Add load_id to arrow tables in extract step instead of normalize

dlt/common/libs/pyarrow.py

@@ -226,7 +226,8 @@ def should_normalize_arrow_schema(
     schema: pyarrow.Schema,
     columns: TTableSchemaColumns,
     naming: NamingConvention,
-) -> Tuple[bool, Mapping[str, str], Dict[str, str], Dict[str, bool], TTableSchemaColumns]:
+    add_load_id: bool = False,
+) -> Tuple[bool, Mapping[str, str], Dict[str, str], Dict[str, bool], bool, TTableSchemaColumns]:
     rename_mapping = get_normalized_arrow_fields_mapping(schema, naming)
     rev_mapping = {v: k for k, v in rename_mapping.items()}
     nullable_mapping = {k: v.get("nullable", True) for k, v in columns.items()}
@@ -240,6 +241,16 @@ def should_normalize_arrow_schema(
 
     dlt_tables = list(map(naming.normalize_table_identifier, ("_dlt_id", "_dlt_load_id")))
 
+    # Do we need to add a load id column?
+    if add_load_id and "_dlt_load_id" in columns:
+        try:
+            schema.field("_dlt_load_id")
+            needs_load_id = False
+        except KeyError:
+            needs_load_id = True
+    else:
+        needs_load_id = False
+
     # remove all columns that are dlt columns but are not present in arrow schema. we do not want to add such columns
     # that should happen in the normalizer
     columns = {
@@ -250,27 +261,38 @@ def should_normalize_arrow_schema(
 
     # check if nothing to rename
     skip_normalize = (
-        list(rename_mapping.keys()) == list(rename_mapping.values()) == list(columns.keys())
-    ) and not nullable_updates
-    return not skip_normalize, rename_mapping, rev_mapping, nullable_updates, columns
+        (list(rename_mapping.keys()) == list(rename_mapping.values()) == list(columns.keys()))
+        and not nullable_updates
+        and not needs_load_id
+    )
+    return (
+        not skip_normalize,
+        rename_mapping,
+        rev_mapping,
+        nullable_updates,
+        needs_load_id,
+        columns,
+    )
 
 
 def normalize_py_arrow_item(
     item: TAnyArrowItem,
     columns: TTableSchemaColumns,
     naming: NamingConvention,
     caps: DestinationCapabilitiesContext,
+    load_id: Optional[str] = None,
 ) -> TAnyArrowItem:
     """Normalize arrow `item` schema according to the `columns`.
 
     1. arrow schema field names will be normalized according to `naming`
     2. arrows columns will be reordered according to `columns`
     3. empty columns will be inserted if they are missing, types will be generated using `caps`
     4. arrow columns with different nullability than corresponding schema columns will be updated
+    5. Add `_dlt_load_id` column if it is missing and `load_id` is provided
     """
     schema = item.schema
-    should_normalize, rename_mapping, rev_mapping, nullable_updates, columns = (
-        should_normalize_arrow_schema(schema, columns, naming)
+    should_normalize, rename_mapping, rev_mapping, nullable_updates, needs_load_id, columns = (
+        should_normalize_arrow_schema(schema, columns, naming, load_id is not None)
     )
     if not should_normalize:
         return item
@@ -307,6 +329,12 @@ def normalize_py_arrow_item(
         new_fields.append(schema.field(idx).with_name(column_name))
         new_columns.append(item.column(idx))
 
+    if needs_load_id and load_id:
+        # Storage efficient type for a column with constant value
+        load_id_type = pyarrow.dictionary(pyarrow.int8(), pyarrow.string())
+        new_fields.append(pyarrow.field("_dlt_load_id", load_id_type, nullable=False))
+        new_columns.append(pyarrow.array([load_id] * item.num_rows, type=load_id_type))
+
     # create desired type
     return item.__class__.from_arrays(new_columns, schema=pyarrow.schema(new_fields))
 
@@ -383,6 +411,30 @@ def from_arrow_scalar(arrow_value: pyarrow.Scalar) -> Any:
 """Sequence of tuples: (field index, field, generating function)"""
 
