Re: [HACKERS] [POC] hash partitioning

Nagata-san,

On 2017/02/28 23:33, Yugo Nagata wrote:
> Hi all,
> 
> Now we have a declarative partitioning, but hash partitioning is not
> implemented yet. Attached is a POC patch to add the hash partitioning
> feature. I know 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.

Great!

> * Description
> 
> In this patch, the hash partitioning implementation is basically based
> on the list partitioning mechanism. However, partition bounds cannot be
> specified explicitly, but this is used internally as hash partition
> index, which is calculated when a partition is created or attached.
> 
> The tentative syntax to create a partitioned table is as bellow;
> 
>  CREATE TABLE h (i int) PARTITION BY HASH(i) PARTITIONS 3 USING hashint4;
> 
> The number of partitions is specified by PARTITIONS, which is currently
> constant and cannot be changed, but I think this is needed to be changed in
> some manner. A hash function is specified by USING. Maybe, specifying hash
> function may be ommitted, and in this case, a default hash function
> corresponding to key type will be used.
> 
> 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.
> 
> When trying create partitions more than the number specified
> by PARTITIONS, it gets an error.
> 
> postgres=# create table h4 partition of h;
> ERROR:  cannot create hash partition more than 3 for h

Instead of having to create each partition individually, wouldn't it be
better if the following command

CREATE TABLE h (i int) PARTITION BY HASH (i) PARTITIONS 3;

created the partitions *automatically*?

It makes sense to provide a way to create individual list and range
partitions separately, because users can specify custom bounds for each.
We don't need that for hash partitions, so why make users run separate
commands (without the FOR VALUES clause) anyway?  We may perhaps need to
offer a way to optionally specify a user-defined name for each partition
in the same command, along with tablespace, storage options, etc.  By
default, the names would be generated internally and the user can ALTER
individual partitions after the fact to specify tablespace, etc.

Thanks,
Amit