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.

For information on Delta Lake SQL commands, see

Create a table

Delta Lake supports creating two types of tables—tables defined in the metastore and tables defined by path.

You can create tables in the following ways.

  • SQL DDL commands: You can use standard SQL DDL commands supported in Apache Spark (for example, CREATE TABLE and REPLACE TABLE) to create Delta tables.

    CREATE IF NOT EXISTS TABLE default.people10m (
      id INT,
      firstName STRING,
      middleName STRING,
      lastName STRING,
      gender STRING,
      birthDate TIMESTAMP,
      ssn STRING,
      salary INT
    ) USING DELTA
    
    CREATE OR REPLACE TABLE default.people10m (
      id INT,
      firstName STRING,
      middleName STRING,
      lastName STRING,
      gender STRING,
      birthDate TIMESTAMP,
      ssn STRING,
      salary INT
    ) USING DELTA
    

    Note

    In Databricks Runtime 8.0 and above, Delta Lake is the default format and you don’t need USING DELTA.

    Note

    In Databricks Runtime 7.0 and above, to avoid eventual consistency issues on AWS S3, Databricks recommends using the CREATE OR REPLACE syntax instead of DROP TABLE followed by a CREATE TABLE.

    In Databricks Runtime 7.0 and above, SQL also supports a creating table at a path without creating an entry in the Hive metastore.

    -- Create or replace table with path
    CREATE OR REPLACE TABLE delta.`/mnt/delta/people10m` (
      id INT,
      firstName STRING,
      middleName STRING,
      lastName STRING,
      gender STRING,
      birthDate TIMESTAMP,
      ssn STRING,
      salary INT
    ) USING DELTA
    
  • DataFrameWriter API: If you want to simultaneously create a table and insert data into it from Spark DataFrames or Datasets, you can use the Spark DataFrameWriter (Scala or Java and Python).

    # Create table in the metastore using DataFrame's schema and write data to it
    df.write.format("delta").saveAsTable("default.people10m")
    
    # Create or replace partitioned table with path using DataFrame's schema and write/overwrite data to it
    df.write.format("delta").mode("overwrite").save("/mnt/delta/people10m")
    
    // Create table in the metastore using DataFrame's schema and write data to it
    df.write.format("delta").saveAsTable("default.people10m")
    
    // Create table with path using DataFrame's schema and write data to it
    df.write.format("delta").mode("overwrite").save("/mnt/delta/people10m")
    
    • In Databricks Runtime 8.0 and above, Delta Lake is the default format and you don’t need to specify USING DELTA, format("delta"), or using("delta").
    • In Databricks Runtime 7.0 and above, you can also create Delta tables using the Spark DataFrameWriterV2 API.
  • DeltaTableBuilder API: You can also use the DeltaTableBuilder API in Delta Lake to create tables. Compared to the DataFrameWriter APIs, this API makes it easier to specify additional information like column comments, table properties, and generated columns.

    Preview

    This feature is in Public Preview.

    Note

    This feature is available on Databricks Runtime 8.3 and above.

    # Create table in the metastore
    DeltaTable.createIfNotExists(spark) \
      .tableName("default.people10m") \
      .addColumn("id", "INT") \
      .addColumn("firstName", "STRING") \
      .addColumn("middleName", "STRING") \
      .addColumn("lastName", "STRING", comment = "surname") \
      .addColumn("gender", "STRING") \
      .addColumn("birthDate", "TIMESTAMP") \
      .addColumn("ssn", "STRING") \
      .addColumn("salary", "INT") \
      .execute()
    
    # Create or replace table with path and add properties
    DeltaTable.createOrReplace(spark) \
      .addColumn("id", "INT") \
      .addColumn("firstName", "STRING") \
      .addColumn("middleName", "STRING") \
      .addColumn("lastName", "STRING", comment = "surname") \
      .addColumn("gender", "STRING") \
      .addColumn("birthDate", "TIMESTAMP") \
      .addColumn("ssn", "STRING") \
      .addColumn("salary", "INT") \
      .property("description", "table with people data") \
      .location("/mnt/delta/people10m") \
      .execute()
    
    // Create table in the metastore
    DeltaTable.createOrReplace(spark)
      .tableName("default.people10m")
      .addColumn("id", "INT")
      .addColumn("firstName", "STRING")
      .addColumn("middleName", "STRING")
      .addColumn(
        DeltaTable.columnBuilder("lastName")
          .dataType("STRING")
          .comment("surname")
          .build())
      .addColumn("lastName", "STRING", comment = "surname")
      .addColumn("gender", "STRING")
      .addColumn("birthDate", "TIMESTAMP")
      .addColumn("ssn", "STRING")
      .addColumn("salary", "INT")
      .execute()
    