 
+def add_constant_column(
+    item: TAnyArrowItem,
+    name: str,
+    data_type: pyarrow.DataType,
+    value: Any = None,
+    nullable: bool = True,
+    index: int = -1,
+) -> TAnyArrowItem:
+    """Add column with a single value to the table.
+
+    Args:
+        item: Arrow table or record batch
+        name: The new column name
+        data_type: The data type of the new column
+        nullable: Whether the new column is nullable
+        value: The value to fill the new column with
+        index: The index at which to insert the new column. Defaults to -1 (append)
+    """
+    field = pyarrow.field(name, pyarrow.dictionary(pyarrow.int8(), data_type), nullable=nullable)
+    if index == -1:
+        return item.append_column(field, pyarrow.array([value] * item.num_rows, type=field.type))
+    return item.add_column(index, field, pyarrow.array([value] * item.num_rows, type=field.type))
+
+
 def pq_stream_with_new_columns(
     parquet_file: TFileOrPath, columns: TNewColumns, row_groups_per_read: int = 1
 ) -> Iterator[pyarrow.Table]:

dlt/extract/extractors.py

@@ -1,6 +1,7 @@
 from copy import copy
 from typing import Set, Dict, Any, Optional, List
 
+from dlt.common.configuration import known_sections
 from dlt.common import logger
 from dlt.common.configuration.inject import with_config
 from dlt.common.configuration.specs import BaseConfiguration, configspec
@@ -21,6 +22,7 @@
 from dlt.extract.resource import DltResource
 from dlt.extract.items import TableNameMeta
 from dlt.extract.storage import ExtractorItemStorage
+from dlt.normalize.configuration import ItemsNormalizerConfiguration
 
 try:
     from dlt.common.libs import pyarrow
@@ -215,13 +217,26 @@ class ObjectExtractor(Extractor):
 class ArrowExtractor(Extractor):
     """Extracts arrow data items into parquet. Normalizes arrow items column names.
     Compares the arrow schema to actual dlt table schema to reorder the columns and to
-    insert missing columns (without data).
+    insert missing columns (without data). Adds _dlt_load_id column to the table if
+    `add_dlt_load_id` is set to True in normalizer config.
 
     We do things that normalizer should do here so we do not need to load and save parquet
     files again later.
 
+    Handles the following types:
+    - `pyarrow.Table`
+    - `pyarrow.RecordBatch`
+    - `pandas.DataFrame` (is converted to arrow `Table` before processing)
     """
 
+    # Inject the parts of normalize configuration that are used here
+    @with_config(
+        spec=ItemsNormalizerConfiguration, sections=(known_sections.NORMALIZE, "parquet_normalizer")
+    )
+    def __init__(self, *args: Any, add_dlt_load_id: bool = False, **kwargs: Any) -> None:
+        super().__init__(*args, **kwargs)
+        self.add_dlt_load_id = add_dlt_load_id
+
     def write_items(self, resource: DltResource, items: TDataItems, meta: Any) -> None:
         static_table_name = self._get_static_table_name(resource, meta)
         items = [
@@ -294,7 +309,13 @@ def _write_item(
         columns = columns or self.schema.get_table_columns(table_name)
         # Note: `items` is always a list here due to the conversion in `write_table`
         items = [
-            pyarrow.normalize_py_arrow_item(item, columns, self.naming, self._caps)
+            pyarrow.normalize_py_arrow_item(
+                item,
+                columns,
+                self.naming,
+                self._caps,
+                load_id=self.load_id if self.add_dlt_load_id else None,
+            )
             for item in items
         ]
         # write items one by one
@@ -318,6 +339,13 @@ def _compute_table(
             arrow_table["columns"] = pyarrow.py_arrow_to_table_schema_columns(item.schema)
             # normalize arrow table before merging
             arrow_table = self.schema.normalize_table_identifiers(arrow_table)
+            # Add load_id column
+            if self.add_dlt_load_id and "_dlt_load_id" not in arrow_table["columns"]:
+                arrow_table["columns"]["_dlt_load_id"] = {
+                    "name": "_dlt_load_id",
+                    "data_type": "text",
+                    "nullable": False,
+                }
             # issue warnings when overriding computed with arrow
             override_warn: bool = False
             for col_name, column in arrow_table["columns"].items():

dlt/normalize/items_normalizers.py

@@ -228,37 +228,13 @@ class ArrowItemsNormalizer(ItemsNormalizer):
     REWRITE_ROW_GROUPS = 1
 
     def _write_with_dlt_columns(
-        self, extracted_items_file: str, root_table_name: str, add_load_id: bool, add_dlt_id: bool
+        self, extracted_items_file: str, root_table_name: str, add_dlt_id: bool
     ) -> List[TSchemaUpdate]:
         new_columns: List[Any] = []
         schema = self.schema
         load_id = self.load_id
         schema_update: TSchemaUpdate = {}
 
