diff --git a/src/muz/rel/udoc_relation.cpp b/src/muz/rel/udoc_relation.cpp
index 269780a94..d3c1f929d 100644
--- a/src/muz/rel/udoc_relation.cpp
+++ b/src/muz/rel/udoc_relation.cpp
@@ -221,7 +221,8 @@ namespace datalog {
         relation_plugin(udoc_plugin::get_name(), rm),
         m(rm.get_context().get_manager()),
         bv(m),
-        dl(m) {
+        dl(m),
+        m_disable_fast_pass(false) {
     }
     udoc_plugin::~udoc_plugin() {
         u_map<doc_manager*>::iterator it = m_dms.begin(), end = m_dms.end();
@@ -1079,13 +1080,22 @@ namespace datalog {
         virtual void operator()(relation_base& tb, const relation_base& negb) {
             udoc_relation& t = get(tb);
             udoc_relation const& n = get(negb);
+            udoc_plugin& p = t.get_plugin();
+            IF_VERBOSE(3, t.display(verbose_stream() << "dst:"););
+            IF_VERBOSE(3, n.display(verbose_stream() << "neg:"););
+            if (!t.fast_empty() && !n.fast_empty() && !p.m_disable_fast_pass) {
+                fast_pass(t, n);
+            }
+            if (!t.fast_empty() && !n.fast_empty()) {
+                slow_pass(t, n);
+            }
+        }
+
+        void fast_pass(udoc_relation& t, const udoc_relation& n) {
             udoc & dst = t.get_udoc();
             udoc const & neg = n.get_udoc();
             doc_manager& dmt = t.get_dm();
             doc_manager& dmn = n.get_dm();
-            IF_VERBOSE(3, t.display(verbose_stream() << "dst:"););
-            IF_VERBOSE(3, n.display(verbose_stream() << "neg:"););
-
             udoc result;
             for (unsigned i = 0; i < dst.size(); ++i) {
                 bool subsumed = false;
@@ -1100,12 +1110,13 @@ namespace datalog {
                     result.push_back(&dst[i]);
             }
             std::swap(dst, result);
-            if (dst.is_empty()) {
-                IF_VERBOSE(3, tb.display(verbose_stream() << "fast empty"););
-                return;
-            }
+        }
 
-            // slow case
+        void slow_pass(udoc_relation& t, udoc_relation const& n) {
+            udoc & dst = t.get_udoc();
+            udoc const & neg = n.get_udoc();
+            doc_manager& dmt = t.get_dm();
+            doc_manager& dmn = n.get_dm();
             udoc renamed_neg;
             for (unsigned i = 0; i < neg.size(); ++i) {
                 doc& neg_i = neg[i];
@@ -1142,8 +1153,9 @@ namespace datalog {
             TRACE("doc", dst.display(dmt, tout) << "\n";);
             SASSERT(dst.well_formed(dmt));
             renamed_neg.reset(dmt);
-            IF_VERBOSE(3, tb.display(verbose_stream() << "slow case:"););
+            IF_VERBOSE(3, t.display(verbose_stream() << "slow case:"););
         }
+        
         bool copy_columns(
             tbv& dst, tbv const& src, 
             udoc_relation const& dstt,
diff --git a/src/muz/rel/udoc_relation.h b/src/muz/rel/udoc_relation.h
index 0adc61b64..f040e4745 100644
--- a/src/muz/rel/udoc_relation.h
+++ b/src/muz/rel/udoc_relation.h
@@ -91,10 +91,12 @@ namespace datalog {
         class filter_by_union_fn;
         class filter_proj_fn;
         class negation_filter_fn;
-        ast_manager& m;
-        bv_util      bv;
-        dl_decl_util dl;
+        ast_manager&        m;
+        bv_util             bv;
+        dl_decl_util        dl;
         u_map<doc_manager*> m_dms;
+        bool                m_disable_fast_pass;
+
         doc_manager& dm(unsigned sz);
         doc_manager& dm(relation_signature const& sig);
         static udoc_relation& get(relation_base& r);
@@ -136,6 +138,7 @@ namespace datalog {
         virtual relation_transformer_fn * mk_filter_interpreted_and_project_fn(
             const relation_base & t, app * condition,
             unsigned removed_col_cnt, const unsigned * removed_cols);
+        void disable_fast_pass() { m_disable_fast_pass = true; }
     };
 };
        
diff --git a/src/test/udoc_relation.cpp b/src/test/udoc_relation.cpp
index 26858c12c..dd77db9f8 100644
--- a/src/test/udoc_relation.cpp
+++ b/src/test/udoc_relation.cpp
@@ -144,13 +144,17 @@ public:
         udoc_relation* t1, *t2, *t3;
         expr_ref fml(m);
 
-        test_join(1000);
+        test_filter_neg4(false);
+        test_filter_neg4(true);
 
-        test_rename();
         test_filter_neg();
         test_filter_neg2();
         test_filter_neg3();
 
+        test_join(1000);
+
+        test_rename();
+
 
         // empty
         {
@@ -635,6 +639,25 @@ public:
         t2->deallocate();
     }
 
+    void test_filter_neg4(bool disable_fast) {
+        relation_signature sig1, sig2;
+        sig1.push_back(bv.mk_sort(2));
+        sig1.push_back(bv.mk_sort(2));
+        sig2.push_back(bv.mk_sort(2));
+        unsigned_vector cols1, cols2;
+
+        cols1.push_back(0);
+        cols1.push_back(1);
+        cols2.push_back(0);
+        cols2.push_back(0);
+        udoc_relation* tgt = mk_full(sig1);
+        udoc_relation* neg = mk_full(sig2);
+        if (disable_fast) p.disable_fast_pass();
+        apply_filter_neg(*tgt, *neg, cols1, cols2);
+        tgt->deallocate();
+        neg->deallocate();        
+    }
+
     void set_random(udoc_relation& r, unsigned num_vals) {
         unsigned num_bits = r.get_dm().num_tbits();
         udoc_relation* full = mk_full(r.get_signature());