At 00:32 27/01/2012, you wrote:
There are cases where intraquery parallelism would be helpful. As
far as I understand it, PostgreSQL is the only major, solid (i.e.
excluding MySQL) RDBMS which does not offer some sort of intraquery
parallelism, and when running queries across very large databases,
it might be helpful to be able to, say, scan different partitions
simultaneously using different threads. So I think it is wrong to
simply dismiss the need out of hand. The thing though is that I am
not sure that where this need really comes to the fore, it is
typical of single-server instances, and so this brings me to the
bigger question.
The question in my mind though is a more basic one: How should
intraquery parallelism be handled? Is it something PostgreSQL needs
to do or is it something that should be the work of an external
project like Postgres-XC? Down the road is there value in merging
the codebases, perhaps making stand-alone/data/coordination node a
compile time option?
I still don't think threads are the solution for this scenary. You
can do intraquery parallelism with multiprocess easier and safer than
with multithread. You launch a process with the whole query, it
divide the work in chunks and assigns them to different process
instead of threads. You can use shared resources for communicattion
between process. When all work is done, they pass results to the
original process and it join them. The principal advantage doing it
with process is that if one of the child subprocess dies, it can be
killed/slained and relaunched without any damage to the work of the
other brothers, but if you use threads, the whole process and all the
work done is lost.
It's not the unique advantage of using process vs threads. Some years
ago, one of the problems on multi socket servers was with the shared
memory and communications between the sockets. The inter cpu speed
was too much slow and latency too much high. Now, we have multi cpus
in one socket and faster intersocket communications and this is not a
problem anymore. Even better, the speed and latency communicating 2
or more servers (not sockets or cpus) is reaching levels where a
postgresql could have a shared memory between them, for example using
Hypertransport cards or modern FC, and it's easier, lot easier,
launch a remote process than a remote thread.
Obviously such is not a question that needs to be addressed now. We
can wait until someone has something that is production-ready and
relatively feature-complete before discussing merging projects.
Best Wishes,
Chris Travers
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