Make merge write-disposition fall back to append if no primary or merge keys are specified

dlt/common/destination/reference.py

@@ -353,6 +353,14 @@ def _verify_schema(self) -> None:
                         f'"{table["x-merge-strategy"]}" is not a valid merge strategy. '  # type: ignore[typeddict-item]
                         f"""Allowed values: {', '.join(['"' + s + '"' for s in MERGE_STRATEGIES])}."""
                     )
+                if not has_column_with_prop(table, "primary_key") and not has_column_with_prop(
+                    table, "merge_key"
+                ):
+                    logger.warning(
+                        f"Table {table_name} has write_disposition set to merge, but no primary or"
+                        " merge keys defined. "
+                        + "dlt will fall back to append for this table."
+                    )
             if has_column_with_prop(table, "hard_delete"):
                 if len(get_columns_names_with_prop(table, "hard_delete")) > 1:
                     raise SchemaException(

dlt/destinations/sql_jobs.py

@@ -145,7 +145,10 @@ def generate_sql(
 
 
 class SqlMergeJob(SqlBaseJob):
-    """Generates a list of sql statements that merge the data from staging dataset into destination dataset."""
+    """
+    Generates a list of sql statements that merge the data from staging dataset into destination dataset.
+    If no merge keys are discovered, falls back to append.
+    """
 
     failed_text: str = "Tried to generate a merge sql job for the following tables:"
 
@@ -382,68 +385,74 @@ def gen_merge_sql(
                 get_columns_names_with_prop(root_table, "merge_key"),
             )
         )
-        key_clauses = cls._gen_key_table_clauses(primary_keys, merge_keys)
 
-        unique_column: str = None
-        root_key_column: str = None
+        # if we do not have any merge keys to select from, we will fall back to a staged append, i.E.
+        # just skip the delete part
+        append_fallback = (len(primary_keys) + len(merge_keys)) == 0
 
-        if len(table_chain) == 1:
-            key_table_clauses = cls.gen_key_table_clauses(
-                root_table_name, staging_root_table_name, key_clauses, for_delete=True
-            )
-            # if no child tables, just delete data from top table
-            for clause in key_table_clauses:
-                sql.append(f"DELETE {clause};")
-        else:
-            key_table_clauses = cls.gen_key_table_clauses(
-                root_table_name, staging_root_table_name, key_clauses, for_delete=False
-            )
-            # use unique hint to create temp table with all identifiers to delete
-            unique_columns = get_columns_names_with_prop(root_table, "unique")
-            if not unique_columns:
-                raise MergeDispositionException(
-                    sql_client.fully_qualified_dataset_name(),
-                    staging_root_table_name,
-                    [t["name"] for t in table_chain],
-                    f"There is no unique column (ie _dlt_id) in top table {root_table['name']} so"
-                    " it is not possible to link child tables to it.",
-                )
-            # get first unique column
-            unique_column = escape_id(unique_columns[0])
-            # create temp table with unique identifier
-            create_delete_temp_table_sql, delete_temp_table_name = cls.gen_delete_temp_table_sql(
-                unique_column, key_table_clauses, sql_client
-            )
-            sql.extend(create_delete_temp_table_sql)
+        if not append_fallback:
+            key_clauses = cls._gen_key_table_clauses(primary_keys, merge_keys)
 
