condition of given WHERE
> > > clause at get_matching_range_bounds(). In this example,
> > > scan_default is set true according to id > 0 because id >= 200
> > > matches the default partition. Similarly, according to id < 30,
> scan_default is set true.
> > > Then, these results are combined according to AND/OR at
> perform_pruning_combine_step().
> > > In this case, final result's scan_default is set true.
> > >
> > > The modifications I made are as follows:
> > > - get_matching_range_bounds() determines only offsets of range bounds
> > > according to each condition
> > > - These results are combined at perform_pruning_combine_step()
> > > - Whether the default partition is scanned or not is determined at
> > > get_matching_partitions()
> > >
> > > Attached the patch. Any feedback is greatly appreciated.
> >
> > Thank you for reporting. Can you please add this to March CF in Bugs
> > category so as not to lose
> track
> > of this?
> >
> > I will try to send review comments soon.
> >
> Thank you for your reply. I added this to March CF.

I tested with simple use case and I confirmed it works correctly like below.

In case using between clause:
postgres=# create table test1(id int, val text) partition by range (id);
postgres=# create table test1_1 partition of test1 for values from (0) to (100);
postgres=# create table test1_2 partition of test1 for values from (150) to (200);
postgres=# create table test1_def partition of test1 default;

[HEAD]
postgres=# explain analyze select * from test1 where id between 0 and 50;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------
Append (cost=0.00..58.16 rows=12 width=36) (actual time=0.008..0.008 rows=0 loops=1)
-> Seq Scan on test1_1 (cost=0.00..29.05 rows=6 width=36) (actual time=0.005..0.005 rows=0 loops=1)
Filter: ((id >= 0) AND (id <= 50))
-> Seq Scan on test1_def (cost=0.00..29.05 rows=6 width=36) (actual time=0.002..0.002 rows=0 loops=1)
Filter: ((id >= 0) AND (id <= 50))

[patched]
postgres=# explain analyze select * from test1 where id between 0 and 50;
QUERY PLAN
---------------------------------------------------------------------------------------------------------
Append (cost=0.00..29.08 rows=6 width=36) (actual time=0.006..0.006 rows=0 loops=1)
-> Seq Scan on test1_1 (cost=0.00..29.05 rows=6 width=36) (actual time=0.004..0.005 rows=0 loops=1)
Filter: ((id >= 0) AND (id <= 50))

I considered about another use case. If default partition contains rows whose id = 300 and then we add another partition which have constraints like id >= 300 and id < 400, I thought we won't scan the rows anymore. But I noticed we simply can't add such a partition.

postgres=# insert into test1 values (300);
INSERT 0 1
postgres=# create table test1_3 partition of test1 for values from (300) to (400);
ERROR: updated partition constraint for default partition "test1_def" would be violated by some row

So I haven't come up with bad cases so far :)

--
Yoshikazu Imai