    // Create or replace table with path and add properties
    DeltaTable.createOrReplace(spark)
      .addColumn("id", "INT")
      .addColumn("firstName", "STRING")
      .addColumn("middleName", "STRING")
      .addColumn(
        DeltaTable.columnBuilder("lastName")
          .dataType("STRING")
          .comment("surname")
          .build())
      .addColumn("lastName", "STRING", comment = "surname")
      .addColumn("gender", "STRING")
      .addColumn("birthDate", "TIMESTAMP")
      .addColumn("ssn", "STRING")
      .addColumn("salary", "INT")
      .property("description", "table with people data")
      .location("/mnt/delta/people10m")
      .execute()
    

See the API documentation for details.

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 a partition by columns. The following example partitions by gender.

-- Create table in the metastore
CREATE TABLE default.people10m (
  id INT,
  firstName STRING,
  middleName STRING,
  lastName STRING,
  gender STRING,
  birthDate TIMESTAMP,
  ssn STRING,
  salary INT
)
USING DELTA
PARTITIONED BY (gender)
df.write.format("delta").partitionBy("gender").saveAsTable("default.people10m")

DeltaTable.create(spark) \
  .tableName("default.people10m") \
  .addColumn("id", "INT") \
  .addColumn("firstName", "STRING") \
  .addColumn("middleName", "STRING") \
  .addColumn("lastName", "STRING", comment = "surname") \
  .addColumn("gender", "STRING") \
  .addColumn("birthDate", "TIMESTAMP") \
  .addColumn("ssn", "STRING") \
  .addColumn("salary", "INT") \
  .partitionedBy("gender") \
  .execute()
df.write.format("delta").partitionBy("gender").saveAsTable("default.people10m")

DeltaTable.createOrReplace(spark)
  .tableName("default.people10m")
  .addColumn("id", "INT")
  .addColumn("firstName", "STRING")
  .addColumn("middleName", "STRING")
  .addColumn(
    DeltaTable.columnBuilder("lastName")
      .dataType("STRING")
      .comment("surname")
      .build())
  .addColumn("lastName", "STRING", comment = "surname")
  .addColumn("gender", "STRING")
  .addColumn("birthDate", "TIMESTAMP")
  .addColumn("ssn", "STRING")
  .addColumn("salary", "INT")
  .partitionedBy("gender")
  .execute()

Control data location

For tables defined in the metastore, you can optionally specify the LOCATION as a path. 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 default.people10m
    USING DELTA
    LOCATION '/mnt/delta/people10m'
    

    the table in the 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.

Note

The metastore is not the source of truth about the latest information of a Delta table. In fact, the table definition in the metastore may not contain all the metadata like schema and properties. It contains the location of the table, and the table’s transaction log at the location is the source of truth. If you query the metastore from a system that is not aware of this Delta-specific customization, you may see incomplete or stale table information.

Use generated columns

Preview

This feature is in Public Preview.

Note

This feature is available on Databricks Runtime 8.3 and above.

Delta Lake supports generated columns which are a special type of columns whose values are automatically generated based on a user-specified function over other columns in the Delta table. When you write to a table with generated columns and you do not explicitly provide values for them, Delta Lake automatically computes the values. For example, you can automatically generate a date column (for partitioning the table by date) from the timestamp column; any writes into the table need only specify the data for the timestamp column. However, if you explicitly provide values for them, the values must satisfy the constraint (<value> <=> <generation expression>) IS TRUE or the write will fail with an error.

The following example shows how to create a table with generated columns:

CREATE TABLE default.people10m (
  id INT,
  firstName STRING,
  middleName STRING,
  lastName STRING,
  gender STRING,
  birthDate TIMESTAMP,
  dateOfBirth DATE GENERATED ALWAYS AS (CAST(birthDate AS DATE)),
  ssn STRING,
  salary INT
)
USING DELTA
PARTITIONED BY (gender)
DeltaTable.create(spark) \
  .tableName("default.people10m") \
  .addColumn("id", "INT") \
  .addColumn("firstName", "STRING") \
  .addColumn("middleName", "STRING") \
  .addColumn("lastName", "STRING", comment = "surname") \
  .addColumn("gender", "STRING") \
  .addColumn("birthDate", "TIMESTAMP") \
  .addColumn("dateOfBirth", DateType(), generatedAlwaysAs="CAST(birthDate AS DATE)") \
  .addColumn("ssn", "STRING") \
  .addColumn("salary", "INT") \
  .partitionedBy("gender") \
  .execute()
DeltaTable.create(spark)
  .tableName("default.people10m")
  .addColumn("id", "INT")
  .addColumn("firstName", "STRING")
  .addColumn("middleName", "STRING")
  .addColumn(
    DeltaTable.columnBuilder("lastName")
      .dataType("STRING")
      .comment("surname")
      .build())
  .addColumn("lastName", "STRING", comment = "surname")
  .addColumn("gender", "STRING")
  .addColumn("birthDate", "TIMESTAMP")
  .addColumn(
    DeltaTable.columnBuilder("dateOfBirth")
     .dataType(DateType)
     .generatedAlwaysAs("CAST(dateOfBirth AS DATE)")
     .build())
  .addColumn("ssn", "STRING")
  .addColumn("salary", "INT")
  .partitionedBy("gender")
  .execute()

