On 02/03/2011 10:00 PM, Greg Smith wrote:
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.
Yes, I agree... for today. If you gaze into 5 years... double the core count
(but not the speed), double the IO rate. What do you see?
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!"
What if... the nodes were run in separate threads, and interconnected via
queues? A node would not have to run to completion either. A queue could be
setup to have a max items. When a node adds 5 out of 5 items it would go to
sleep. Its parent node, removing one of the items could wake it up.
-Andy
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