Re: add_path optimization

On Sun, Feb 1, 2009 at 12:03 PM, David Fetter <david@fetter.org> wrote:
> On Sat, Jan 31, 2009 at 11:37:39PM -0500, Robert Haas wrote:
>> I've been doing some benchmarking and profiling on the PostgreSQL
>> query analyzer, and it seems that (at least for the sorts of queries
>> that I typically run) the dominant cost is add_path().  I've been
>> able to find two optimizations that seem to help significantly:
>
> Are there any cases you've found where this change significantly
> impairs performance, and if so, how did you find them?  If not, would
> you be up for trying to find some?

Basically, the patch is just performing the same operations with less
overhead.  For example, add_similar_path() is pretty much the same
thing as repeated calls to add_path(), but you save the cost of
unnecessary pathkey comparisons and maybe some ListCell alloc/free
cycles.  So I'm not really sure how it could make things worse, but
I'd be interested in knowing if there's a case that you're worried
about.  It's pretty low-level code, so I don't think there's room for
a lot of surprises.

...Robert