Table batch reads and writes

Delta Lake supports most of the options provided by Apache Spark DataFrame read and write APIs for performing batch reads and writes on tables.

In this article:

Configure SparkSession

For many Delta Lake DDL operations, you must enable our integration with Apache Spark DataSourceV2 and Catalog APIs (since 3.0) by setting the following configurations when creating a new SparkSession.

from pyspark.sql import SparkSession

spark = SparkSession \
  .builder \
  .appName("...") \
  .master("...") \
  .config("spark.sql.extensions", "io.delta.sql.DeltaSparkSessionExtension") \
  .config("spark.sql.catalog.spark_catalog", "org.apache.spark.sql.delta.catalog.DeltaCatalog") \
  .getOrCreate()
import org.apache.spark.sql.SparkSession

val spark = SparkSession
  .builder()
  .appName("...")
  .master("...")
  .config("spark.sql.extensions", "io.delta.sql.DeltaSparkSessionExtension")
  .config("spark.sql.catalog.spark_catalog", "org.apache.spark.sql.delta.catalog.DeltaCatalog")
  .getOrCreate()

Create a table

Delta Lake supports creating tables directly based on the path using DataFrameWriter (Scala or Java/Python). Delta Lake also supports creating tables in the metastore using standard DDL CREATE TABLE. When you create a table in the metastore using Delta Lake, it stores the location of the table data in the metastore. This pointer makes it easier for other users to discover and refer to the data without having to worry about exactly where it is stored. However, the metastore is not the source of truth about what is valid in the table. That responsibility stays with Delta Lake.

-- Create table in the metastore
CREATE TABLE events (
  date DATE,
  eventId STRING,
  eventType STRING,
  data STRING)
USING DELTA
df.write.format("delta").saveAsTable("events")      # create table in the metastore


df.write.format("delta").save("/delta/events")  # create table by path
df.write.format("delta").saveAsTable("events")      // create table in the metastore

df.write.format("delta").save("/delta/events")  // create table by path

Partition data

You can partition data to speed up queries or DML that have predicates involving the partition columns. To partition data when you create a Delta table, specify partition by columns. A common pattern is to partition by date, for example:

-- Create table in the metastore
CREATE TABLE events (
 date DATE,
 eventId STRING,
 eventType STRING,
 data STRING)
USING DELTA
PARTITIONED BY (date)
LOCATION '/delta/events'
df.write.format("delta").partitionBy("date").saveAsTable("events")      # create table in the metastore


df.write.format("delta").partitionBy("date").save("/delta/events")  # create table by path
df.write.format("delta").partitionBy("date").saveAsTable("events")      // create table in the metastore

df.write.format("delta").partitionBy("date").save("/delta/events")  // create table by path

Control data location

To control the location of the Delta table files, you can optionally specify the LOCATION as a path on DBFS.

Tables created with a specified LOCATION are considered unmanaged by the metastore. Unlike a managed table, where no path is specified, an unmanaged table’s files are not deleted when you DROP the table.

When you run CREATE TABLE with a LOCATION that already contains data stored using Delta Lake, Delta Lake does the following:

  • If you specify only the table name and location, for example:

    CREATE TABLE events
    USING DELTA
    LOCATION '/delta/events'
    

    the table in the Hive metastore automatically inherits the schema, partitioning, and table properties of the existing data. This functionality can be used to “import” data into the metastore.

  • If you specify any configuration (schema, partitioning, or table properties), Delta Lake verifies that the specification exactly matches the configuration of the existing data.

    Important

    If the specified configuration does not exactly match the configuration of the data, Delta Lake throws an exception that describes the discrepancy.

Read a table

You can load a Delta table as a DataFrame by specifying a path:

SELECT * FROM events   -- query table in the metastore

SELECT * FROM delta.`/delta/events`  -- query table by path
spark.table("events")    # query table in the metastore

spark.read.format("delta").load("/delta/events")  # query table by path
spark.table("events")      // query table in the metastore

spark.read.format("delta").load("/delta/events")  // create table by path

The DataFrame returned automatically reads the most recent snapshot of the table for any query; you never need to run REFRESH TABLE. Delta Lake automatically uses partitioning and statistics to read the minimum amount of data when there are applicable predicates in the query.

