Re: Add SPLIT PARTITION/MERGE PARTITIONS commands

On Wed, May 1, 2024 at 12:14 AM Dmitry Koval <d(dot)koval(at)postgrespro(dot)ru> wrote:
> 30.04.2024 6:00, Alexander Lakhin пишет:
> > Maybe I'm doing something wrong, but the following script:
> > CREATE TABLE t (i int, PRIMARY KEY(i)) PARTITION BY RANGE (i);
> > CREATE TABLE tp_0 PARTITION OF t FOR VALUES FROM (0) TO (1);
> > CREATE TABLE tp_1 PARTITION OF t FOR VALUES FROM (1) TO (2);
> >
> > CREATE TABLE t2 (LIKE t INCLUDING ALL);
> > CREATE TABLE tp2 (LIKE tp_0 INCLUDING ALL);
> > creates tables t2, tp2 without not-null constraints.
>
> To create partitions is used the "CREATE TABLE ... LIKE ..." command
> with the "EXCLUDING INDEXES" modifier (to speed up the insertion of
values).
>
> CREATE TABLE t (i int, PRIMARY KEY(i)) PARTITION BY RANGE(i);
> CREATE TABLE t2 (LIKE t INCLUDING ALL EXCLUDING INDEXES EXCLUDING
IDENTITY);
> \d+ t2;
> ...
> Not-null constraints:
> "t2_i_not_null" NOT NULL "i"
> Access method: heap

I've explored this a little bit more.

If the parent table has explicit not null constraint than results of
MERGE/SPLIT look the same as result of CREATE TABLE ... PARTITION OF. In
every case there is explicit not null constraint in all the cases.

# CREATE TABLE t (i int not null, PRIMARY KEY(i)) PARTITION BY RANGE(i);
# \d+ t
Partitioned table "public.t"
Column | Type | Collation | Nullable | Default | Storage | Compression
| Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
i | integer | | not null | | plain |
| |
Partition key: RANGE (i)
Indexes:
"t_pkey" PRIMARY KEY, btree (i)
Not-null constraints:
"t_i_not_null" NOT NULL "i"
Number of partitions: 0
# CREATE TABLE tp_0_2 PARTITION OF t FOR VALUES FROM (0) TO (2);
# \d+ tp_0_2
Table "public.tp_0_2"
Column | Type | Collation | Nullable | Default | Storage | Compression
| Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
i | integer | | not null | | plain |
| |
Partition of: t FOR VALUES FROM (0) TO (2)
Partition constraint: ((i IS NOT NULL) AND (i >= 0) AND (i < 2))
Indexes:
"tp_0_2_pkey" PRIMARY KEY, btree (i)
Not-null constraints:
"t_i_not_null" NOT NULL "i" (inherited)
Access method: heap
# ALTER TABLE t SPLIT PARTITION tp_0_2 INTO
# (PARTITION tp_0_1 FOR VALUES FROM (0) TO (1),
# PARTITION tp_1_2 FOR VALUES FROM (1) TO (2))
# \d+ tp_0_1
Table "public.tp_0_1"
Column | Type | Collation | Nullable | Default | Storage | Compression
| Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
i | integer | | not null | | plain |
| |
Partition of: t FOR VALUES FROM (0) TO (1)
Partition constraint: ((i IS NOT NULL) AND (i >= 0) AND (i < 1))
Indexes:
"tp_0_1_pkey" PRIMARY KEY, btree (i)
Not-null constraints:
"t_i_not_null" NOT NULL "i" (inherited)
Access method: heap

However, if not null constraint is implicit and derived from primary key,
the situation is different. The partition created by CREATE TABLE ...
PARTITION OF doesn't have explicit not null constraint just like the
parent. But the partition created by MERGE/SPLIT has explicit not null
contraint.

# CREATE TABLE t (i int not null, PRIMARY KEY(i)) PARTITION BY RANGE(i);
# \d+ t
Partitioned table "public.t"
Column | Type | Collation | Nullable | Default | Storage | Compression
| Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
i | integer | | not null | | plain |
| |
Partition key: RANGE (i)
Indexes:
"t_pkey" PRIMARY KEY, btree (i)
Number of partitions: 0
# CREATE TABLE tp_0_2 PARTITION OF t FOR VALUES FROM (0) TO (2);
# \d+ tp_0_2
Table "public.tp_0_2"
Column | Type | Collation | Nullable | Default | Storage | Compression
| Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
i | integer | | not null | | plain |
| |
Partition of: t FOR VALUES FROM (0) TO (2)
Partition constraint: ((i IS NOT NULL) AND (i >= 0) AND (i < 2))
Indexes:
"tp_0_2_pkey" PRIMARY KEY, btree (i)
Access method: heap
# ALTER TABLE t SPLIT PARTITION tp_0_2 INTO
# (PARTITION tp_0_1 FOR VALUES FROM (0) TO (1),
# PARTITION tp_1_2 FOR VALUES FROM (1) TO (2))
# \d+ tp_0_1
Table "public.tp_0_1"
Column | Type | Collation | Nullable | Default | Storage | Compression
| Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
i | integer | | not null | | plain |
| |
Partition of: t FOR VALUES FROM (0) TO (1)
Partition constraint: ((i IS NOT NULL) AND (i >= 0) AND (i < 1))
Indexes:
"tp_0_1_pkey" PRIMARY KEY, btree (i)
Not-null constraints:
"tp_0_1_i_not_null" NOT NULL "i"
Access method: heap

I think this is related to the fact that we create indexes later. The same
applies to CREATE TABLE ... LIKE. If we create indexes immediately, not
explicit not null contraints are created. Not if we do without indexes, we
have an explicit not null constraint.

# CREATE TABLE t2 (LIKE t INCLUDING ALL);
# \d+ t2
Table "public.t2"
Column | Type | Collation | Nullable | Default | Storage | Compression
| Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
i | integer | | not null | | plain |
| |
Not-null constraints:
"t2_i_not_null" NOT NULL "i"
Access method: heap
# CREATE TABLE t3 (LIKE t INCLUDING ALL EXCLUDING IDENTITY);
# \d+ t3
Table "public.t3"
Column | Type | Collation | Nullable | Default | Storage | Compression
| Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
i | integer | | not null | | plain |
| |
Indexes:
"t3_pkey" PRIMARY KEY, btree (i)
Access method: heap

I think this is feasible to avoid. However, it's minor and we exactly
documented how we create new partitions. So, I think it works "as
documented" and we don't have to fix this for v17.

------
Regards,
Alexander Korotkov
Supabase