Re: hash partitioning based on v10Beta2

On Sat, Aug 26, 2017 at 10:10 AM, yangjie(at)highgo(dot)com <yangjie(at)highgo(dot)com>
wrote:

> Hi all,
>
> Now we have had the range / list partition, but hash partitioning is not
> implemented yet.
> Attached is a POC patch based on the v10Beta2 to add the
> hash partitioning feature.
> Although we will need more discussions about the syntax
> and other specifications before going ahead the project,
> but I think this runnable code might help to discuss
> what and how we implement this.
>
>
FYI, there is already an existing commitfest entry for this project.

https://commitfest.postgresql.org/14/1059/

> Description
>
> The hash partition's implement is on the basis of
> the original range / list partition,and using similar syntax.
>
> To create a partitioned table ,use:
>
> CREATE TABLE h (id int) PARTITION BY HASH(id);
>
> The partitioning key supports only one value, and I think
> the partition key can support multiple values,
> which may be difficult to implement when querying, but
> it is not impossible.
>
> A partition table can be create as bellow:
>
> CREATE TABLE h1 PARTITION OF h;
> CREATE TABLE h2 PARTITION OF h;
> CREATE TABLE h3 PARTITION OF h;
>
> FOR VALUES clause cannot be used, and the partition bound
> is calclulated automatically as partition index of single integer value.
>
> An inserted record is stored in a partition whose index equals
> DatumGetUInt32(OidFunctionCall1(lookup_type_cache(key->parttypid[0],
> TYPECACHE_HASH_PROC)->hash_proc, values[0])) % nparts
> /* Number of partitions */
> ;
> In the above example, this is DatumGetUInt32(OidFunctionCall1(lookup_type_
> cache(key->parttypid[0], TYPECACHE_HASH_PROC)->hash_proc, id)) % 3;
>
> postgres=# insert into h select generate_series(1,20);
> INSERT 0 20
> postgres=# select tableoid::regclass,* from h;
> tableoid | id
> ----------+----
> h1 | 3
> h1 | 5
> h1 | 17
> h1 | 19
> h2 | 2
> h2 | 6
> h2 | 7
> h2 | 11
> h2 | 12
> h2 | 14
> h2 | 15
> h2 | 18
> h2 | 20
> h3 | 1
> h3 | 4
> h3 | 8
> h3 | 9
> h3 | 10
> h3 | 13
> h3 | 16
> (20 rows)
>
> The number of partitions here can be dynamically added, and
> if a new partition is created, the number of partitions
> changes, the calculated target partitions will change,
> and the same data is not reasonable in different
> partitions,So you need to re-calculate the existing data
> and insert the target partition when you create a new partition.
>
> postgres=# create table h4 partition of h;
> CREATE TABLE
> postgres=# select tableoid::regclass,* from h;
> tableoid | id
> ----------+----
> h1 | 5
> h1 | 17
> h1 | 19
> h1 | 6
> h1 | 12
> h1 | 8
> h1 | 13
> h2 | 11
> h2 | 14
> h3 | 1
> h3 | 9
> h3 | 2
> h3 | 15
> h4 | 3
> h4 | 7
> h4 | 18
> h4 | 20
> h4 | 4
> h4 | 10
> h4 | 16
> (20 rows)
>
> When querying the data, the hash partition uses the same
> algorithm as the insertion, and filters out the table
> that does not need to be scanned.
>
> postgres=# explain analyze select * from h where id = 1;
> QUERY PLAN
>
> ------------------------------------------------------------
> ----------------------------------------
> Append (cost=0.00..41.88 rows=13 width=4) (actual time=
> 0.020..0.023 rows=1 loops=1)
> -> Seq Scan on h3 (cost=0.00..41.88 rows=13 width=4) (
> actual time=0.013..0.016 rows=1 loops=1)
> Filter: (id = 1)
> Rows Removed by Filter: 3
> Planning time: 0.346 ms
> Execution time: 0.061 ms
> (6 rows)
>
> postgres=# explain analyze select * from h where id in (1,5);;
> QUERY PLAN
>
> ------------------------------------------------------------
> ----------------------------------------
> Append (cost=0.00..83.75 rows=52 width=4) (actual time=
> 0.016..0.028 rows=2 loops=1)
> -> Seq Scan on h1 (cost=0.00..41.88 rows=26 width=4) (
> actual time=0.015..0.018 rows=1 loops=1)
> Filter: (id = ANY ('{1,5}'::integer[]))
> Rows Removed by Filter: 6
> -> Seq Scan on h3 (cost=0.00..41.88 rows=26 width=4) (
> actual time=0.005..0.007 rows=1 loops=1)
> Filter: (id = ANY ('{1,5}'::integer[]))
> Rows Removed by Filter: 3
> Planning time: 0.720 ms
> Execution time: 0.074 ms
> (9 rows)
>
> postgres=# explain analyze select * from h where id = 1 or id = 5;;
> QUERY PLAN
>
> ------------------------------------------------------------
> ----------------------------------------
> Append (cost=0.00..96.50 rows=50 width=4) (actual time=
> 0.017..0.078 rows=2 loops=1)
> -> Seq Scan on h1 (cost=0.00..48.25 rows=25 width=4) (
> actual time=0.015..0.019 rows=1 loops=1)
> Filter: ((id = 1) OR (id = 5))
> Rows Removed by Filter: 6
> -> Seq Scan on h3 (cost=0.00..48.25 rows=25 width=4) (
> actual time=0.005..0.010 rows=1 loops=1)
> Filter: ((id = 1) OR (id = 5))
> Rows Removed by Filter: 3
> Planning time: 0.396 ms
> Execution time: 0.139 ms
> (9 rows)
>
> Can not detach / attach / drop partition table.
>
> Best regards,
> young
>
> ------------------------------
>
> https://yonj1e.github.io/
>
>
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--
Rushabh Lathia