index 9636bb53ddd8d45391535cbad4750ec523ffbae2..3be8fabd7fb99874d4b2f3a78f9bb0aa3e87e02c 100644 (file)
@@ -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
index 707c25289cd74aebcde4671145f38ee3de43c7c4..b7f10a1aed0bb12e2cda6f94f302a84d61a7d18f 100644 (file)
* 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);