Andy Colson wrote:
Cpu's wont get faster, but HD's and SSD's will. To have one database connection, which runs one query, run fast, it's going to need multi-core support.
My point was that situations where people need to run one query on one database connection that aren't in fact limited by disk I/O are far less common than people think. My troublesome database servers aren't ones with a single CPU at its max but wishing there were more workers, they're the ones that have >25% waiting for I/O. And even that crowd is still a subset, distinct from people who don't care about the speed of any one core, they need lots of connections to go at once.
That's not to say we need "parallel query's". Or we need multiple backends to work on one query. We need one backend, working on one query, using mostly the same architecture, to just use more than one core.
That's exactly what we mean when we say "parallel query" in the context of a single server.
My point is, there must be levels of threading, yes? If a backend has data to sort, has it collected, nothing locked, what would it hurt to use multi-core sorting?
Optimizer nodes don't run that way. The executor "pulls" rows out of the top of the node tree, which then pulls from its children, etc. If you just blindly ran off and executed every individual node to completion in parallel, that's not always going to be faster--could be a lot slower, if the original query never even needed to execute portions of the tree.
When you start dealing with all of the types of nodes that are out there it gets very messy in a hurry. Decomposing the nodes of the query tree into steps that can be executed in parallel usefully is the hard problem hiding behind the simple idea of "use all the cores!"
I thought I read a paper someplace that said shared cache (L1/L2/etc) multicore cpu's would start getting really slow at 16/32 cores, and that message passing was the way forward past that. If PG started aiming for 128 core support right now, it should use some kinda message passing with queues thing, yes?
There already is a TupleStore type that is going to serve as the message being sent between the client backends. Unfortunately we won't get anywhere near 128 cores without addressing the known scalability issues that are in the code right now, ones you can easily run into even with 8 cores.
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