Re: [POC] hash partitioning

On Thu, Nov 2, 2017 at 1:35 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Wed, Nov 1, 2017 at 3:46 PM, amul sul <sulamul@gmail.com> wrote:
>> Although partition constraints become more simple, there isn't any performance
>> gain with 0005 patch. Also I am little skeptic about logic in 0005 where we
>> copied extended hash function info from the partition key, what if parent is
>> changed while we are using it? Do we need to keep lock on parent until commit in
>> satisfies_hash_partition?
>
> I don't think it should be possible for the parent to be changed.  I
> mean, the partition key is altogether immutable -- it can't be changed
> after creation time.  The partition bounds can be changed for
> individual partitions but that would require a lock on the partition.
>
> Can you give an example of the kind of scenario about which you are concerned?

Right now partition keys are immutable but we don't have much code
written with that assumption. All the code usually keeps a lock on the
parent till the time they use the information in the partition key. In
a distant future, which may not exist, we may support ALTER TABLE ...
PARTITION BY to change partition keys (albeit at huge cost of data
movement). If we do that, we will have to remember this one-off
instance of code which assumes that the partition keys are immutable.

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company