On 2016-05-11 10:12:26 -0400, Robert Haas wrote:
> > I've to admit I'm not that convinced about the speedups in the !fdw
> > case. There seems to be a lot easier avenues for performance
> > improvements.
> What I'm talking about is a query like this:
> SELECT * FROM inheritance_tree_of_foreign_tables WHERE very_rarely_true;

Note that I said "!fdw case".

> > FWIW, I've even hacked something up for a bunch of simple queries, and
> > the performance improvements were significant.  Besides it only being a
> > weekend hack project, the big thing I got stuck on was considering how
> > to exactly determine when to batch and not to batch.
> Yeah.  I think we need a system for signalling nodes as to when they
> will be run to completion.  But a Boolean is somehow unsatisfying;
> LIMIT 1000000 is more like no LIMIT than it it is like LIMIT 1.  I'm
> tempted to add a numTuples field to every ExecutorState and give upper
> nodes some way to set it, as a hint.

I was wondering whether we should hand down TupleVectorStates to lower
nodes, and their size determines the max batch size...

> >> Some care is required here because any
> >> functions we execute as scan keys are run with the buffer locked, so
> >> we had better not run anything very complicated.  But doing this for
> >> simple things like integer equality operators seems like it could save
> >> quite a few buffer lock/unlock cycles and some other executor overhead
> >> as well.
> >
> > Hm. Do we really have to keep the page locked in the page-at-a-time
> > mode? Shouldn't the pin suffice?
> I think we need a lock to examine MVCC visibility information.  A pin
> is enough to prevent a tuple from being removed, but not from having
> its xmax and cmax overwritten at almost but not quite exactly the same
> time.

We already batch visibility lookups in page-at-a-time
mode. Cf. heapgetpage() / scan->rs_vistuples. So we can evaluate quals
after releasing the lock, but before the pin is released, without that
much effort.  IIRC that isn't used for index lookups, but that's
probably a good idea.


Andres Freund

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