diff options
| author | Noah Misch | 2025-12-15 20:19:49 +0000 |
|---|---|---|
| committer | Noah Misch | 2025-12-15 20:19:49 +0000 |
| commit | 0839fbe400d7807196d1442f4c781f9234ac2a4c (patch) | |
| tree | 917ba6c8926036d3711648616ec77091fae1cb05 | |
| parent | 7db6809ced4406257a80766e4109c8be8e1ea744 (diff) | |
Correct comments of "Fix data loss at inplace update after heap_update()".
This corrects commit a07e03fd8fa7daf4d1356f7cb501ffe784ea6257.
Reported-by: Paul A Jungwirth <pj@illuminatedcomputing.com>
Reported-by: Surya Poondla <s_poondla@apple.com>
Reviewed-by: Paul A Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://postgr.es/m/CA+renyWCW+_2QvXERBQ+mna6ANwAVXXmHKCA-WzL04bZRsjoBA@mail.gmail.com
| -rw-r--r-- | src/backend/access/heap/heapam.c | 11 | ||||
| -rw-r--r-- | src/backend/access/index/genam.c | 9 |
2 files changed, 12 insertions, 8 deletions
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c index 9636bb53ddd..3be8fabd7fb 100644 --- a/src/backend/access/heap/heapam.c +++ b/src/backend/access/heap/heapam.c @@ -6349,10 +6349,13 @@ heap_abort_speculative(Relation relation, const ItemPointerData *tid) * Since this is intended for system catalogs and SERIALIZABLE doesn't cover * DDL, this doesn't guarantee any particular predicate locking. * - * One could modify this to return true for tuples with delete in progress, - * All inplace updaters take a lock that conflicts with DROP. If explicit - * "DELETE FROM pg_class" is in progress, we'll wait for it like we would an - * update. + * heap_delete() is a rarer source of blocking transactions (xwait). We'll + * wait for such a transaction just like for the normal heap_update() case. + * Normal concurrent DROP commands won't cause that, because all inplace + * updaters take some lock that conflicts with DROP. An explicit SQL "DELETE + * FROM pg_class" can cause it. By waiting, if the concurrent transaction + * executed both "DELETE FROM pg_class" and "INSERT INTO pg_class", our caller + * can find the successor tuple. * * Readers of inplace-updated fields expect changes to those fields are * durable. For example, vac_truncate_clog() reads datfrozenxid from diff --git a/src/backend/access/index/genam.c b/src/backend/access/index/genam.c index 707c25289cd..b7f10a1aed0 100644 --- a/src/backend/access/index/genam.c +++ b/src/backend/access/index/genam.c @@ -781,10 +781,11 @@ systable_endscan_ordered(SysScanDesc sysscan) * systable_inplace_update_begin --- update a row "in place" (overwrite it) * * Overwriting violates both MVCC and transactional safety, so the uses of - * this function in Postgres are extremely limited. Nonetheless we find some - * places to use it. See README.tuplock section "Locking to write - * inplace-updated tables" and later sections for expectations of readers and - * writers of a table that gets inplace updates. Standard flow: + * this function in Postgres are extremely limited. This makes no effort to + * support updating cache key columns or other indexed columns. Nonetheless + * we find some places to use it. See README.tuplock section "Locking to + * write inplace-updated tables" and later sections for expectations of + * readers and writers of a table that gets inplace updates. Standard flow: * * ... [any slow preparation not requiring oldtup] ... * systable_inplace_update_begin([...], &tup, &inplace_state); |