Correct comment in GetConflictingVirtualXIDs()

We use Share lock because it is safe to do so.

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 813cea21a5d94cf21b4bfd4b3dffc2dcdeb9500f..bd0c96124e3c77711365333316ce18f397875d6a 100644 (file)
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -2540,8 +2540,11 @@ GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
 
                        /*
                         * We ignore an invalid pxmin because this means that backend has
-                        * no snapshot and cannot get another one while we hold exclusive
-                        * lock.
+                        * no snapshot currently. We hold a Share lock to avoid contention
+                        * with users taking snapshots.  That is not a problem because
+                        * the current xmin is always at least one higher than the latest
+                        * removed xid, so any new snapshot would never conflict with the
+                        * test here.
                         */
                        if (!TransactionIdIsValid(limitXmin) ||
                                (TransactionIdIsValid(pxmin) && !TransactionIdFollows(pxmin, limitXmin)))