Hi, Alexander! On Mon, 13 May 2024 at 05:42, Alexander Korotkov <aekorot...@gmail.com> wrote:
> On Mon, May 13, 2024 at 12:23 AM Alexander Korotkov > <aekorot...@gmail.com> wrote: > > On Sat, May 11, 2024 at 4:13 AM Mark Dilger > > <mark.dil...@enterprisedb.com> wrote: > > > > On May 10, 2024, at 12:05 PM, Alexander Korotkov < > aekorot...@gmail.com> wrote: > > > > The only bt_target_page_check() caller is > > > > bt_check_level_from_leftmost(), which overrides state->target in the > > > > next iteration anyway. I think the patch is just refactoring to > > > > eliminate the confusion pointer by Peter Geoghegan upthread. > > > > > > I find your argument unconvincing. > > > > > > After bt_target_page_check() returns at line 919, and before > bt_check_level_from_leftmost() overrides state->target in the next > iteration, bt_check_level_from_leftmost() conditionally fetches an item > from the page referenced by state->target. See line 963. > > > > > > I'm left with four possibilities: > > > > > > > > > 1) bt_target_page_check() never gets to the code that uses > "rightpage" rather than "state->target" in the same iteration where > bt_check_level_from_leftmost() conditionally fetches an item from > state->target, so the change you're making doesn't matter. > > > > > > 2) The code prior to v2-0003 was wrong, having changed state->target > in an inappropriate way, causing the wrong thing to happen at what is now > line 963. The patch fixes the bug, because state->target no longer gets > overwritten where you are now using "rightpage" for the value. > > > > > > 3) The code used to work, having set up state->target correctly in > the place where you are now using "rightpage", but v2-0003 has broken that. > > > > > > 4) It's been broken all along and your patch just changes from wrong > to wrong. > > > > > > > > > If you believe (1) is true, then I'm complaining that you are relying > far to much on action at a distance, and that you are not documenting it. > Even with documentation of this interrelationship, I'd be unhappy with how > brittle the code is. I cannot easily discern that the two don't ever > happen in the same iteration, and I'm not at all convinced one way or the > other. I tried to set up some Asserts about that, but none of the test > cases actually reach the new code, so adding Asserts doesn't help to > investigate the question. > > > > > > If (2) is true, then I'm complaining that the commit message doesn't > mention the fact that this is a bug fix. Bug fixes should be clearly > documented as such, otherwise future work might assume the commit can be > reverted with only stylistic consequences. > > > > > > If (3) is true, then I'm complaining that the patch is flat busted. > > > > > > If (4) is true, then maybe we should revert the entire feature, or > have a discussion of mitigation efforts that are needed. > > > > > > Regardless of which of 1..4 you pick, I think it could all do with > more regression test coverage. > > > > > > > > > For reference, I said something similar earlier today in another email > to this thread: > > > > > > This patch introduces a change that stores a new page into variable > "rightpage" rather than overwriting "state->target", which the old > implementation most certainly did. That means that after returning from > bt_target_page_check() into the calling function > bt_check_level_from_leftmost() the value in state->target is not what it > would have been prior to this patch. Now, that'd be irrelevant if nobody > goes on to consult that value, but just 44 lines further down in > bt_check_level_from_leftmost() state->target is clearly used. So the > behavior at that point is changing between the old and new versions of the > code, and I think I'm within reason to ask if it was wrong before the > patch, wrong after the patch, or something else? Is this a bug being > introduced, being fixed, or ... ? > > > > Thank you for your analysis. I'm inclined to believe in 2, but not > > yet completely sure. It's really pity that our tests don't cover > > this. I'm investigating this area. > > It seems that I got to the bottom of this. Changing > BtreeCheckState.target for a cross-page unique constraint check is > wrong, but that happens only for leaf pages. After that > BtreeCheckState.target is only used for setting the low key. The low > key is only used for non-leaf pages. So, that didn't lead to any > visible bug. I've revised the commit message to reflect this. > I agree with your analysis regarding state->target: - when the unique check is on, state->target was reassigned only for the leaf pages (under P_ISLEAF(topaque) in bt_target_page_check). - in this level (leaf) in bt_check_level_from_leftmost() this value of state->target was used to get state->lowkey. Then it was reset (in the next iteration of do loop in in bt_check_level_from_leftmost() - state->lowkey lives until the end of pages level (leaf) iteration cycle. Then, low-key is reset (state->lowkey = NULL in the end of bt_check_level_from_leftmost()) - state->lowkey is used only in bt_child_check/bt_child_highkey_check. Both are called only from non-leaf pages iteration cycles (under P_ISLEAF(topaque)) - Also there is a check (rightblock_number != P_NONE) in before getting rightpage into state->target in bt_target_page_check() that ensures us that rightpage indeed exists and getting this (unused) lowkey in bt_check_level_from_leftmost will not invoke any page reading errors. I'm pretty sure that there was no bug in this, not just the bug was hidden. Indeed re-assigning state->target in leaf page iteration for cross-page unique check was not beautiful, and Peter pointed out this. In my opinion the patch 0003 is a pure code refactoring. As for the cross-page check regression/TAP testing, this test had problems since the btree page layout is not fixed (especially it's different on 32-bit arch). I had a variant for testing cross-page check when the test was yet regression one upthread for both 32/64 bit architectures. I remember it was decided not to include it due to complications and low impact for testing the corner case of very rare cross-page duplicates. (There were also suggestions to drop cross-page duplicates check at all, which I didn't agree 2 years ago, but still it can make sense) Separately, I propose to avoid getting state->lowkey for leaf pages at all as it's unused. PFA is a simple patch for this. (I don't add it to the current patch set as I believe it has nothing to do with UNIQUE constraint check, rather it improves the previous btree amcheck code) Best regards, Pavel Borisov, Supabase
XXXX-amcheck-Get-lowkey-only-for-internal-pages-of-btree-.patch
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