diff --git a/edb/ir/statypes.py b/edb/ir/statypes.py
index 8c834d7622bb..09f122161e97 100644
--- a/edb/ir/statypes.py
+++ b/edb/ir/statypes.py
@@ -801,3 +801,13 @@ def get_translation_map(cls) -> Mapping[TransactionIsolationEnum, str]:
             TransactionIsolationEnum.Serializable: "serializable",
             TransactionIsolationEnum.RepeatableRead: "repeatable read",
         }
+
+    def to_qltypes(self) -> qltypes.TransactionIsolationLevel:
+        from edb.edgeql import qltypes
+        match self._val:
+            case TransactionIsolationEnum.Serializable:
+                return qltypes.TransactionIsolationLevel.SERIALIZABLE
+            case TransactionIsolationEnum.RepeatableRead:
+                return qltypes.TransactionIsolationLevel.REPEATABLE_READ
+            case _:
+                raise AssertionError(f"unexpected value: {self._val!r}")
diff --git a/edb/lib/cfg.edgeql b/edb/lib/cfg.edgeql
index a60eac426bb5..bf8cb298c1f6 100644
--- a/edb/lib/cfg.edgeql
+++ b/edb/lib/cfg.edgeql
@@ -191,6 +191,7 @@ ALTER TYPE cfg::AbstractConfig {
     CREATE REQUIRED PROPERTY default_transaction_isolation
         -> sys::TransactionIsolation
     {
+        CREATE ANNOTATION cfg::affects_compilation := 'true';
         CREATE ANNOTATION cfg::backend_setting :=
             '"default_transaction_isolation"';
         CREATE ANNOTATION std::description :=
diff --git a/edb/server/compiler/compiler.py b/edb/server/compiler/compiler.py
index 9b51c86f87d3..5e779e2e2298 100644
--- a/edb/server/compiler/compiler.py
+++ b/edb/server/compiler/compiler.py
@@ -2167,7 +2167,14 @@ def _compile_ql_transaction(
                     hint=f"specify READ ONLY access mode",
                 )
             sqls += f' ISOLATION LEVEL {iso.value}'
-        if ql.access is None:
+        else:
+            iso_config: statypes.TransactionIsolation = _get_config_val(
+                ctx, "default_transaction_isolation"
+            )
+            iso = iso_config.to_qltypes()
+        if iso is not qltypes.TransactionIsolationLevel.SERIALIZABLE:
+            sqls += f' {qltypes.TransactionAccessMode.READ_ONLY.value}'
+        elif ql.access is None:
             access_mode: statypes.TransactionAccessMode = _get_config_val(
                 ctx, "default_transaction_access_mode"
             )