-            # delete from child tables first. This is important for databricks which does not support temporary tables,
-            # but uses temporary views instead
-            for table in table_chain[1:]:
-                table_name = sql_client.make_qualified_table_name(table["name"])
-                root_key_columns = get_columns_names_with_prop(table, "root_key")
-                if not root_key_columns:
+            unique_column: str = None
+            root_key_column: str = None
+
+            if len(table_chain) == 1:
+                key_table_clauses = cls.gen_key_table_clauses(
+                    root_table_name, staging_root_table_name, key_clauses, for_delete=True
+                )
+                # if no child tables, just delete data from top table
+                for clause in key_table_clauses:
+                    sql.append(f"DELETE {clause};")
+            else:
+                key_table_clauses = cls.gen_key_table_clauses(
+                    root_table_name, staging_root_table_name, key_clauses, for_delete=False
+                )
+                # use unique hint to create temp table with all identifiers to delete
+                unique_columns = get_columns_names_with_prop(root_table, "unique")
+                if not unique_columns:
                     raise MergeDispositionException(
                         sql_client.fully_qualified_dataset_name(),
                         staging_root_table_name,
                         [t["name"] for t in table_chain],
-                        "There is no root foreign key (ie _dlt_root_id) in child table"
-                        f" {table['name']} so it is not possible to refer to top level table"
-                        f" {root_table['name']} unique column {unique_column}",
+                        "There is no unique column (ie _dlt_id) in top table"
+                        f" {root_table['name']} so it is not possible to link child tables to it.",
                     )
-                root_key_column = escape_id(root_key_columns[0])
+                # get first unique column
+                unique_column = escape_id(unique_columns[0])
+                # create temp table with unique identifier
+                create_delete_temp_table_sql, delete_temp_table_name = (
+                    cls.gen_delete_temp_table_sql(unique_column, key_table_clauses, sql_client)
+                )
+                sql.extend(create_delete_temp_table_sql)
+
+                # delete from child tables first. This is important for databricks which does not support temporary tables,
+                # but uses temporary views instead
+                for table in table_chain[1:]:
+                    table_name = sql_client.make_qualified_table_name(table["name"])
+                    root_key_columns = get_columns_names_with_prop(table, "root_key")
+                    if not root_key_columns:
+                        raise MergeDispositionException(
+                            sql_client.fully_qualified_dataset_name(),
+                            staging_root_table_name,
+                            [t["name"] for t in table_chain],
+                            "There is no root foreign key (ie _dlt_root_id) in child table"
+                            f" {table['name']} so it is not possible to refer to top level table"
+                            f" {root_table['name']} unique column {unique_column}",
+                        )
+                    root_key_column = escape_id(root_key_columns[0])
+                    sql.append(
+                        cls.gen_delete_from_sql(
+                            table_name, root_key_column, delete_temp_table_name, unique_column
+                        )
+                    )
+
+                # delete from top table now that child tables have been prcessed
                 sql.append(
                     cls.gen_delete_from_sql(
-                        table_name, root_key_column, delete_temp_table_name, unique_column
+                        root_table_name, unique_column, delete_temp_table_name, unique_column
                     )
                 )
 
-            # delete from top table now that child tables have been prcessed
-            sql.append(
-                cls.gen_delete_from_sql(
-                    root_table_name, unique_column, delete_temp_table_name, unique_column
-                )
-            )
-
         # get name of column with hard_delete hint, if specified
         not_deleted_cond: str = None
         hard_delete_col = get_first_column_name_with_prop(root_table, "hard_delete")