-        if add_load_id:
-            table_update = schema.update_table(
-                {
-                    "name": root_table_name,
-                    "columns": {
-                        "_dlt_load_id": {
-                            "name": "_dlt_load_id",
-                            "data_type": "text",
-                            "nullable": False,
-                        }
-                    },
-                }
-            )
-            table_updates = schema_update.setdefault(root_table_name, [])
-            table_updates.append(table_update)
-            load_id_type = pa.dictionary(pa.int8(), pa.string())
-            new_columns.append(
-                (
-                    -1,
-                    pa.field("_dlt_load_id", load_id_type, nullable=False),
-                    lambda batch: pa.array([load_id] * batch.num_rows, type=load_id_type),
-                )
-            )
-
         if add_dlt_id:
             table_update = schema.update_table(
                 {
@@ -292,9 +268,9 @@ def _write_with_dlt_columns(
                 items_count += batch.num_rows
                 # we may need to normalize
                 if is_native_arrow_writer and should_normalize is None:
-                    should_normalize, _, _, _, _ = pyarrow.should_normalize_arrow_schema(
+                    should_normalize = pyarrow.should_normalize_arrow_schema(
                         batch.schema, columns_schema, schema.naming
-                    )
+                    )[0]
                     if should_normalize:
                         logger.info(
                             f"When writing arrow table to {root_table_name} the schema requires"
@@ -366,25 +342,22 @@ def __call__(self, extracted_items_file: str, root_table_name: str) -> List[TSch
         base_schema_update = self._fix_schema_precisions(root_table_name, arrow_schema)
 
         add_dlt_id = self.config.parquet_normalizer.add_dlt_id
-        add_dlt_load_id = self.config.parquet_normalizer.add_dlt_load_id
         # if we need to add any columns or the file format is not parquet, we can't just import files
-        must_rewrite = (
-            add_dlt_id or add_dlt_load_id or self.item_storage.writer_spec.file_format != "parquet"
-        )
+        must_rewrite = add_dlt_id or self.item_storage.writer_spec.file_format != "parquet"
         if not must_rewrite:
             # in rare cases normalization may be needed
-            must_rewrite, _, _, _, _ = pyarrow.should_normalize_arrow_schema(
+            must_rewrite = pyarrow.should_normalize_arrow_schema(
                 arrow_schema, self.schema.get_table_columns(root_table_name), self.schema.naming
-            )
+            )[0]
         if must_rewrite:
             logger.info(
                 f"Table {root_table_name} parquet file {extracted_items_file} must be rewritten:"
-                f" add_dlt_id: {add_dlt_id} add_dlt_load_id: {add_dlt_load_id} destination file"
+                f" add_dlt_id: {add_dlt_id} destination file"
                 f" format: {self.item_storage.writer_spec.file_format} or due to required"
                 " normalization "
             )
             schema_update = self._write_with_dlt_columns(
-                extracted_items_file, root_table_name, add_dlt_load_id, add_dlt_id
+                extracted_items_file, root_table_name, add_dlt_id
             )
             return base_schema_update + schema_update
 

docs/website/docs/dlt-ecosystem/verified-sources/arrow-pandas.md

@@ -84,7 +84,10 @@ add_dlt_load_id = true
 add_dlt_id = true
 ```
 
-Keep in mind that enabling these incurs some performance overhead because the `parquet` file needs to be read back from disk in chunks, processed and rewritten with new columns.
+Keep in mind that enabling these incurs some performance overhead:
+
+- `add_dlt_load_id` has minimal overhead since the column is added to arrow table in memory during `extract` stage, before parquet file is written to disk
+- `add_dlt_id` adds the column during `normalize` stage after file has been extracted to disk. The file needs to be read back from disk in chunks, processed and rewritten with new columns
 
 ## Incremental loading with Arrow tables