On Mon, Jun 19, 2023 at 4:28 PM Peter Geoghegan <p...@bowt.ie> wrote: > We still fall short when it comes to handling boundary cases optimally > during backwards scans. This is at least true for a subset of > backwards scans that request "goback=true" processing inside > _bt_first. Attached patch improves matters here. Again, the simplest > way of explaining what this does is through a practical worked > example.
On further reflection, we should go even further in the direction of teaching _bt_search (and related routines) about the initial leaf level positioning requirements of backwards scans. In fact, we should give _bt_search and friends exclusive responsibility for dealing with initial positioning on behalf of _bt_first. Attached revision shows how this can work. This new direction is partly based on the observation that "goback" is really just a synonym of "backwards scan initial positioning behavior": all backwards scans already use "goback", while all forward scans use "!goback". So why shouldn't we just change any "goback" symbol names to "backward", and be done with it? Then we can move the "step back one item on leaf level" logic from _bt_first over to _bt_binsrch. Now _bt_search/_bt_binsrch/_bt_compare own everything to do with initial positioning. The main benefit of this approach is that it allows _bt_first to describe how its various initial positioning strategies work using high level language, while pretty much leaving the implementation details up to _bt_search. I've always thought that it was confusing that the "<= strategy" uses "nextkey=true" -- how can it be "next key" while also returning keys that directly match those from the insertion scankey? It only makes sense once you see that the "<= strategy" uses both "nextkey=true" and "backwards/goback = true" -- something that the structure in the patch makes clear and explicit. This revision also adds test coverage for the aforementioned "<= strategy" (not to be confused with the strategy that we're optimizing), since it was missing before now. It also adds test coverage for the "< strategy" (which *is* the strategy affected by the new optimization). The "< strategy" already has decent enough coverage -- it just doesn't have coverage that exercises the new optimization. (Note that I use the term "new optimization" advisedly here -- the new behavior is closer to "how it's really supposed to work".) I'm happy with the way that v2 came out, since the new structure makes a lot more sense to me. The refactoring is probably the most important aspect of this patch. The new structure seems like it might become important in a world with skip scan or other new MDAM techniques added to B-Tree. The important principle here is "think in terms of logical key space, not in terms of physical pages". Adding this to the next CF. -- Peter Geoghegan
v2-0001-Optimize-nbtree-backward-scan-boundary-cases.patch
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