Generated columns are stored as if they were normal columns. That is, they occupy storage.

The following restrictions apply to generated columns:

  • A generation expression can use any SQL functions in Spark that always return the same result when given the same argument values, except the following types of functions:
    • User-defined functions.
    • Aggregate functions.
    • Window functions.
    • Functions returning multiple rows.
  • MERGE operations do not support generated columns.

Read a table

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

SELECT * FROM default.people10m   -- query table in the metastore

SELECT * FROM delta.`/mnt/delta/people10m`  -- query table by path
spark.table("default.people10m")    # query table in the metastore

spark.read.format("delta").load("/mnt/delta/people10m")  # query table by path
spark.table("default.people10m")      // query table in the metastore

spark.read.format("delta").load("/mnt/delta/people10m")  // create table by path

import io.delta.implicits._
spark.read.delta("/mnt/delta/people10m")

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.

SQL AS OF syntax

SELECT * FROM table_name TIMESTAMP AS OF timestamp_expression
SELECT * FROM table_name VERSION AS OF version

where

  • timestamp_expression can be any one of:
    • '2018-10-18T22:15:12.013Z', that is, a string that can be cast to a timestamp
    • cast('2018-10-18 13:36:32 CEST' as timestamp)
    • '2018-10-18', that is, a date string
    • In Databricks Runtime 6.6 and above:
      • current_timestamp() - interval 12 hours
      • date_sub(current_date(), 1)
      • Any other expression that is or can be cast to a timestamp
  • version is a long value that can be obtained from the output of DESCRIBE HISTORY table_spec.

Neither timestamp_expression nor version can be subqueries.

Example

SELECT * FROM default.people10m TIMESTAMP AS OF '2018-10-18T22:15:12.013Z'
SELECT * FROM delta.`/mnt/delta/people10m` VERSION AS OF 123

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("/mnt/delta/people10m")
df2 = spark.read.format("delta").option("versionAsOf", version).load("/mnt/delta/people10m")

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.`/mnt/delta/people10m`)").collect()
df = spark.read.format("delta").option("versionAsOf", latest_version[0][0]).load("/mnt/delta/people10m")

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))

Data retention

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

The data files backing a Delta table are never deleted automatically; data files are deleted only when you run VACUUM. VACUUM does not delete Delta log files; log files are automatically cleaned up after checkpoints are written.

By default you can time travel to a Delta table up to 30 days old unless you have:

  • Run VACUUM on your Delta table.

  • Changed the data or log file retention periods using the following table properties:

    • delta.logRetentionDuration = "interval <interval>": controls how long the history for a table is kept. The default is interval 30 days.

      Each time 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.

    • 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.

      To access 30 days of historical data even if you run VACUUM on the Delta table, set delta.deletedFileRetentionDuration = "interval 30 days". This setting may cause your storage costs to go up.

Write to a table

Append

To atomically add new data to an existing Delta table, use append mode:

INSERT INTO default.people10m SELECT * FROM morePeople
df.write.format("delta").mode("append").save("/mnt/delta/people10m")
df.write.format("delta").mode("append").saveAsTable("default.people10m")
df.write.format("delta").mode("append").save("/mnt/delta/people10m")
df.write.format("delta").mode("append").saveAsTable("default.people10m")

import io.delta.implicits._
df.write.mode("append").delta("/mnt/delta/people10m")

Overwrite

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

INSERT OVERWRITE TABLE default.people10m SELECT * FROM morePeople
df.write.format("delta").mode("overwrite").save("/mnt/delta/people10m")
df.write.format("delta").mode("overwrite").saveAsTable("default.people10m")
df.write.format("delta").mode("overwrite").save("/mnt/delta/people10m")
df.write.format("delta").mode("overwrite").saveAsTable("default.people10m")

import io.delta.implicits._
df.write.mode("overwrite").delta("/mnt/delta/people10m")