docs/website/docs/general-usage/incremental-loading.md

@@ -132,6 +132,12 @@ def github_repo_events(last_created_at = dlt.sources.incremental("created_at", "
     yield from _get_rest_pages("events")
 ```
 
+:::note
+If you use the `merge` write disposition, but do not specify merge or primary keys, merge will fallback to `append`.
+The appended data will be inserted from a staging table in one transaction for most destinations in this case.
+:::
+
+
 #### Delete records
 The `hard_delete` column hint can be used to delete records from the destination dataset. The behavior of the delete mechanism depends on the data type of the column marked with the hint:
 1) `bool` type: only `True` leads to a delete—`None` and `False` values are disregarded

tests/load/pipeline/test_filesystem_pipeline.py

@@ -29,30 +29,14 @@
 skip_if_not_active("filesystem")
 
 
-def assert_file_matches(
-    layout: str, job: LoadJobInfo, load_id: str, client: FilesystemClient
-) -> None:
-    """Verify file contents of load job are identical to the corresponding file in destination"""
-    local_path = Path(job.file_path)
-    filename = local_path.name
-    destination_fn = create_path(
-        layout,
-        filename,
-        client.schema.name,
-        load_id,
-        extra_placeholders=client.config.extra_placeholders,
-    )
-    destination_path = posixpath.join(client.dataset_path, destination_fn)
-
-    assert local_path.read_bytes() == client.fs_client.read_bytes(destination_path)
-
-
 def test_pipeline_merge_write_disposition(default_buckets_env: str) -> None:
     """Run pipeline twice with merge write disposition
-    Resource with primary key falls back to append. Resource without keys falls back to replace.
+    Regardless wether primary key is set or not, filesystem appends
     """
     import pyarrow.parquet as pq  # Module is evaluated by other tests
 
+    os.environ["DATA_WRITER__DISABLE_COMPRESSION"] = "True"
+
     pipeline = dlt.pipeline(
         pipeline_name="test_" + uniq_id(),
         destination="filesystem",
@@ -71,54 +55,25 @@ def other_data():
     def some_source():
         return [some_data(), other_data()]
 
-    info1 = pipeline.run(some_source(), write_disposition="merge")
-    info2 = pipeline.run(some_source(), write_disposition="merge")
-
-    client: FilesystemClient = pipeline.destination_client()  # type: ignore[assignment]
-    layout = client.config.layout
-
-    append_glob = list(client._get_table_dirs(["some_data"]))[0]
-    replace_glob = list(client._get_table_dirs(["other_data"]))[0]
-
-    append_files = client.fs_client.ls(append_glob, detail=False, refresh=True)
-    replace_files = client.fs_client.ls(replace_glob, detail=False, refresh=True)
-
-    load_id1 = info1.loads_ids[0]
-    load_id2 = info2.loads_ids[0]
-
-    # resource with pk is loaded with append and has 1 copy for each load
-    assert len(append_files) == 2
-    assert any(load_id1 in fn for fn in append_files)
-    assert any(load_id2 in fn for fn in append_files)
-
-    # resource without pk is treated as append disposition
-    assert len(replace_files) == 2
-    assert any(load_id1 in fn for fn in replace_files)
-    assert any(load_id2 in fn for fn in replace_files)
-
-    # Verify file contents
-    assert info2.load_packages
-    for pkg in info2.load_packages:
-        assert pkg.jobs["completed_jobs"]
-        for job in pkg.jobs["completed_jobs"]:
-            assert_file_matches(layout, job, pkg.load_id, client)
-
-    complete_fn = f"{client.schema.name}__%s.jsonl"
+    pipeline.run(some_source(), write_disposition="merge")
+    assert load_table_counts(pipeline, "some_data", "other_data") == {
+        "some_data": 3,
+        "other_data": 5,
+    }
 
-    # Test complete_load markers are saved
-    assert client.fs_client.isfile(
-        posixpath.join(client.dataset_path, client.schema.loads_table_name, complete_fn % load_id1)
-    )
-    assert client.fs_client.isfile(
-        posixpath.join(client.dataset_path, client.schema.loads_table_name, complete_fn % load_id2)
-    )
+    # second load shows that merge always appends on filesystem
+    pipeline.run(some_source(), write_disposition="merge")
+    assert load_table_counts(pipeline, "some_data", "other_data") == {
+        "some_data": 6,
+        "other_data": 10,
+    }
 
-    # Force replace
+    # Force replace, back to initial values
     pipeline.run(some_source(), write_disposition="replace")
-    append_files = client.fs_client.ls(append_glob, detail=False, refresh=True)
-    replace_files = client.fs_client.ls(replace_glob, detail=False, refresh=True)
-    assert len(append_files) == 1
-    assert len(replace_files) == 1
+    assert load_table_counts(pipeline, "some_data", "other_data") == {
+        "some_data": 3,
+        "other_data": 5,
+    }
 
 
 @pytest.mark.parametrize("item_type", ALL_TEST_DATA_ITEM_FORMATS)

tests/load/pipeline/test_merge_disposition.py

@@ -240,10 +240,17 @@ def test_merge_no_child_tables(destination_config: DestinationTestConfiguration)
     assert github_2_counts["issues"] == 100 if destination_config.supports_merge else 115
 
 
+# mark as essential for now
+@pytest.mark.essential
 @pytest.mark.parametrize(
