diff --git a/sql/emaj--next_version.sql b/sql/emaj--next_version.sql
index 5fa09a98..3c41aa78 100644
--- a/sql/emaj--next_version.sql
+++ b/sql/emaj--next_version.sql
@@ -4907,7 +4907,7 @@ r record;
   BEGIN
 -- check supplied group names and mark parameters
     SELECT emaj._rlbk_check(v_groupNames,v_mark) INTO v_markName;
--- compute a random negative rollback-id
+-- compute a random negative rollback-id (not to interfere with ids of real rollbacks)
     SELECT (random() * -10000000000)::bigint INTO v_rlbkId; 
 --
 -- simulate a rollback planning
@@ -4919,7 +4919,7 @@ r record;
 -- call the _rlbk_planning function
       PERFORM emaj._rlbk_planning(v_rlbkId);
 -- compute the sum of the duration estimates of all elementary steps 
-      SELECT sum(rlbp_estimated_duration) INTO v_estimDuration
+      SELECT coalesce(sum(rlbp_estimated_duration), '0 SECONDS'::INTERVAL) INTO v_estimDuration
         FROM emaj.emaj_rlbk_plan
         WHERE rlbp_rlbk_id = v_rlbkId;
 -- cancel the effect of the rollback planning
diff --git a/sql/emaj.sql b/sql/emaj.sql
index 995d083f..e9f11f62 100644
--- a/sql/emaj.sql
+++ b/sql/emaj.sql
@@ -4983,7 +4983,7 @@ r record;
   BEGIN
 -- check supplied group names and mark parameters
     SELECT emaj._rlbk_check(v_groupNames,v_mark) INTO v_markName;
--- compute a random negative rollback-id
+-- compute a random negative rollback-id (not to interfere with ids of real rollbacks)
     SELECT (random() * -10000000000)::bigint INTO v_rlbkId; 
 --
 -- simulate a rollback planning
@@ -4995,7 +4995,7 @@ r record;
 -- call the _rlbk_planning function
       PERFORM emaj._rlbk_planning(v_rlbkId);
 -- compute the sum of the duration estimates of all elementary steps 
-      SELECT sum(rlbp_estimated_duration) INTO v_estimDuration
+      SELECT coalesce(sum(rlbp_estimated_duration), '0 SECONDS'::INTERVAL) INTO v_estimDuration
         FROM emaj.emaj_rlbk_plan
         WHERE rlbp_rlbk_id = v_rlbkId;
 -- cancel the effect of the rollback planning
diff --git a/sql/emaj_src.sql b/sql/emaj_src.sql
index 1586b731..7d339673 100644
--- a/sql/emaj_src.sql
+++ b/sql/emaj_src.sql
@@ -5006,7 +5006,7 @@ r record;
   BEGIN
 -- check supplied group names and mark parameters
     SELECT emaj._rlbk_check(v_groupNames,v_mark) INTO v_markName;
--- compute a random negative rollback-id
+-- compute a random negative rollback-id (not to interfere with ids of real rollbacks)
     SELECT (random() * -10000000000)::bigint INTO v_rlbkId; 
 --
 -- simulate a rollback planning
@@ -5018,7 +5018,7 @@ r record;
 -- call the _rlbk_planning function
       PERFORM emaj._rlbk_planning(v_rlbkId);
 -- compute the sum of the duration estimates of all elementary steps 
-      SELECT sum(rlbp_estimated_duration) INTO v_estimDuration
+      SELECT coalesce(sum(rlbp_estimated_duration), '0 SECONDS'::INTERVAL) INTO v_estimDuration
         FROM emaj.emaj_rlbk_plan
         WHERE rlbp_rlbk_id = v_rlbkId;
 -- cancel the effect of the rollback planning
diff --git a/test/83/expected/instPsql.out b/test/83/expected/instPsql.out
index 3bbc0d38..d4418546 100644
--- a/test/83/expected/instPsql.out
+++ b/test/83/expected/instPsql.out
@@ -5075,7 +5075,7 @@ r record;
   BEGIN
 -- check supplied group names and mark parameters
     SELECT emaj._rlbk_check(v_groupNames,v_mark) INTO v_markName;
--- compute a random negative rollback-id
+-- compute a random negative rollback-id (not to interfere with ids of real rollbacks)
     SELECT (random() * -10000000000)::bigint INTO v_rlbkId; 
 --
 -- simulate a rollback planning
@@ -5087,7 +5087,7 @@ r record;
 -- call the _rlbk_planning function
       PERFORM emaj._rlbk_planning(v_rlbkId);
 -- compute the sum of the duration estimates of all elementary steps 
-      SELECT sum(rlbp_estimated_duration) INTO v_estimDuration
+      SELECT coalesce(sum(rlbp_estimated_duration), '0 SECONDS'::INTERVAL) INTO v_estimDuration
         FROM emaj.emaj_rlbk_plan
         WHERE rlbp_rlbk_id = v_rlbkId;
 -- cancel the effect of the rollback planning
diff --git a/test/83/expected/misc.out b/test/83/expected/misc.out
index 2062d151..3761f09f 100644
--- a/test/83/expected/misc.out
+++ b/test/83/expected/misc.out
@@ -457,6 +457,13 @@ select reltuples from pg_class, pg_namespace where relnamespace=pg_namespace.oid
 