Query an older snapshot of a table (time travel)

In this section:

Delta Lake time travel allows you to query an older snapshot of a Delta table. Time travel has many use cases, including:

  • Re-creating analyses, reports, or outputs (for example, the output of a machine learning model). This could be useful for debugging or auditing, especially in regulated industries.
  • Writing complex temporal queries.
  • Fixing mistakes in your data.
  • Providing snapshot isolation for a set of queries for fast changing tables.

This section describes the supported methods for querying older versions of tables, data retention concerns, and provides examples.

Syntax

This section shows how to query an older version of a Delta table.

DataFrameReader options

DataFrameReader options allow you to create a DataFrame from a Delta table that is fixed to a specific version of the table.

df1 = spark.read.format("delta").option("timestampAsOf", timestamp_string).load("/delta/events")
df2 = spark.read.format("delta").option("versionAsOf", version).load("/delta/events")

For timestamp_string, only date or timestamp strings are accepted. For example, "2019-01-01" and "2019-01-01T00:00:00.000Z".

A common pattern is to use the latest state of the Delta table throughout the execution of <a Databricks> job to update downstream applications.

Because Delta tables auto update, a DataFrame loaded from a Delta table may return different results across invocations if the underlying data is updated. By using time travel, you can fix the data returned by the DataFrame across invocations:

latest_version = spark.sql("SELECT max(version) FROM (DESCRIBE HISTORY delta.`/delta/events`)").collect()
df = spark.read.format("delta").option("versionAsOf", latest_version[0][0]).load("/delta/events")

Data retention

By default, Delta tables retain the commit history for 30 days. This means that you can specify a version from 30 days ago. However, there are some caveats:

  • You did not run VACUUM on your Delta table. If you run VACUUM, you lose the ability to go back to a version older than the default 7 day data retention period.

You can configure retention periods using the following table properties:

  • delta.logRetentionDuration = "interval <interval>": controls how long the history for a table is kept. Each time a a checkpoint is written, <Databricks> automatically cleans up log entries older than the retention interval. If you set this config to a large enough value, many log entries are retained. This should not impact performance as operations against the log are constant time. Operations on history are parallel (but will become more expensive as the log size increases). The default is interval 30 days.

  • delta.deletedFileRetentionDuration = "interval <interval>": controls how long ago a file must have been deleted before being a candidate for VACUUM. The default is interval 7 days. For access to 30 days of historical data, set delta.deletedFileRetentionDuration = "interval 30 days". This setting may cause your storage costs to go up.

    Note

    VACUUM doesn’t clean up log files; log files are automatically cleaned up after checkpoints are written.

To time travel to a previous version, you must retain both the log and the data files for that version.

Examples

  • Fix accidental deletes to a table for the user 111:

    INSERT INTO my_table
      SELECT * FROM my_table TIMESTAMP AS OF date_sub(current_date(), 1)
      WHERE userId = 111
    
  • Fix accidental incorrect updates to a table:

    MERGE INTO my_table target
      USING my_table TIMESTAMP AS OF date_sub(current_date(), 1) source
      ON source.userId = target.userId
      WHEN MATCHED THEN UPDATE SET *
    
  • Query the number of new customers added over the last week.

    SELECT count(distinct userId) - (
      SELECT count(distinct userId)
      FROM my_table TIMESTAMP AS OF date_sub(current_date(), 7))
    

Write to a table

Append

Using append mode you can atomically add new data to an existing Delta table:

INSERT INTO events SELECT * FROM newEvents
df.write.format("delta").mode("append").save("/delta/events")
df.write.format("delta").mode("append").saveAsTable("events")
df.write.format("delta").mode("append").save("/delta/events")
df.write.format("delta").mode("append").saveAsTable("events")

Overwrite

To atomically replace all of the data in a table, you can use overwrite mode:

INSERT OVERWRITE events SELECT * FROM newEvents
df.write.format("delta").mode("overwrite").save("/delta/events")
df.write.format("delta").mode("overwrite").saveAsTable("events")
df.write.format("delta").mode("overwrite").save("/delta/events")
df.write.format("delta").mode("overwrite").saveAsTable("events")

Using DataFrames, you can also selectively overwrite only the data that matches predicates over partition columns. The following command atomically replaces the month of January with the data in df:

df.write \
  .format("delta") \
  .mode("overwrite") \
  .option("replaceWhere", "date >= '2017-01-01' AND date <= '2017-01-31'") \
  .save("/delta/events")
df.write
  .format("delta")
  .mode("overwrite")
  .option("replaceWhere", "date >= '2017-01-01' AND date <= '2017-01-31'")
  .save("/delta/events")

This sample code writes out the data in df, validates that it all falls within the specified partitions, and performs an atomic replacement.

