Re: Hash partitioning.

On 26.06.2013 18:34, Kevin Grittner wrote:
> Markus Wanner<markus(at)bluegap(dot)ch> wrote:
>> On 06/25/2013 11:52 PM, Kevin Grittner wrote:
>>> At least until we have parallel
>>> query execution. At *that* point this all changes.
>>
>> Can you elaborate on that, please? I currently have a hard time
>> imagining how partitions can help performance in that case,
>> either.
>
> Well, partitioning will *still* be a net loss for overall
> throughput on a machine with enough active connections to keep all
> the resources busy. Where it will help is when you have a machine
> with a lot of cores and a few big "reporting" style queries. Since
> we currently can only use one core for a single query, we leave a
> lot of CPU time (often the bottleneck for such queries) unused. If
> we allow a large query to search multiple partitions in parallel, a
> big query can complete sooner.

We could also allow a large query to search a single table in parallel.
A seqscan would be easy to divide into N equally-sized parts that can be
scanned in parallel. It's more difficult for index scans, but even then
it might be possible at least in some limited cases.

- Heikki