Transformations: Reshape data after loading#

import { DltHubFeatureAdmonition } from '@theme@.../DltHubFeatureAdmonition';

dlt transformations let you build new tables or full datasets from datasets that have already been ingested with dlt. dlt transformations are written and run in a very similar fashion to dlt source and resources. dlt transformations require you to have loaded data to a location, for example a local duckdb database, a bucket or a warehouse on which the transformations may be executed. dlt transformations are fully supported for all of our sql destinations including all filesystem and bucket formats.

You create them with the @dlt.hub.transformation decorator, which has the same signature as the @dlt.resource decorator but yields a SQL query, including the resulting
column schema, rather than data items. dlt transformations support the same write_dispositions per destination as dlt resources do.

Motivations#

A few real-world scenarios where dlt transformations can be useful:

Build one-stop reporting tables – Flatten and enrich raw data into a wide table that analysts can pivot, slice, and dice without writing SQL each time.
Clean data – Remove irrelevant columns or anonymize sensitive information before sending it to a layer with lower privacy protections.
Normalize JSON into 3-NF – Break out repeating attributes from nested JSON so updates are consistent and storage isn't wasted.
Create dimensional (star-schema) models – Produce fact and dimension tables so BI users can drag-and-drop metrics and break them down by any dimension.
Generate task-specific feature sets – Deliver slim tables tailored for personalization, forecasting, or other ML workflows.
Apply shared business definitions – Encode rules such as "a sale is a transaction whose status became paid this month," ensuring every metric is counted the same way.
Merge heterogeneous sources – Combine Shopify, Amazon, WooCommerce (etc.) into one canonical orders feed for unified inventory and revenue reporting.
Run transformations during ingestion pre-warehouse – Pre-aggregate or pre-filter data before it hits the warehouse to cut compute and storage costs.
…and more – Any scenario where reshaping, enriching, or aggregating existing data unlocks faster insight or cleaner downstream pipelines.

Quick-start in three simple steps#

For the example below, you can copy–paste everything into one script and run it.

1. Load some example data#

The snippets below assume that we have a simple fruitshop dataset as produced by the dlt fruitshop template:

2. Inspect the dataset#

3. Write and run a transformation#

3.1 Alternatively use pure SQL for the transformation#

That's it — copied_customers is now a new table in the same DuckDB schema with the first 5 customers when ordered by name. dlt has detected that we are loading into the same dataset
and executed this transformation in SQL - no data was transferred to and from the machine executing this pipeline. Additionally, the new destination table copied_customers was automatically evolved
to the correct new schema, and you could also set a different write disposition and even merge data from a transformation.

Defining a transformation#

Decorator arguments mirror those accepted by @dlt.resource.
The transformation function signature must contain at least one dlt.Dataset which is used inside the function to create the transformation SQL statements and calculate the resulting schema update.
A transformation yields a Relation created with ibis expressions or a select query which will be materialized into the destination table. If the first item yielded is a valid sql query or relation object, data will be interpreted as a transformation. In all other cases, the transformation decorator will work like any other resource.

Loading to other datasets#

Loading to another dataset on the same physical location#

Below we load to the same DuckDB instance with a new pipeline that points to another dataset. dlt will be able to detect that both datasets live on the same destination and
will run the transformation as pure SQL.

Loading to another dataset on a different physical location#

Below we load the data from our local DuckDB instance to a Postgres instance. dlt will use the query to extract the data as Parquet files and will do a regular dlt load, pushing the data to Postgres. Note that you can use the exact same transformation functions for both scenarios. This can be extremely useful when you want to avoid compute costs in warehouses by running transformations directly from a local duckdb instance or raw data in a bucket into the warehouse, as the compute will happen on the machine executing the pipeline that runs the transformations.

Using transformations#

Grouping multiple transformations in a source#

dlt transformations can be grouped like all other resources into sources and will be executed together. You can even mix regular resources and transformations in one pipeline load.

Yielding multiple transformations from one transformation resource#

dlt transformations may also yield more than one transformation instruction. If no further table name hints are supplied, the result will be a union of the yielded transformation instructions. dlt will take care of the necessary schema migrations, you will just need to ensure that no columns are marked as non-nullable that are missing from one of the transformation instructions:

Supplying additional hints#

You may supply column and table hints the same way you do for regular resources. dlt will derive schema hints from your query, but in some cases you may need to modify or extend them — for example, making columns nullable as in the example above, or adjusting the precision or type of a column to ensure compatibility with a specific target destination (if it differs from the source).

Writing your queries in SQL#

If you prefer to write your queries in SQL, you can omit ibis expressions by simply creating a Relation from a query on your dataset:

The identifiers (table and column names) used in these raw SQL expressions must correspond to the identifiers as they are present in your dlt schema, NOT in your destination database schema.

Using Pandas or Polars DataFrames and Arrow tables#

You can also write transformations directly using Pandas or Polars DataFrames and Arrow tables. Note that in this case your transformation resource behaves like a regular resource: column-level hints will not be propagated, and dlt will simply treat the yielded DataFrames or Arrow tables like data from any other resource. This behavior may change in the future.

Schema evolution and hints lineage#

When executing transformations, dlt computes the resulting schema before the transformation is executed. This allows dlt to:

Migrate the destination schema accordingly, creating new columns or tables as needed
Fail early if there are schema mismatches that cannot be resolved
Preserve column-level hints from source to destination

Schema evolution#

For example, if your transformation joins two tables and creates new columns, dlt will automatically update the destination schema to accommodate these changes. If your transformation would result in incompatible schema changes (like changing a column's data type in a way that could lose data), dlt will fail before executing the transformation, protecting your data and saving execution and debug time.