 -- estimate with empty rollback statistics and default parameters
 delete from emaj.emaj_rlbk_stat;
+select emaj.emaj_estimate_rollback_group('myGroup2','EMAJ_LAST_MARK',FALSE);
+ emaj_estimate_rollback_group 
+------------------------------
+ @ 0.00 secs
+(1 row)
+
+-- should return 0.004000 sec (nothing to rollback => only the fixed costs)
 select emaj.emaj_estimate_rollback_group('myGroup2','Mark21',FALSE);
  emaj_estimate_rollback_group 
 ------------------------------
diff --git a/test/84/expected/instPsql.out b/test/84/expected/instPsql.out
index 3d1d1d3a..ad247f1f 100644
--- a/test/84/expected/instPsql.out
+++ b/test/84/expected/instPsql.out
@@ -5075,7 +5075,7 @@ r record;
   BEGIN
 -- check supplied group names and mark parameters
     SELECT emaj._rlbk_check(v_groupNames,v_mark) INTO v_markName;
--- compute a random negative rollback-id
+-- compute a random negative rollback-id (not to interfere with ids of real rollbacks)
     SELECT (random() * -10000000000)::bigint INTO v_rlbkId; 
 --
 -- simulate a rollback planning
@@ -5087,7 +5087,7 @@ r record;
 -- call the _rlbk_planning function
       PERFORM emaj._rlbk_planning(v_rlbkId);
 -- compute the sum of the duration estimates of all elementary steps 
-      SELECT sum(rlbp_estimated_duration) INTO v_estimDuration
+      SELECT coalesce(sum(rlbp_estimated_duration), '0 SECONDS'::INTERVAL) INTO v_estimDuration
         FROM emaj.emaj_rlbk_plan
         WHERE rlbp_rlbk_id = v_rlbkId;
 -- cancel the effect of the rollback planning
diff --git a/test/84/expected/misc.out b/test/84/expected/misc.out
index 99a41e88..d0493d99 100644
--- a/test/84/expected/misc.out
+++ b/test/84/expected/misc.out
@@ -457,6 +457,13 @@ select reltuples from pg_class, pg_namespace where relnamespace=pg_namespace.oid
 
 -- estimate with empty rollback statistics and default parameters
 delete from emaj.emaj_rlbk_stat;
+select emaj.emaj_estimate_rollback_group('myGroup2','EMAJ_LAST_MARK',FALSE);
+ emaj_estimate_rollback_group 
+------------------------------
+ @ 0.004 secs
+(1 row)
+
+-- should return 0.004000 sec (nothing to rollback => only the fixed costs)
 select emaj.emaj_estimate_rollback_group('myGroup2','Mark21',FALSE);
  emaj_estimate_rollback_group 
 ------------------------------
diff --git a/test/90/expected/instPsql.out b/test/90/expected/instPsql.out
index 7d58d7c5..47f7c09b 100644
--- a/test/90/expected/instPsql.out
+++ b/test/90/expected/instPsql.out
@@ -5075,7 +5075,7 @@ r record;
   BEGIN
 -- check supplied group names and mark parameters
     SELECT emaj._rlbk_check(v_groupNames,v_mark) INTO v_markName;
--- compute a random negative rollback-id
+-- compute a random negative rollback-id (not to interfere with ids of real rollbacks)
     SELECT (random() * -10000000000)::bigint INTO v_rlbkId; 
 --
 -- simulate a rollback planning
@@ -5087,7 +5087,7 @@ r record;
 -- call the _rlbk_planning function
       PERFORM emaj._rlbk_planning(v_rlbkId);
 -- compute the sum of the duration estimates of all elementary steps 
-      SELECT sum(rlbp_estimated_duration) INTO v_estimDuration
+      SELECT coalesce(sum(rlbp_estimated_duration), '0 SECONDS'::INTERVAL) INTO v_estimDuration
         FROM emaj.emaj_rlbk_plan
         WHERE rlbp_rlbk_id = v_rlbkId;
 -- cancel the effect of the rollback planning
diff --git a/test/90/expected/misc.out b/test/90/expected/misc.out
index c6afb38c..63dad0d0 100644
--- a/test/90/expected/misc.out
+++ b/test/90/expected/misc.out
@@ -457,6 +457,13 @@ select reltuples from pg_class, pg_namespace where relnamespace=pg_namespace.oid
 
