Re: IRe: BUG #16792: silent corruption of GIN index resulting in SELECTs returning non-matching rows

On Sun, Jul 25, 2021 at 9:08 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>
> On 23/07/2021 18:04, Pawel Kudzia wrote:
> > Thanks a lot for your patience and multiple patches that you've
> > provided. Please pardon my ignorance - I don't have low-level
> > understanding of how the query is being executed - but are you sure
> > that index is missing entries and not the other way around - that it
> > has too many entries?
>
> To be precise, the index has an extra entry for row (4002784,1) with key
> 1373, and it's missing the entry with key 38048120. And for row
> (4002869,14), it has an extra row for key 1373, and it's missing the
> entry for key 95333744.
>
> > To recap - SELECT, answered based on the GIN, reports rows that
> > actually do not match the criteria provided in WHERE. Just lowering
> > work_mem makes the problem go away, whith GIN still being used.
>
> The reason that lowering work_mem hides the problem is that GIN collects
> all the matches in a so called TID bitmap, and if the bitmap grows too
> large compared to work_mem, it becomes lossy to save memory. When it's
> lossy, it only stores the heap block numbers of the matches. For a
> regular, non-lossy, match, the Bitmap Heap Scan just returns the row
> that the index says is a match. For the lossy matches, the Bitmap Heap
> Scan node needs to check every row on the page to see which ones
> actually match. This re-checking hides the problem that some of the
> matches that the index reported were not real.

Thank you for the explanation!


>
> Note that you are also getting incorrect results with missing row for
> other queries. You can try it with e.g.:
>
> -- Using the index
> set enable_seqscan=off;
> set enable_bitmapscan=on;
> SELECT count(*) FROM entity WHERE attribute_name_ids && '{95333744}';
> SELECT count(*) FROM entity WHERE attribute_name_ids && '{38048120}';
>
> -- Without index
> set enable_seqscan=on;
> set enable_bitmapscan=off;
> SELECT count(*) FROM entity WHERE attribute_name_ids && '{95333744}';
> SELECT count(*) FROM entity WHERE attribute_name_ids && '{38048120}';
>
>

Actually - both give identical results - count(*) = 0.

I think you actually wanted me to run those:

data=# set enable_seqscan=off;
SET
data=# set enable_bitmapscan=on;
SET
data=# SELECT count(*) FROM entity WHERE attribute_name_ids && '{1737}';
 count
-------
 79565
(1 row)

data=# set enable_seqscan=on;
SET
data=# set enable_bitmapscan=off;
SET
data=# SELECT count(*) FROM entity WHERE attribute_name_ids && '{1737}';
 count
-------
 79560
(1 row)

Results indeed differ.

> I'll work on a patch to add more sanity checks to the GIN code when it
> traverses the tree, to catch the case that it accidentally steps on a
> wrong kind of a page (I'm pretty busy next week, so might not get to
> that until the week after though). I don't think that will help here,
> but who knows, and at least we can rule out some kinds of bugs.
> Alexander, can you finish the fixes to the ternary logic? That doesn't
> explain this corruption either, but we should fix it anyway.
>

Thanks a lot! I'm happy to test.

Greetings!


-- 
regards,
Pawel Kudzia