Add merge incremental strategy for postgres

.changes/unreleased/Features-20230211-163236.yaml

@@ -0,0 +1,6 @@
+kind: Features
+body: Add merge as valid incremental strategy for postgres
+time: 2023-02-11T16:32:36.2260502+01:00
+custom:
+  Author: rainermensing
+  Issue: "1880"

plugins/postgres/dbt/adapters/postgres/impl.py

@@ -140,7 +140,7 @@ def valid_incremental_strategies(self):
         """The set of standard builtin strategies which this adapter supports out-of-the-box.
         Not used to validate custom strategies defined by end users.
         """
-        return ["append", "delete+insert"]
+        return ["append", "delete+insert", "merge"]
 
     def debug_query(self):
         self.execute("select 1 as id")

tests/functional/materializations/test_incremental.py

@@ -42,11 +42,12 @@ def test_basic(project):
     assert macro_func
     assert type(macro_func).__name__ == "MacroGenerator"
 
-    # These two incremental strategies are not valid for Postgres
-    with pytest.raises(DbtRuntimeError) as excinfo:
-        macro_func = project.adapter.get_incremental_strategy_macro(context, "merge")
-    assert "merge" in str(excinfo.value)
+    # This incremental strategy only works for Postgres >= 15
+    macro_func = project.adapter.get_incremental_strategy_macro(context, "merge")
+    assert macro_func
+    assert type(macro_func).__name__ == "MacroGenerator"
 
+    # This incremental strategy is not valid for Postgres
     with pytest.raises(DbtRuntimeError) as excinfo:
         macro_func = project.adapter.get_incremental_strategy_macro(context, "insert_overwrite")
     assert "insert_overwrite" in str(excinfo.value)