Thread: Re: Corrupt btree index includes rows that don't match
Hi again,
Thanks very much for the replies last week. We’ve been continuing to investigate this problem, and I thought I’d share an update on where we are.
To recap: the situation is that, looking at our backup from 2025-06-26 via pageinspect, we have btree index rows which point to either non-existent heap TIDs, or to heap TIDs with data which does not correspond to the index row. In fact it looks like we have entire index pages which point only to non-existent heap TIDs.
(I previously said that these index rows were marked as ‘dead’ in the backup. We now suspect this is an artifact of the restore process: we believe they are live in the backup, but were marked as dead during the restore.)
Empirically, and surprisingly to us, when one does a SELECT from an index entry that points to a non-existent TID, the index entry is quietly ignored.
We therefore suspect that this index corruption has been present for some time (possibly years); more recently those non-existent heap TIDs have been recycled, and that is when we have noticed the effects of the problem.
As far as we can tell, the corruption only affects one index on one table, and only a specific region of that index/table. Specifically, it only appears to affect rows which would have been inserted between 2018 and January 2021. At least 1B rows appear to be affected (the table as a whole has 29B rows).
One thing that surprised us is that ‘amcheck’ didn’t find any sign of the corruption. We’re not completely sure if this is because we are holding it wrong, or because it’s simply out of scope or unsupported for amcheck. Any advice on this, or suggestions for other tooling we could use to check the consistency of our other indexes, would be much appreciated.
We’re still very interested in trying to understand the root cause of the corruption, mostly to confirm that it’s not an ongoing problem. Thanks Tom for the suggestion of https://git.postgresql.org/gitweb/?p=postgresql.git&a=commitdiff&h=4934d3875. We agree with your assessment that this is unlikely. For one thing, it looks like that bug could only conceivably cause this corruption if it affected an UPDATE query, and we’re reasonably sure we never do any UPDATE queries on that table. (The table is mostly append-only. We do sometimes run cleanup/compression jobs which amount to large amounts of interleaved DELETEs and INSERTs, but no UPDATEs.)
Back in 2021, we were running Postgres 10.11. We’ve taken a pass through the release notes since then to see if we can find any likely-looking bugs. We found the one that causes BRIN index corruption (this is not a BRIN index), and the one that causes CREATE INDEX CONCURRENTLY to end up with too *few* entries (this one has the opposite problem), but no particularly likely candidate. Any other suggestions would be welcome here.
At the moment, a historical hardware-level problem seems like it might be the most likely culprit, though we are a bit mystified about how any hardware failure could have caused such widespread damage to a single index, whilst apparently leaving the rest of the database intact.
Any thoughts or suggestions are very much appreciated.
Thanks,
Erik
On Fri, 4 Jul 2025, 15:38 Ron Johnson, <ronljohnsonjr@gmail.com> wrote:On Fri, Jul 4, 2025 at 9:49 AM Erik Johnston <erikj@element.io> wrote:Hi, a quick update:
- We have discovered that the corruption was present from before libicu update.
- We ran `pg_amcheck --index state_groups_state_type_idx --heapallindexed matrix`, which returned nothing
- We believe that means that (and matches what we see sampling) the index has gained extra entries, i.e. that for a given state group it does return all the relevant rows in the table plus extra rows.We are also seeing old state groups starting to point at rows that have only just been inserted. For example, querying for 353864583 on the primary it returns that row plus four rows that have been inserted today, but on the backup from last week an index only scan for 353864583 only returns one row. This makes it feel like the corruption is ongoing? Nothing should have modified that state group in the interim (they are generally immutable).
This naively feels like when inserting a new row we sometimes add the row to the index twice: once pointing from the correct state group to the new row, and once from an old state group to the new row?
Are checksums enabled in the instance?Alas not.We've also now found that the index on the backup does in fact point to those ctids after all, but they are marked as dead. So at some point between then and when we inserted the new row at that ctid today those entries were marked undead.
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This message is intended for the addressee only and may contain private and confidential information or material which may be privileged. If this message has come to you in error please delete it immediately and do not copy it or show it to any other person.