Re: Buffering GiST leaf pages too

On Aug 23, 2011, at 2:03 AM, Heikki Linnakangas wrote:
> While looking at Alexander's GiST fastbuild patch, which adds buffers to internal nodes to avoid random I/O during
indexbuild, it occurred to me that inserting the tuples to the leaf pages one at a time is quite inefficient too, even
ifthe leaf pages are in cache. There's still the overhead of locking and WAL-logging each insertion separately. I think
wecould get a nice further speedup if we attach a small buffer (one block or so) to every leaf page we're currently
writingtuples to, and update the leaf page in bulk. Conveniently, the code to insert multiple tuples to a page already
existsin GiST code (because inserting a tuple sometimes splits the page into more than two parts, so you need to insert
multipledownlinks to the parent), so this requires no changes to the low-level routines and WAL-logging. 
>
> Let's finish off the main fastbuild patch first, but I wanted to get the idea out there.

I've often wondered about the per-tuple overhead of all kinds of operations, not just GiST index builds. For example,
ifyou're doing a seqscan, ISTM it would be a lot more efficient to memcpy an entire page into backend-local memory and
operateoff of that lock-free. Similarly for an index scan, you'd want to copy a full leaf page if you think you'll be
hittingit more than once or twice. 
--
Jim C. Nasby, Database Architect                   jim@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net