-    "destination_config", destinations_configs(default_sql_configs=True), ids=lambda x: x.name
+    "destination_config",
+    destinations_configs(default_sql_configs=True, local_filesystem_configs=True),
+    ids=lambda x: x.name,
 )
 def test_merge_no_merge_keys(destination_config: DestinationTestConfiguration) -> None:
+    # NOTE: we can test filesystem destination merge behavior here too, will also fallback!
+    if destination_config.file_format == "insert_values":
+        pytest.skip("Insert values row count checking is buggy, skipping")
     p = destination_config.setup_pipeline("github_3", full_refresh=True)
     github_data = github()
     # remove all keys
@@ -264,8 +271,8 @@ def test_merge_no_merge_keys(destination_config: DestinationTestConfiguration) -
     info = p.run(github_data, loader_file_format=destination_config.file_format)
     assert_load_info(info)
     github_1_counts = load_table_counts(p, *[t["name"] for t in p.default_schema.data_tables()])
-    # only ten rows remains. merge falls back to replace when no keys are specified
-    assert github_1_counts["issues"] == 10 if destination_config.supports_merge else 100 - 45
+    # we have 10 rows more, merge falls back to append if no keys present
+    assert github_1_counts["issues"] == 100 - 45 + 10
 
 
 @pytest.mark.parametrize(
@@ -291,14 +298,14 @@ def test_merge_keys_non_existing_columns(destination_config: DestinationTestConf
     if not destination_config.supports_merge:
         return
 
-    # all the keys are invalid so the merge falls back to replace
+    # all the keys are invalid so the merge falls back to append
     github_data = github()
     github_data.load_issues.apply_hints(merge_key=("mA1", "Ma2"), primary_key=("123-x",))
     github_data.load_issues.add_filter(take_first(1))
     info = p.run(github_data, loader_file_format=destination_config.file_format)
     assert_load_info(info)
     github_2_counts = load_table_counts(p, *[t["name"] for t in p.default_schema.data_tables()])
-    assert github_2_counts["issues"] == 1
+    assert github_2_counts["issues"] == 100 - 45 + 1
     with p._sql_job_client(p.default_schema) as job_c:
         _, table_schema = job_c.get_storage_table("issues")
         assert "url" in table_schema
@@ -589,8 +596,10 @@ def r(data):
     destinations_configs(default_sql_configs=True, supports_merge=True),
     ids=lambda x: x.name,
 )
-@pytest.mark.parametrize("key_type", ["primary_key", "merge_key"])
+@pytest.mark.parametrize("key_type", ["primary_key", "merge_key", "no_key"])
 def test_hard_delete_hint(destination_config: DestinationTestConfiguration, key_type: str) -> None:
+    # no_key setting will have the effect that hard deletes have no effect, since hard delete records
+    # can not be matched
     table_name = "test_hard_delete_hint"
 
     @dlt.resource(
@@ -605,6 +614,9 @@ def data_resource(data):
         data_resource.apply_hints(primary_key="id", merge_key="")
     elif key_type == "merge_key":
         data_resource.apply_hints(primary_key="", merge_key="id")
+    elif key_type == "no_key":
+        # we test what happens if there are no merge keys
+        pass
 
     p = destination_config.setup_pipeline(f"abstract_{key_type}", full_refresh=True)
 
@@ -623,24 +635,25 @@ def data_resource(data):
     ]
     info = p.run(data_resource(data), loader_file_format=destination_config.file_format)
     assert_load_info(info)
-    assert load_table_counts(p, table_name)[table_name] == 1
+    assert load_table_counts(p, table_name)[table_name] == (1 if key_type != "no_key" else 2)
 
     # update one record (None for hard_delete column is treated as "not True")
     data = [
         {"id": 2, "val": "baz", "deleted": None},
     ]
     info = p.run(data_resource(data), loader_file_format=destination_config.file_format)
     assert_load_info(info)
-    assert load_table_counts(p, table_name)[table_name] == 1
+    assert load_table_counts(p, table_name)[table_name] == (1 if key_type != "no_key" else 3)
 
     # compare observed records with expected records
-    qual_name = p.sql_client().make_qualified_table_name(table_name)
-    observed = [
-        {"id": row[0], "val": row[1], "deleted": row[2]}
-        for row in select_data(p, f"SELECT id, val, deleted FROM {qual_name}")
-    ]
-    expected = [{"id": 2, "val": "baz", "deleted": None}]
-    assert sorted(observed, key=lambda d: d["id"]) == expected
+    if key_type != "no_key":
+        qual_name = p.sql_client().make_qualified_table_name(table_name)
+        observed = [
+            {"id": row[0], "val": row[1], "deleted": row[2]}
+            for row in select_data(p, f"SELECT id, val, deleted FROM {qual_name}")
+        ]
+        expected = [{"id": 2, "val": "baz", "deleted": None}]
+        assert sorted(observed, key=lambda d: d["id"]) == expected
 
     # insert two records with same key
     data = [
@@ -654,6 +667,12 @@ def data_resource(data):
         assert counts == 2
     elif key_type == "merge_key":
         assert counts == 3
+    elif key_type == "no_key":
+        assert counts == 5
+
+    # we do not need to test "no_key" further
+    if key_type == "no_key":
+        return
 
     # delete one key, resulting in one (primary key) or two (merge key) deleted records
     data = [