REINDEX on vchordrq fails with "missing chunk ... for toast value" after 0.5.3 -> 1.0.0 upgrade, when table has dead tuples

No source file or test is named. Start by reproducing the upgrade sequence with a TOASTed vector column, dead tuples, and the vchordrq index, then compare REINDEX INDEX clip_index with and without VACUUM (ANALYZE) before ALTER EXTENSION vchord UPDATE. Done means the upgraded index can be reindexed successfully without requiring a pre-upgrade vacuum, while the reported TOAST error remains covered by a regression test.

Summary

REINDEX INDEX on a vchordrq index fails with missing chunk number N for toast value M in pg_toast_X after upgrading from 0.5.3 to 1.0.0, when the underlying table has dead tuples. The embeddings themselves are not corrupt — every row detoasts fine and the same reindex succeeds on 0.5.3. Running VACUUM on the table before the extension upgrade avoids the failure entirely.

Environment

• PostgreSQL 16.14 (Debian 16.14-1.pgdg13+1), Debian 13 (trixie), amd64
• vchord 0.5.3 → 1.0.0
• vector (pgvector) 0.8.1 → 0.8.2
• Table with a TOASTed vector column and a vchordrq index:

  Table "public.smart_search"
    Column   |     Type     | Storage
  -----------+--------------+----------
   assetId   | uuid         | plain
   embedding | vector(1152) | external
  
  Indexes:
      "smart_search_pkey" PRIMARY KEY, btree ("assetId")
      "clip_index" vchordrq (embedding vector_cosine_ops) WITH (
          residual_quantization = false,
          build.internal.lists = [512],
          spherical_centroids = true,
          build_threads = 4,
          sampling_factor = 1024)
  

  ~270k rows.

Steps to reproduce

1. On vchord 0.5.3, have a table like the above with some accumulated dead tuples (normal insert/update churn without a recent vacuum/autovacuum pass).
2. Install vchord 1.0.0, restart postgres, then run:

   ALTER EXTENSION vector UPDATE;
   ALTER EXTENSION vchord UPDATE;
   REINDEX INDEX clip_index;
   

3. Observe the reindex fail partway through clustering:

   INFO:  clustering: estimated memory usage is 2.26 GiB
   ERROR:  missing chunk number 0 for toast value 5517461 in pg_toast_17330
   

What we ruled out

• Not corrupt data. A full scan of every row (SELECT embedding FROM smart_search per row, forcing detoast) completes with zero failures, both before and after the reported error.
• Not a fluke. Reproduced deterministically from an identical snapshot: same starting DB restored twice, reindex fails every time without a prior vacuum, and succeeds every time with one. The toast value/chunk number differs slightly between runs (5517461/chunk 0, 5517449/chunk 1 in our tests), consistent with it pointing at whichever dead tuples happen to be present rather than a single fixed corrupt row.

Workaround

Running this before ALTER EXTENSION vchord UPDATE (i.e. still on 0.5.3) reliably prevents the failure:

VACUUM (ANALYZE) smart_search;

Note the ordering matters and there's a bit of a catch-22: if you vacuum after the extension is already on 1.0.0 (with the index still in 0.5.3's on-disk format), VACUUM itself errors out:

ERROR:  deserialization: bad version number; after upgrading VectorChord, please use REINDEX

So recovery has to happen in this order: vacuum while still on the old version → upgrade → reindex.

Expected behavior

REINDEX (or the upgrade path in general) shouldn't fail due to ordinary dead tuples in the heap — index builds normally tolerate this via the visibility map / MVCC snapshot rather than needing every dead row's TOAST data to still be intact. This came up via the Immich community-scripts LXC installer, where several users hit it on the 0.5.3 → 1.0.0 bump (community-scripts/ProxmoxVE#15588) and initially assumed it meant DB corruption.