 -- estimate with empty rollback statistics and default parameters
 delete from emaj.emaj_rlbk_stat;
+select emaj.emaj_estimate_rollback_group('myGroup2','EMAJ_LAST_MARK',FALSE);
+ emaj_estimate_rollback_group 
+------------------------------
+ @ 0.004 secs
+(1 row)
+
+-- should return 0.004000 sec (nothing to rollback => only the fixed costs)
 select emaj.emaj_estimate_rollback_group('myGroup2','Mark21',FALSE);
  emaj_estimate_rollback_group 
 ------------------------------
diff --git a/test/91/expected/instPsql.out b/test/91/expected/instPsql.out
index 525c02a2..90d65e85 100644
--- a/test/91/expected/instPsql.out
+++ b/test/91/expected/instPsql.out
@@ -5075,7 +5075,7 @@ r record;
   BEGIN
 -- check supplied group names and mark parameters
     SELECT emaj._rlbk_check(v_groupNames,v_mark) INTO v_markName;
--- compute a random negative rollback-id
+-- compute a random negative rollback-id (not to interfere with ids of real rollbacks)
     SELECT (random() * -10000000000)::bigint INTO v_rlbkId; 
 --
 -- simulate a rollback planning
@@ -5087,7 +5087,7 @@ r record;
 -- call the _rlbk_planning function
       PERFORM emaj._rlbk_planning(v_rlbkId);
 -- compute the sum of the duration estimates of all elementary steps 
-      SELECT sum(rlbp_estimated_duration) INTO v_estimDuration
+      SELECT coalesce(sum(rlbp_estimated_duration), '0 SECONDS'::INTERVAL) INTO v_estimDuration
         FROM emaj.emaj_rlbk_plan
         WHERE rlbp_rlbk_id = v_rlbkId;
 -- cancel the effect of the rollback planning
diff --git a/test/91/expected/misc.out b/test/91/expected/misc.out
index 38a361e8..04726b17 100644
--- a/test/91/expected/misc.out
+++ b/test/91/expected/misc.out
@@ -457,6 +457,13 @@ select reltuples from pg_class, pg_namespace where relnamespace=pg_namespace.oid
 
 -- estimate with empty rollback statistics and default parameters
 delete from emaj.emaj_rlbk_stat;
+select emaj.emaj_estimate_rollback_group('myGroup2','EMAJ_LAST_MARK',FALSE);
+ emaj_estimate_rollback_group 
+------------------------------
+ @ 0.004 secs
+(1 row)
+
+-- should return 0.004000 sec (nothing to rollback => only the fixed costs)
 select emaj.emaj_estimate_rollback_group('myGroup2','Mark21',FALSE);
  emaj_estimate_rollback_group 
 ------------------------------
diff --git a/test/92/expected/instPsql.out b/test/92/expected/instPsql.out
index 2f879b61..34046f17 100644
--- a/test/92/expected/instPsql.out
+++ b/test/92/expected/instPsql.out
@@ -5075,7 +5075,7 @@ r record;
   BEGIN
 -- check supplied group names and mark parameters
     SELECT emaj._rlbk_check(v_groupNames,v_mark) INTO v_markName;
--- compute a random negative rollback-id
+-- compute a random negative rollback-id (not to interfere with ids of real rollbacks)
     SELECT (random() * -10000000000)::bigint INTO v_rlbkId; 
 --
 -- simulate a rollback planning
@@ -5087,7 +5087,7 @@ r record;
 -- call the _rlbk_planning function
       PERFORM emaj._rlbk_planning(v_rlbkId);
 -- compute the sum of the duration estimates of all elementary steps 
-      SELECT sum(rlbp_estimated_duration) INTO v_estimDuration
+      SELECT coalesce(sum(rlbp_estimated_duration), '0 SECONDS'::INTERVAL) INTO v_estimDuration
         FROM emaj.emaj_rlbk_plan
         WHERE rlbp_rlbk_id = v_rlbkId;
 -- cancel the effect of the rollback planning
diff --git a/test/92/expected/misc.out b/test/92/expected/misc.out
index 5cf3980d..02f322a9 100644
--- a/test/92/expected/misc.out
+++ b/test/92/expected/misc.out
@@ -457,6 +457,13 @@ select reltuples from pg_class, pg_namespace where relnamespace=pg_namespace.oid
 
 -- estimate with empty rollback statistics and default parameters
 delete from emaj.emaj_rlbk_stat;
+select emaj.emaj_estimate_rollback_group('myGroup2','EMAJ_LAST_MARK',FALSE);
+ emaj_estimate_rollback_group 
+------------------------------
+ @ 0.004 secs
+(1 row)
+
+-- should return 0.004000 sec (nothing to rollback => only the fixed costs)
 select emaj.emaj_estimate_rollback_group('myGroup2','Mark21',FALSE);
  emaj_estimate_rollback_group 
 ------------------------------
diff --git a/test/sql/misc.sql b/test/sql/misc.sql
index 61e83fc4..eccb4bc3 100644
--- a/test/sql/misc.sql
+++ b/test/sql/misc.sql
@@ -164,6 +164,9 @@ select reltuples from pg_class, pg_namespace where relnamespace=pg_namespace.oid
 -- estimate with empty rollback statistics and default parameters
 delete from emaj.emaj_rlbk_stat;
 
+select emaj.emaj_estimate_rollback_group('myGroup2','EMAJ_LAST_MARK',FALSE);
+-- should return 0.004000 sec (nothing to rollback => only the fixed costs)
+
 select emaj.emaj_estimate_rollback_group('myGroup2','Mark21',FALSE);
 -- should return 1.436620 sec