Using DataFrames, you can also selectively overwrite only the data that matches an arbitrary expression. This feature is available in Databricks Runtime 9.1 LTS and Databricks Runtime 9.1 LTS Photon and above. The following command atomically replaces events in January in the target table, which is partitioned by start_date, with the data in df:

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

This sample code writes out the data in df, validates that it all matches the predicate, and performs an atomic replacement. If you want to write out data that doesn’t all match the predicate, to replace the matching rows in the target table, you can disable the constraint check by setting spark.databricks.delta.replaceWhere.constraintCheck.enabled to false:

spark.conf.set("spark.databricks.delta.replaceWhere.constraintCheck.enabled", False)
spark.conf.set("spark.databricks.delta.replaceWhere.constraintCheck.enabled", false)

In Databricks Runtime 9.0 and below, replaceWhere overwrites data matching a predicate over partition columns only. The following command atomically replaces the month in January in the target table, which is partitioned by date, with the data in df:

df.write \
  .format("delta") \
  .mode("overwrite") \
  .option("replaceWhere", "birthDate >= '2017-01-01' AND birthDate <= '2017-01-31'") \
  .save("/mnt/delta/people10m")
df.write
  .format("delta")
  .mode("overwrite")
  .option("replaceWhere", "birthDate >= '2017-01-01' AND birthDate <= '2017-01-31'")
  .save("/mnt/delta/people10m")

In Databricks Runtime 9.1 and above, if you want to fall back to the old behavior, you can disable the spark.databricks.delta.replaceWhere.dataColumns.enabled flag:

spark.conf.set("spark.databricks.delta.replaceWhere.dataColumns.enabled", False)
spark.conf.set("spark.databricks.delta.replaceWhere.dataColumns.enabled", false)

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 default.people10m SELECT * FROM morePeople
df.write.format("delta") \
  .mode("overwrite") \
  .option("userMetadata", "overwritten-for-fixing-incorrect-data") \
  .save("/mnt/delta/people10m")
df.write.format("delta")
  .mode("overwrite")
  .option("userMetadata", "overwritten-for-fixing-incorrect-data")
  .save("/mnt/delta/people10m")

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 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.

Note

In Databricks Runtime 7.0 and above, INSERT syntax provides schema enforcement and supports schema evolution. If a column’s data type cannot be safely cast to your Delta Lake table’s data type, then a runtime exception is thrown. If schema evolution is enabled, new columns can exist as the last columns of your schema (or nested columns) for the schema to evolve.

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.

For recommended methods, see Structured Streaming in production.

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 nested_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("birthDate", col("birthDate").cast("date")) \
  .write \
  .format("delta") \
  .mode("overwrite")
  .option("overwriteSchema", "true") \
  .saveAsTable(...)
Change a column name
spark.read.table(...) \
  .withColumnRenamed("dateOfBirth", "birthDate") \
  .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.delta.schema.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.

Note

  • mergeSchema is not supported when table access control is enabled (as it elevates a request that requires MODIFY to one that requires ALL PRIVILEGES).
  • mergeSchema cannot be used with INSERT INTO or .write.insertInto().

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.

These views integrate with table access control to allow for column and row level security.

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.
  • Configure the number of columns for which statistics are collected: delta.dataSkippingNumIndexedCols=<number-of-columns>. This property takes affect only for new data that is written out.
  • Randomize file prefixes to avoid hot spots in S3 metadata: delta.randomizeFilePrefixes=true. For tables that require a lot (thousands of requests per second) of fast read/write operations, we strongly recommend dedicating an S3 bucket to a table (locating table at the root of the bucket), and enabling randomized file prefixes to get the best experience.

Note

  • 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")

Table metadata

Delta Lake has rich features for exploring table metadata.

It supports SHOW [PARTITIONS | COLUMNS] and DESCRIBE TABLE. See

It also provides the following unique commands:

DESCRIBE DETAIL

Provides information about schema, partitioning, table size, and so on. For details, see Retrieve Delta table details.

DESCRIBE HISTORY

Provides provenance information, including the operation, user, and so on, and operation metrics for each write to a table. Table history is retained for 30 days. For details, see Retrieve Delta table history.

The Data sidebar provides a visual view of this detailed table information and history for Delta tables. In addition to the table schema and sample data, you can click the History tab to see the table history that displays with DESCRIBE HISTORY.

Notebook

For an example of the various Delta table metadata commands, see the end of the following notebook:

Delta Lake batch commands notebook

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