Note

Unlike the file APIs in Apache Spark, Delta Lake remembers and enforces the schema of a table. This means that by default overwrites do not replace the schema of an existing table.

For Delta Lake support for updating tables, see Table deletes, updates, and merges.

Set user-defined commit metadata

You can specify user-defined strings as metadata in commits made by these operations, either using the DataFrameWriter option userMetadata or the SparkSession configuration spark.databricks.delta.commitInfo.userMetadata. If both of them have been specified, then the option takes preference. This user-defined metadata is readable in the history operation.

SET spark.databricks.delta.commitInfo.userMetadata=overwritten-for-fixing-incorrect-data
INSERT OVERWRITE events SELECT * FROM newEvents
df.write.format("delta") \
  .mode("overwrite") \
  .option("userMetadata", "overwritten-for-fixing-incorrect-data") \
  .save("/delta/events")
df.write.format("delta")
  .mode("overwrite")
  .option("userMetadata", "overwritten-for-fixing-incorrect-data")
  .save("/delta/events")

Schema validation

Delta Lake automatically validates that the schema of the DataFrame being written is compatible with the schema of the table. Delta Lake uses the following rules to determine whether a write from a DataFrame to a table is compatible:

  • All DataFrame columns must exist in the target table. If there are columns in the DataFrame not present in the table, an exception is raised. Columns present in the table but not in the DataFrame are set to null.
  • DataFrame column data types must match the column data types in the target table. If they don’t match, an exception is raised.
  • DataFrame column names cannot differ only by case. This means that you cannot have columns such as “Foo” and “foo” defined in the same table. While you can use Spark in case sensitive or insensitive (default) mode, Parquet is case sensitive when storing and returning column information. Delta Lake is case-preserving but insensitive when storing the schema and has this restriction to avoid potential mistakes, data corruption, or loss issues.

Delta Lake support DDL to explicitly add new columns explicitly and the ability to update schema automatically.

If you specify other options, such as partitionBy, in combination with append mode, Delta Lake validates that they match and throws an error for any mismatch. When partitionBy is not present, appends automatically follow the partitioning of the existing data.

Update table schema

Delta Lake lets you update the schema of a table. The following types of changes are supported:

  • Adding new columns (at arbitrary positions)
  • Reordering existing columns

You can make these changes explicitly using DDL or implicitly using DML.

Important

When you update a Delta table schema, streams that read from that table terminate. If you want the stream to continue you must restart it.

Explicitly update schema

You can use the following DDL to explicitly change the schema of a table.

Add columns

  ALTER TABLE table_name ADD COLUMNS (col_name data_type [COMMENT col_comment] [FIRST|AFTER colA_name], ...)

By default, nullability is true.

To add a column to a nested field, use:

  ALTER TABLE table_name ADD COLUMNS (col_name.nested_col_name data_type [COMMENT col_comment] [FIRST|AFTER colA_name], ...)

##### Example

If the schema before running ALTER TABLE boxes ADD COLUMNS (colB.nested STRING AFTER field1) is:

  - root
  | - colA
  | - colB
  | +-field1
  | +-field2

the schema after is:

  - root
  | - colA
  | - colB
  | +-field1
  | +-nested
  | +-field2

.. note:

Adding nested columns is supported only for structs. Arrays and maps are not supported.

Change column comment or ordering

  ALTER TABLE table_name CHANGE [COLUMN] col_name col_name data_type [COMMENT col_comment] [FIRST|AFTER colA_name]

To change a column in a nested field, use:

  ALTER TABLE table_name CHANGE [COLUMN] col_name.nested_col_name col_name data_type [COMMENT col_comment] [FIRST|AFTER colA_name]

##### Example

If the schema before running ALTER TABLE boxes CHANGE COLUMN colB.field2 field2 STRING FIRST is:

  - root
  | - colA
  | - colB
  | +-field1
  | +-field2

the schema after is:

  - root
  | - colA
  | - colB
  | +-field2
  | +-field1

Replace columns

  ALTER TABLE table_name REPLACE COLUMNS (col_name1 col_type1 [COMMENT col_comment1], ...)

