The analysis in the beginning of the discussion seems to be right, but
the fix v2 looks too invasive for me.
Personally, I'd like not to remove snapshot even if transaction is
read-only. I propose to consider "xid < TransactionXmin" as a legit case
and just promote xid to TransactionXmin.
It's annoying this old bug still not fixed. What do you think?
---
Best regards,
Maxim Orlov.
diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index 6a8e521f894..ce7c90aa38f 100644
--- a/src/backend/access/transam/subtrans.c
+++ b/src/backend/access/transam/subtrans.c
@@ -29,6 +29,7 @@
#include "postgres.h"
#include "access/slru.h"
+#include "access/parallel.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "pg_trace.h"
@@ -152,6 +153,21 @@ SubTransGetTopmostTransaction(TransactionId xid)
TransactionId parentXid = xid,
previousXid = xid;
+ /*
+ * We may have different xmins in active and transaction snapshots in
+ * parallel workers. And TransactionXmin can be promoted to the max of them.
+ * So, we do this to avoid assert below when restore active snapshot with
+ * less xmin value.
+ *
+ * XXX: the same may be fixed by prohibiting read-only transactions (like
+ * parallel scans) to increment TransactionXmin.
+ */
+ if (IsParallelWorker())
+ {
+ xid = xid > TransactionXmin ? xid : TransactionXmin;
+ parentXid = xid;
+ previousXid = xid;
+ }
/* Can't ask about stuff that might not be around anymore */
Assert(TransactionIdFollowsOrEquals(xid, TransactionXmin));