You can inspect the computed result schema during development by looking at the result of compute_columns_schema on your Relation:

Column level hint forwarding#

When creating or updating tables with transformation resources, dlt will also forward certain column hints to the new tables. In our fruitshop source, we have applied a custom hint named
x-annotation-pii set to True for the name column, which indicates that this column contains PII (personally identifiable information).
Downstream of the transformation layer, we may want to know which columns originate from columns that contain private data:

Features and limitations:#

dlt will only forward certain types of hints to the resulting tables: custom hints starting with x-annotation... and type hints such as nullable, data_type, precision, scale, and timezone. Other hints, such as primary_key or merge_keys, will need to be set via the columns argument on the transformation decorator, since dlt does not know how the transformed tables will be used.
dlt cannot forward hints for columns that result from combining multiple origin columns, such as when they are concatenated or produced through other SQL operations.

Lifecycle of a SQL transformation#

In this section, we focus on the lifecycle of transformations that yield a Relation object, which we call SQL transformations here. This is in contrast to Python-based transformations that yield dataframes, arrow tables, or polars frames, which go through the regular extract, normalize, and load lifecycle of a dlt resource.

Extract#

In the extract stage, a Relation yielded by a transformation is converted into a SQL string and saved as a .model file along with its source SQL dialect.
At this stage, the SQL string is just the user's original query — either the string that was explicitly provided or the one generated by Relation.to_sql(). No dlt-specific columns like _dlt_id or _dlt_load_id are added yet.

Normalize#

In the normalize stage, .model files are read and processed. The normalization process modifies your SQL queries to ensure they execute correctly and integrate with dlt's features.

Adding `dlt` columns#

During normalization, dlt adds internal dlt columns to your SQL queries depending on the configuration:

_dlt_load_id, which tracks which load operation created or modified each row, is added by default. Even if present in your query, the _dlt_load_id column will be replaced with a constant value corresponding to the current load ID. To disable this behavior, set:
```
[normalize.model_normalizer]
add_dlt_load_id = false
```
In this case, the column will not be added or replaced.
_dlt_id, a unique identifier for each row, is not added by default. If your query already includes a _dlt_id column, it will be left unchanged. To enable automatic generation of this column when it’s missing, set:
```
[normalize.model_normalizer]
add_dlt_id = true
```
When enabled and the column is not in the query, dlt will generate a _dlt_id. Note that if the column is already present, it will not be replaced.

The _dlt_id column is generated using the destination's UUID function, such as generateUUIDv4() in ClickHouse. For dialects without native UUID support:
- In Redshift, _dlt_id is generated using an MD5 hash of the load ID and row number.
- In SQLite, _dlt_id is simulated using lower(hex(randomblob(16))).

Query transformations#

The normalization process also applies the following transformations to ensure your queries work correctly:

Fully qualifies all identifiers with database and dataset prefixes
Quotes and adjusts identifier casing to match destination requirements
Normalizes column names according to the selected naming convention
Aliases columns and tables to handle naming convention differences
Reorders columns to match the destination table schema
Fills in NULL values for columns that exist in the destination but aren't in your query

Load#

In the load stage, the normalized queries from .model files are wrapped in INSERT statements and executed on the destination.
For example, given this query from the extract stage:

SELECT
    "my_table"."id" AS "id",
    "my_table"."value" AS "value"
FROM "my_pipeline_dataset"."my_table" AS "my_table"

After the normalize stage processes it (adding dlt columns, wrapping in subquery, etc.) and results in:

SELECT
    _dlt_subquery."id" AS "id",
    _dlt_subquery."value" AS "value",
    '1749134128.17655' AS "_dlt_load_id",
    UUID() AS "_dlt_id"
FROM (
    SELECT
        "my_table"."id" AS "id",
        "my_table"."value" AS "value"
    FROM "my_pipeline_dataset"."my_table" AS "my_table"
    )
AS _dlt_subquery

The load stage executes:

INSERT INTO
    "my_pipeline_dataset"."my_transformation" ("id", "value", "_dlt_load_id", "_dlt_id")
SELECT
    _dlt_subquery."id" AS "id",
    _dlt_subquery."value" AS "value",
    '1749134128.17655' AS "_dlt_load_id",
    UUID() AS "_dlt_id"
FROM (
    SELECT
        "my_table"."id" AS "id",
        "my_table"."value" AS "value"
    FROM "my_pipeline_dataset"."my_table" AS "my_table"
    )
AS _dlt_subquery

The query is executed via the destination's SQL client, materializing the transformation result directly in the database.

Examples#

Local in-transit transformations example#

If you require aggregated or otherwise transformed data in your warehouse, but would like to avoid or reduce the costs of running queries across many rows in your warehouse tables, you can run some or all of your transformations "in transit" while loading data from your source. The code below demonstrates how you can extract data with our rest_api source to a local DuckDB instance and then forward aggregated data to a warehouse destination.

This script demonstrates:

Fetching data from a REST API using dlt's rest_api_source
Loading raw data into a local DuckDB instance as an intermediate step
Transforming the data by joining orders with stores and aggregating order counts directly on the local DuckDB instance, not in the destination warehouse
Loading only the aggregated results to a production warehouse (Postgres)
Reducing warehouse compute costs by performing transformations locally in DuckDB
Using multiple pipelines in a single workflow for different stages of processing

Incremental transformations example#

This example demonstrates how you can incrementally transform incoming new data for the customers table into a cleaned_customers table where the name column has been removed. It:

Uses primary key-based incremental loading to process only new data
Tracks the highest ID processed so far to filter out already processed records
Handles first-time runs with the PipelineNeverRan exception
Removes sensitive data (name column) during the transformation
Uses write_disposition="append" to add only new records to the destination table
Can be run repeatedly as new data arrives, processing only the delta