[t:5067] Reduce GC-system overhead for read-only transactions

git-svn-id: file:///svn/toku/tokudb@44750 c7de825b-a66e-492c-adef-691d508d4ae1

ft/txn_manager.c

@@ -136,8 +136,8 @@ verify_snapshot_system(TXN_MANAGER txn_manager UU()) {
                     // Only committed entries have return a youngest.
                     invariant(youngest == TXNID_NONE);
                 }
-                else {
-                    invariant(youngest != TXNID_NONE);
+                else if (youngest != TXNID_NONE) {
+                    // A committed entry might have been read-only, in which case it won't return anything.
                     // This snapshot reads 'live_xid' so it's youngest cannot be older than snapshot_xid.
                     invariant(youngest >= snapshot_xid);
                 }
@@ -586,8 +586,10 @@ void toku_txn_manager_finish_txn(TXN_MANAGER txn_manager, TOKUTXN txn) {
         r = toku_omt_delete_at(txn_manager->live_root_txns, idx);
         invariant_zero(r);
 
+        if (txn->begin_was_logged) {
             if (!is_snapshot) {
                 // If it's a snapshot, we already calculated index_in_snapshot_txnids.
+                // Otherwise, calculate it now.
                 r = toku_omt_find_zero(txn_manager->snapshot_txnids, toku_find_xid_by_xid, (OMTVALUE) txn->txnid64, NULL, &index_in_snapshot_txnids);
                 invariant(r == DB_NOTFOUND);
             }
@@ -608,6 +610,7 @@ void toku_txn_manager_finish_txn(TXN_MANAGER txn_manager, TOKUTXN txn) {
                 lazy_assert_zero(r);
             }
         }
+    }
 
     assert(txn_manager->oldest_living_xid <= txn->txnid64);
     if (txn->txnid64 == txn_manager->oldest_living_xid) {