Migrate workloads to Delta Lake
When you migrate workloads to Delta Lake, you should be aware of the following simplifications and differences compared with the data sources provided by Apache Spark and Apache Hive.
Delta Lake handles the following operations automatically, which you should never perform manually:
REFRESH TABLE: Delta tables always return the most up-to-date information, so there is no need to manually call
REFRESH TABLEafter changes.
Add and remove partitions: Delta Lake automatically tracks the set of partitions present in a table and updates the list as data is added or removed. As a result, there is no need to run
ALTER TABLE [ADD|DROP] PARTITIONor
Load a single partition: As an optimization, you may sometimes directly load the partition of data you are interested in. For example,
spark.read.parquet("/data/date=2017-01-01"). This is unnecessary with Delta Lake, since it can quickly read the list of files from the transaction log to find the relevant ones. If you are interested in a single partition, specify it using a
WHEREclause. For example,
spark.read.delta("/data").where("date = '2017-01-01'"). For large tables with many files in the partition, this can be much faster than loading a single partition (with direct partition path, or with
WHERE) from a Parquet table because listing the files in the directory is often slower than reading the list of files from the transaction log.
When you port an existing application to Delta Lake, you should avoid the following operations, which bypass the transaction log:
Manually modify data: Delta Lake uses the transaction log to atomically commit changes to the table. Because the log is the source of truth, files that are written out but not added to the transaction log are not read by Spark. Similarly, even if you manually delete a file, a pointer to the file is still present in the transaction log. Instead of manually modifying files stored in a Delta table, always use the commands that are described in this guide.
External readers: The data stored in Delta Lake is encoded as Parquet files. However, accessing these files using an external reader is not safe. You’ll see duplicates and uncommitted data and the read may fail when someone runs Remove files no longer referenced by a Delta table.
Because the files are encoded in an open format, you always have the option to move the files outside Delta Lake. At that point, you can run
VACUUM RETAIN 0 and delete the transaction log. This leaves the table’s files in a consistent state that can be read by the external reader of your choice.
Suppose you have Parquet data stored in a directory named
/data-pipeline, and you want to create a Delta table named
The first example shows how to:
- Read the Parquet data from its original location,
/data-pipeline, into a DataFrame.
- Save the DataFrame’s contents in Delta format in a separate location,
- Create the
eventstable based on that separate location,
The second example shows how to use
CONVERT TO TABLE to convert data from Parquet to Delta format without changing its original location,
Each of these examples create an unmanaged table, where you continue to manage the data in its specified location. Azure Databricks records the table’s name and its specified location in the metastore.
Save as Delta table
Read the Parquet data into a DataFrame and then save the DataFrame’s contents to a new directory in
data = spark.read.parquet("/data-pipeline") data.write.format("delta").save("/tmp/delta/data-pipeline/")
Create a Delta table named
eventsthat refers to the files in the new directory:
spark.sql("CREATE TABLE events USING DELTA LOCATION '/tmp/delta/data-pipeline/'")
Convert to Delta table
You have three options for converting a Parquet table to a Delta table:
Convert files to Delta Lake format and then create a Delta table:
CONVERT TO DELTA parquet.`/data-pipeline/` CREATE TABLE events USING DELTA LOCATION '/data-pipeline/'
Create a Parquet table and then convert it to a Delta table:
CREATE TABLE events USING PARQUET OPTIONS (path '/data-pipeline/') CONVERT TO DELTA events
Convert a Parquet table to a Delta table:
CONVERT TO DELTA events
This assumes that the table named
eventsis a Parquet table.
For details, see Convert a Parquet table to a Delta table.