Add C comment about the fact that the autovacuum limit can go backwards

by 3, but that is it OK.

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 9fa63a4e6e15ba8e1ede9ef26cf875b7cfb567c4..fcc912f8e3f5e5f9595319830d57efa3683e0936 100644 (file)
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -1108,6 +1108,7 @@ do_start_worker(void)
        recentXid = ReadNewTransactionId();
        xidForceLimit = recentXid - autovacuum_freeze_max_age;
        /* ensure it's a "normal" XID, else TransactionIdPrecedes misbehaves */
+       /* this can cause the limit to go backwards by 3, but that's OK */
        if (xidForceLimit < FirstNormalTransactionId)
                xidForceLimit -= FirstNormalTransactionId;