Databricks Delta supports most of the options provided by Spark SQL DataFrame read and write APIs for performing batch reads and writes on tables.
In this topic:
- Create a table
- Read a table
- Write to a table
- Schema validation
- Update table schema
- Replace table schema
- Views on tables
- Table properties
- Table metadata
Delta supports creating tables in the metastore using standard DDL:
CREATE TABLE events ( date DATE, eventId STRING, eventType STRING, data STRING) USING DELTA
When you create a table in the metastore using Delta, it creates a symlink-like pointer in the metastore to the transaction log and data that are stored on DBFS. 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.
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 events ( date DATE, eventId STRING, eventType STRING, data STRING) USING DELTA PARTITIONED BY (date) LOCATION '/delta/events'
You can also specify partition columns when writing a DataFrame to a new Delta table:
To control the location of the 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, Delta 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 verifies that the specification exactly matches the configuration of the existing data.
If the specified configuration does not exactly match the configuration of the data, Delta throws an exception that describes the discrepancy.
You can load a Delta table as a DataFrame by specifying either its path:
SELECT * FROM delta.`/delta/events`
or table name:
SELECT * FROM 'events'
The DataFrame returned automatically reads the most recent snapshot of the table for any query; you never need to run
REFRESH TABLE. Delta automatically uses partitioning and statistics to read the minimum amount of data when there are applicable predicates in the query.
This feature is available with Databricks Runtime 5.2 and above and is Experimental.
Delta 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 fields.
- 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.
There are several ways to query an older version of a Delta table.
SELECT * FROM events TIMESTAMP AS OF timestamp_expression SELECT * FROM events VERSION AS OF version
timestamp_expressioncan 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
current_timestamp() - interval 12 hours
- Any other expression that is or can be cast to a timestamp
versionis a long value that can be obtained from the output of
DESCRIBE HISTORY events.
version can be subqueries.
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")
timestamp_string, only date or timestamp strings are accepted. For example,
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).load("/delta/events") # Every query that stems off df will use the same snapshot
You may have a parametrized pipeline, where the input path of your pipeline is a parameter of your job. After the execution of your job, you may want to reproduce the output some time in the future. In this case, you can use the
@ syntax to specify the timestamp or version:
df1 = spark.read.format("delta").load("/delta/events@20190101000000000") # table on 2019-01-01 00:00:00.000 df2 = spark.read.format("delta").load("/delta/events@v123") # table on version 123
SELECT * FROM events@20190101000000000 SELECT * FROM events@v123
The timestamp must be in
yyyyMMddHHmmssSSS format. You can obtain table versions by running
DESCRIBE HISTORY <table> and specify a version after
@ by prepending a
v to the version. For example, to query version
123 for the table
By default, Delta tables keep a commit history of 30 days. This means that you can potentially specify a version from 30 days ago. However, there are some caveats:
- All writers to the Delta table must be using Databricks Runtime 5.1 or above.
- You must not have run
VACUUMon your Delta table. If you have run
VACUUM, then you may lose the ability to go back to a version older than the default 7 day data retention period.
You configure retention periods using the following table properties:
delta.logRetentionDuration = "interval <interval>": Configure how long you can go back in time. Default is
interval 30 days.
delta.deletedFileRetentionDuration = "interval <interval>": Configure how long stale data files are kept around before being deleted with
VACUUM. Default is
interval 1 week.
For full access to 30 days of historical data, set
delta.deletedFileRetentionDuration = "interval 30 days" on your table. This setting may cause your storage costs to go up.
Fix accidental deletes to a table for the user
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))
In this section:
append mode you can atomically add new data to an existing Delta table:
To atomically replace all of the data in a table, you can use
You can 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.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.
Unlike the file APIs in Apache Spark, Delta remembers and enforces the schema of a table. This means that by default overwrites do not replace the schema of an existing table.
UPDATE statement allows you to apply expressions to change the value of columns when a row matches a predicate. For example, you can use
UPDATE to fix a spelling mistake in the
UPDATE events SET eventType = 'click' WHERE eventType = 'clck'
Similar to delete, update operations automatically make use of the partitioning of the table when possible.
