diff --git a/contrib/test_decoding/expected/truncate.out b/contrib/test_decoding/expected/truncate.out
index be85178206..1cf2ae835c 100644
--- a/contrib/test_decoding/expected/truncate.out
+++ b/contrib/test_decoding/expected/truncate.out
@@ -1,3 +1,5 @@
+-- predictability
+SET synchronous_commit = on;
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
  ?column? 
 ----------
diff --git a/contrib/test_decoding/specs/oldest_xmin.spec b/contrib/test_decoding/specs/oldest_xmin.spec
index 141fe2b145..6cb13e85ce 100644
--- a/contrib/test_decoding/specs/oldest_xmin.spec
+++ b/contrib/test_decoding/specs/oldest_xmin.spec
@@ -17,6 +17,7 @@ teardown
 }
 
 session "s0"
+setup { SET synchronous_commit=on; }
 step "s0_begin" { BEGIN; }
 step "s0_getxid" { SELECT txid_current() IS NULL; }
 step "s0_alter" { ALTER TYPE basket DROP ATTRIBUTE mangos; }
@@ -26,6 +27,7 @@ step "s0_vacuum" { VACUUM pg_attribute; }
 step "s0_get_changes" { SELECT data FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1'); }
 
 session "s1"
+setup { SET synchronous_commit=on; }
 step "s1_begin" { BEGIN; }
 step "s1_insert" { INSERT INTO harvest VALUES ((1, 2, 3)); }
 step "s1_commit" { COMMIT; }
diff --git a/contrib/test_decoding/specs/snapshot_transfer.spec b/contrib/test_decoding/specs/snapshot_transfer.spec
index 47db7fd90a..8fb70e6567 100644
--- a/contrib/test_decoding/specs/snapshot_transfer.spec
+++ b/contrib/test_decoding/specs/snapshot_transfer.spec
@@ -17,6 +17,7 @@ teardown
 }
 
 session "s0"
+setup { SET synchronous_commit=on; }
 step "s0_begin" { BEGIN; }
 step "s0_begin_sub0" { SAVEPOINT s0; }
 step "s0_log_assignment" { SELECT txid_current() IS NULL; }
@@ -30,6 +31,7 @@ step "s0_commit" { COMMIT; }
 step "s0_get_changes" { SELECT data FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1'); }
 
 session "s1"
+setup { SET synchronous_commit=on; }
 step "s1_produce_new_snap" { ALTER TABLE harvest ADD COLUMN mangos int; }
 
 # start top-level without base snap, get base snap in subxact, then create new
diff --git a/contrib/test_decoding/sql/truncate.sql b/contrib/test_decoding/sql/truncate.sql
index 88f113fd5b..5aecdf0881 100644
--- a/contrib/test_decoding/sql/truncate.sql
+++ b/contrib/test_decoding/sql/truncate.sql
@@ -1,3 +1,6 @@
+-- predictability
+SET synchronous_commit = on;
+
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
 
 CREATE TABLE tab1 (id serial unique, data int);