Thread: Query regarding RANGE Partitioning

Hi Nitin,

On Fri, May 7, 2021 at 4:21 PM Nitin Jadhav <nitinjadhavpostgres@gmail.com> wrote:

Hi,

I am not convinced with the following behaviour of RANGE Partitioning.

Kindly let me know if this is expected behaviour or it should be changed.

Case-1:

postgres@68941=#create table r(a int, b int) partition by range(a,b);

CREATE TABLE

postgres@68941=#create table r1 partition of r for values from (100,0) to (200,100);

CREATE TABLE

postgres@68941=#create table r2 partition of r for values from (400,200) to (500,300);

CREATE TABLE

postgres@68941=#create table r3 partition of r for values from (0,100) to (100,200);

ERROR:  partition "r3" would overlap partition "r1"

LINE 1: ...able r3 partition of r for values from (0,100) to (100,200);

As we can see here, I am trying to create a partition table with ranges from (0,100) to (100,200)

which is actually not overlapped with any of the existing partitions. But I am getting error saying,

it overlaps with partition 'r1'. 

 

Case-2:

postgres@68941=#\d+ r

                                      Partitioned table "public.r"

 Column |  Type   | Collation | Nullable | Default | Storage | Compression | Stats target | Description 

--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------

 a      | integer |           |          |         | plain   |             |              | 

 b      | integer |           |          |         | plain   |             |              | 

Partition key: RANGE (a, b)

Partitions: r1 FOR VALUES FROM (100, 0) TO (200, 100),

            r2 FOR VALUES FROM (400, 200) TO (500, 300),

            r3 FOR VALUES FROM (200, 100) TO (300, 200)

postgres@68941=#insert into r values(300, 50);

INSERT 0 1

postgres@68941=#select * from r3;

  a  |  b  

-----+-----

 300 |  50

(2 rows)

As per my understanding, in the range partitioned table, lower bound is included and upper bound is excluded.

and in case of multi-column partition keys, the row comparison operator is used for tuple routing which means

the columns are compared left to right. If the partition key value is equal to the upper bound of that column then 

the next column will be considered.

So, In case of insertion of row (300, 50). Based on the understanding, partition 'r3' should have rejected it.

Kindly confirm whether the above is expected or not. If expected, kindly explain.

If you describe the partition r3, you can see the way partition

constraints are formed:

postgres=# \d+ r3

                                           Table "public.r3"

 Column |  Type   | Collation | Nullable | Default | Storage | Compression | Stats target | Description 

--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------

 a      | integer |           |          |         | plain   |             |              | 

 b      | integer |           |          |         | plain   |             |              | 

Partition of: r FOR VALUES FROM (200, 100) TO (300, 200)

Partition constraint: ((a IS NOT NULL) AND (b IS NOT NULL) AND ((a > 200) OR ((a = 200) AND (b >= 100))) AND ((a < 300) OR ((a = 300) AND (b < 200))))

Access method: heap

The above constraint very well fits the tuple you are trying to insert

that is: (a, b) = (300, 50) (where (a = 300) AND (b < 200))

Also, the table partition syntax documentation[1] clarifies this (look

for "partition_bound_expr"):

"When creating a range partition, the lower bound specified with

FROM is an inclusive bound, whereas the upper bound specified with

TO is an exclusive bound. That is, the values specified in the FROM

list are valid values of the corresponding partition key columns

for this partition, whereas those in the TO list are not. Note that

this statement must be understood according to the rules of row-wise

comparison (Section 9.24.5). For example, given PARTITION BY RANGE

(x,y), a partition bound FROM (1, 2) TO (3, 4) allows x=1 with any y>=2,

x=2 with any non-null y, and x=3 with any y<4."

So, in your case the partition (a, b) for bound (200, 100) TO (300, 200)

would transform to allowing:

a = 200 with any b >= 100 OR

a > 200 and a < 300 with any non-null b

OR a=300 with any b<200

Your particular tuple (300, 50) fits in the last part of the OR i.e

(a=300 with any b<200).

So, IMHO, the range partitioning is behaving as expected.

Similarly, for the case-1 you mention above:

create table r1 partition of r for values from (100,0) to (200,100);

create table r3 partition of r for values from (0,100) to (100,200);

here, (100, 0) or r1 would overlap with (100, 200) of r3.

[1] https://www.postgresql.org/docs/current/sql-createtable.html

Regards,

Jeevan Ladhe

Hi Nitin,

On Sat, May 8, 2021 at 5:20 PM Nitin Jadhav
<nitinjadhavpostgres@gmail.com> wrote:
> Thanks Ashutosh and Jeevan for replying.
>
> >  "When creating a range partition, the lower bound specified with
> > FROM is an inclusive bound, whereas the upper bound specified with
> > TO is an exclusive bound. That is, the values specified in the FROM
> > list are valid values of the corresponding partition key columns
> > for this partition, whereas those in the TO list are not. Note that
> > this statement must be understood according to the rules of row-wise
> > comparison (Section 9.24.5). For example, given PARTITION BY RANGE
> > (x,y), a partition bound FROM (1, 2) TO (3, 4) allows x=1 with any y>=2,
> > x=2 with any non-null y, and x=3 with any y<4."
>
> Thanks for the detailed explanation. I understood more about how the
> partition constraints are prepared based on the RANGE bound values and
> how the tuple routing takes place based on that.
>
> > overlapping range is (100, 0), (100, 200)
>
> > Similarly, for the case-1 you mention above:
> > create table r1 partition of r for values from (100,0) to (200,100);
> > create table r3 partition of r for values from (0,100) to (100,200);
> > here, (100, 0) or r1 would overlap with (100, 200) of r3.

Thanks Jeevan, that's right.

Another way to look at this: the partition key (100, 0) would be
insertable into r3, because the key satisfies its proposed exclusive
upper bound (< (100, 200)).  The same key is also insertable into r1,
because it satisfies the latter's inclusive upper bound (>= (100, 0)).
That is, the key (100, 0) is insertable into both r1 and r3, so the
error that the proposed range of r3 would overlap r1's.

> postgres@68941=#\d+ r1
>                                            Table "public.r1"
>  Column |  Type   | Collation | Nullable | Default | Storage | Compression | Stats target | Description
> --------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
>  a      | integer |           |          |         | plain   |             |              |
>  b      | integer |           |          |         | plain   |             |              |
> Partition of: r FOR VALUES FROM (100, 0) TO (200, 100)
> Partition constraint: ((a IS NOT NULL) AND (b IS NOT NULL) AND ((a > 100) OR ((a = 100) AND (b >= 0))) AND ((a < 200) OR ((a = 200) AND (b < 100))))
> Access method: heap
>
> I understand that partition 'r1' says, when column 'a' value is '100', column 'b'
> values should be greater than '0'. Because of this constraint, creation of
> partition 'r3' for values from (0,100) to (100,200) failed since the condition
> when value of column 'a' is 100, column 'b' should be less than '200' which
> overlaps with the constraints of 'r1'. So, based on the documentation, the
> behaviour is correct.
>
> So in the above scenarios, users cannot create a partition for column 'a' values
> from (0) to (100). If user tries insert any values for column 'a' between '0' to '100',
> either it should go to default partition if exists. Otherwise it should fail saying, no partition
> found. I feel there should be some way to create partitions in these scenarios.

Well, you simply need to come up with bound values for r3 that don't
overlap with existing partitions' ranges; the following will work for
example:

create table r3 partition of r for values from (0,100) to (100,0);

--
Amit Langote
EDB: http://www.enterprisedb.com