Re: [HACKERS] Poor memory context performance in large hash joins

Jeff Janes <jeff.janes@gmail.com> writes:
> The number of new chunks can be almost as as large as the number of old
> chunks, especially if there is a very popular value.  The problem is that
> every time an old chunk is freed, the code in aset.c around line 968 has to
> walk over all the newly allocated chunks in the linked list before it can
> find the old one being freed.  This is an N^2 operation, and I think it has
> horrible CPU cache hit rates as well.

Maybe it's time to convert that to a doubly-linked list.  Although if the
hash code is producing a whole lot of requests that are only a bit bigger
than the separate-block threshold, I'd say It's Doing It Wrong.  It should
learn to aggregate them into larger requests.
        regards, tom lane