##### Example

When running the following DSL:

  ALTER TABLE boxes REPLACE COLUMNS (colC STRING, colB STRUCT<field2:STRING, nested:STRING, field1:STRING>, colA STRING)

if the schema before is:

  - root
  | - colA
  | - colB
  | +-field1
  | +-field2

the schema after is:

  - root
  | - colC
  | - colB
  | +-field2
  | +-nested
  | +-field1
  | - colA

Change column type or name

Changing a column’s type or name or dropping a column requires rewriting the table. To do this, use the overwriteSchema option:

##### Change a column type

  spark.read.table(...)
    .withColumn("date", col("date").cast("date"))
    .write
    .format("delta")
    .mode("overwrite")
    .option("overwriteSchema", "true")
    .saveAsTable(...)

##### Change a column name

  spark.read.table(...)
    .withColumnRenamed("date", "date_created")
    .write
    .format("delta")
    .mode("overwrite")
    .option("overwriteSchema", "true")
    .saveAsTable(...)

Automatic schema update

Delta Lake can automatically update the schema of a table as part of a DML transaction (either appending or overwriting), and make the schema compatible with the data being written.

Add columns

Columns that are present in the DataFrame but missing from the table are automatically added as part of a write transaction when:

  • write or writeStream have .option("mergeSchema", "true")
  • spark.databricks.deltaschema.autoMerge.enabled is true

When both options are specified, the option from the DataFrameWriter takes precedence. The added columns are appended to the end of the struct they are present in. Case is preserved when appending a new column.

NullType columns

Because Parquet doesn’t support NullType, NullType columns are dropped from the DataFrame when writing into Delta tables, but are still stored in the schema. When a different data type is received for that column, Delta Lake merges the schema to the new data type. If Delta Lake receives a NullType for an existing column, the old schema is retained and the new column is dropped during the write.

NullType in streaming is not supported. Since you must set schemas when using streaming this should be very rare. NullType is also not accepted for complex types such as ArrayType and MapType.

Replace table schema

By default, overwriting the data in a table does not overwrite the schema. When overwriting a table using mode("overwrite") without replaceWhere, you may still want to overwrite the schema of the data being written. You replace the schema and partitioning of the table by setting the overwriteSchema option to true:

df.write.option("overwriteSchema", "true")

Views on tables

Delta Lake supports the creation of views on top of Delta tables just like you might with a data source table.

The core challenge when you operate with views is resolving the schemas. If you alter a Delta table schema, you must recreate derivative views to account for any additions to the schema. For instance, if you add a new column to a Delta table, you must make sure that this column is available in the appropriate views built on top of that base table.

Table properties

You can store your own metadata as a table property using TBLPROPERTIES in CREATE and ALTER.

TBLPROPERTIES are stored as part of Delta table metadata. You cannot define new TBLPROPERTIES in a CREATE statement if a Delta table already exists in a given location. See table creation for more details.

In addition, to tailor behavior and performance, Delta Lake supports certain Delta table properties:

  • Block deletes and updates in a Delta table: delta.appendOnly=true.
  • Configure the time travel retention properties: delta.logRetentionDuration=<interval-string> and delta.deletedFileRetentionDuration=<interval-string>. For details, see Data retention.

Note

  • These are the only supported delta.-prefixed table properties.
  • Modifying a Delta table property is a write operation that will conflict with other concurrent write operations, causing them to fail. We recommend that you modify a table property only when there are no concurrent write operations on the table.

You can also set delta.-prefixed properties during the first commit to a Delta table using Spark configurations. For example, to initialize a Delta table with the property delta.appendOnly=true, set the Spark configuration spark.databricks.delta.properties.defaults.appendOnly to true. For example:

spark.sql("SET spark.databricks.delta.properties.defaults.appendOnly = true")
spark.conf.set("spark.databricks.delta.properties.defaults.appendOnly", "true")
spark.conf.set("spark.databricks.delta.properties.defaults.appendOnly", "true")