Re: Index Skip Scan

> On Mon, Jan 20, 2020 at 05:05:33PM -0800, Peter Geoghegan wrote:
>
> I suggest that you find a way to add assertions to code like
> _bt_readpage() that verify that we do in fact have the buffer content
> lock. Actually, there is an existing assertion here that covers the
> pin, but not the buffer content lock:
>
> static bool
> _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum)
> {
> <declare variables>
> ...
>
> /*
> * We must have the buffer pinned and locked, but the usual macro can't be
> * used here; this function is what makes it good for currPos.
> */
> Assert(BufferIsValid(so->currPos.buf));
>
> You can add another assertion that calls a new utility function in
> bufmgr.c. That can use the same logic as this existing assertion in
> FlushOneBuffer():
>
> Assert(LWLockHeldByMe(BufferDescriptorGetContentLock(bufHdr)));
>
> We haven't needed assertions like this so far because it's usually it
> is clear whether or not a buffer lock is held (plus the bufmgr.c
> assertions help on their own). The fact that it isn't clear whether or
> not a buffer lock will be held by caller here suggests a problem. Even
> still, having some guard rails in the form of these assertions could
> be helpful. Also, it seems like _bt_scankey_within_page() should have
> a similar set of assertions.

Thanks for suggestion. Agree, we will add such guards. It seems that in
general I need to go through the locking in the patch one more time,
since there are some gaps I din't notice/didn't know about before.

> BTW, there is a paper that describes optimizations like loose index
> scan and skip scan together, in fairly general terms: "Efficient
> Search of Multidimensional B-Trees". Loose index scans are given the
> name "MDAM duplicate elimination" in the paper. See:
>
> http://vldb.org/conf/1995/P710.PDF
>
> Goetz Graefe told me about the paper. It seems like the closest thing
> that exists to a taxonomy or conceptual framework for these
> techniques.

Yes, I've read this paper, as it's indeed the only reference I found
about this topic in literature. But unfortunately it's not much and (at
least from the first read) gives only an overview of the idea.