MERGE INTO statement allows you to merge a set of updates and insertions into an existing dataset. For example, the following statement takes a stream of updates and merges it into the
events table. When there is already an event present with the same
eventId, Delta updates the data column using the given expression. When there is no matching event, Delta adds a new row.
Here’s a worked example:
MERGE INTO events USING updates ON events.eventId = updates.eventId WHEN MATCHED THEN UPDATE SET events.data = updates.data WHEN NOT MATCHED THEN INSERT (date, eventId, data) VALUES (date, eventId, data)
You must specify a value for every column in your table when you perform an
INSERT (for example, when there is no matching row in the existing dataset). However, you do not need to update all values.
MERGE INTO requires that the update table is small. There are no requirements on the destination table size. If your workload does not satisfy this requirement, try using separate
Delta tables allow you to remove data that matches a predicate. For instance, to delete all events from before 2017, you can run the following DML:
DELETE FROM events WHERE date < '2017-01-01'
Delete operations automatically make use of the partitioning of the table when possible. This optimization means that it will be significantly faster to delete data based on partition predicates.
Delta automatically validates that the schema of the
DataFrame being written is compatible with the schema of the table. Columns that are present in the table but not in the DataFrame are set to null. If there are extra columns in the DataFrame that are not present in the table, this operation throws an exception. Delta has DDL to explicitly add new columns explicitly and the ability to update the schema automatically.
If you specify other options, such as
partitionBy, in combination with append mode, Delta 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.
This feature requires Databricks Runtime 4.1 or above.
Delta 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.
When you update a Delta table schema, streams that read from that table terminate. If you want the stream to continue you will need to restart it. For recommended methods, see Structured Streaming in Production.
You can use the following DDL to explicitly change the schema of a table.
ALTER TABLE table_name ADD COLUMNS (col_name data_type [COMMENT col_comment] [FIRST|AFTER colA_name], ...)
By default, nullability is
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], ...)
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
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]
If the schema before running
ALTER TABLE boxes CHANGE COLUMN colB.field2 field2 STRING FIRSTis:
- root | - colA | - colB | +-field1 | +-field2
the schema after is:
- root | - colA | - colB | +-field2 | +-field1
ALTER TABLE table_name REPLACE COLUMNS (col_name1 col_type1 [COMMENT col_comment1], ...)
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
Delta 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.
Columns that are present in the DataFrame but missing from the table are automatically added as part of a write transaction when either of the following 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.
mergeSchemais not supported when table access control is enabled (as it elevates a request that requires
MODIFYto one that requires
mergeSchemacannot be used with
Columns that are
NullType are dropped from the DataFrame when writing into Delta (because Parquet doesn’t support
NullType), but are still stored in the schema. When a different data type is received for that column, Delta merges the schema to the new data type. If we receive a
NullType for an existing column, we will keep the old schema, and drop the new column 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
By default, overwriting the data in a table does not overwrite the schema. When overwriting a table using
replaceWhere, you may still want to override the schema of the data being written. You can choose to replace the schema and partitioning of the table by setting:
Delta 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.
You can store your own metadata as a table property using
TBLPROPERTIES are stored as part of Delta’s 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, Databricks Delta supports certain Delta table properties:
- Block deletes and modifications of a table:
- 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.
- Configure the time travel retention properties:
These are the only
delta.-prefixed table properties that Delta lets you set.
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 = true.
Delta has rich features for exploring table metadata. It supports
DESC TABLE, and so on. It also provides the following unique commands:
Provides information about schema, partitioning, table size, and so on. For example, you can see the current reader and writer versions of a table:
Provides provenance information, including the operation, user, and so on, for each write to a table. This information is not recorded by versions of Databricks Runtime below 4.1 and tables created using these versions will show this information as
null. Table history is retained for 30 days.
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
For an example of the various Delta table metadata commands, see the end of the following notebook: