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Delta Lake

Table deletes, updates, and merges

Delta Lake supports several statements to facilitate deleting data from and updating data in Delta tables.

Delete from a table

Section titled “Delete from a table”

You can remove data that matches a predicate from a Delta table. For instance, in a table named people10m or a path at /tmp/delta/people-10m, to delete all rows corresponding to people with a value in the birthDate column from before 1955, you can run the following:

DELETE FROM people10m WHERE birthDate < '1955-01-01'


DELETE FROM delta.`/tmp/delta/people-10m` WHERE birthDate < '1955-01-01'

See Configure SparkSession for the steps to enable support for SQL commands.

from delta.tables import *
from pyspark.sql.functions import *


deltaTable = DeltaTable.forPath(spark, '/tmp/delta/people-10m')


# Declare the predicate by using a SQL-formatted string.
deltaTable.delete("birthDate < '1955-01-01'")


# Declare the predicate by using Spark SQL functions.
deltaTable.delete(col('birthDate') < '1960-01-01')

See the Delta Lake APIs for details.

Update a table

Section titled “Update a table”

You can update data that matches a predicate in a Delta table. For example, in a table named people10m or a path at /tmp/delta/people-10m, to change an abbreviation in the gender column from M or F to Male or Female, you can run the following:

UPDATE people10m SET gender = 'Female' WHERE gender = 'F';
UPDATE people10m SET gender = 'Male' WHERE gender = 'M';


UPDATE delta.`/tmp/delta/people-10m` SET gender = 'Female' WHERE gender = 'F';
UPDATE delta.`/tmp/delta/people-10m` SET gender = 'Male' WHERE gender = 'M';

See Configure SparkSession for the steps to enable support for SQL commands.

from delta.tables import *
from pyspark.sql.functions import *


deltaTable = DeltaTable.forPath(spark, '/tmp/delta/people-10m')


# Declare the predicate by using a SQL-formatted string.
deltaTable.update(
  condition = "gender = 'F'",
  set = { "gender": "'Female'" }
)


# Declare the predicate by using Spark SQL functions.
deltaTable.update(
  condition = col('gender') == 'M',
  set = { 'gender': lit('Male') }
)

See the Delta Lake APIs for details.

Upsert into a table using merge

Section titled “Upsert into a table using merge”

You can upsert data from a source table, view, or DataFrame into a target Delta table by using the MERGE SQL operation. Delta Lake supports inserts, updates and deletes in MERGE, and it supports extended syntax beyond the SQL standards to facilitate advanced use cases.

Suppose you have a source table named people10mupdates or a source path at /tmp/delta/people-10m-updates that contains new data for a target table named people10m or a target path at /tmp/delta/people-10m. Some of these new records may already be present in the target data. To merge the new data, you want to update rows where the person’s id is already present and insert the new rows where no matching id is present. You can run the following:

MERGE INTO people10m
USING people10mupdates
ON people10m.id = people10mupdates.id
WHEN MATCHED THEN
  UPDATE SET
    id = people10mupdates.id,
    firstName = people10mupdates.firstName,
    middleName = people10mupdates.middleName,
    lastName = people10mupdates.lastName,
    gender = people10mupdates.gender,
    birthDate = people10mupdates.birthDate,
    ssn = people10mupdates.ssn,
    salary = people10mupdates.salary
WHEN NOT MATCHED
  THEN INSERT (
    id,
    firstName,
    middleName,
    lastName,
    gender,
    birthDate,
    ssn,
    salary
  )
  VALUES (
    people10mupdates.id,
    people10mupdates.firstName,
    people10mupdates.middleName,
    people10mupdates.lastName,
    people10mupdates.gender,
    people10mupdates.birthDate,
    people10mupdates.ssn,
    people10mupdates.salary
  )

See Configure SparkSession for the steps to enable support for SQL commands.

from delta.tables import *


deltaTablePeople = DeltaTable.forPath(spark, '/tmp/delta/people-10m')
deltaTablePeopleUpdates = DeltaTable.forPath(spark, '/tmp/delta/people-10m-updates')


dfUpdates = deltaTablePeopleUpdates.toDF()


deltaTablePeople.alias('people') \
  .merge(
    dfUpdates.alias('updates'),
    'people.id = updates.id'
  ) \
  .whenMatchedUpdate(set =
    {
      "id": "updates.id",
      "firstName": "updates.firstName",
      "middleName": "updates.middleName",
      "lastName": "updates.lastName",
      "gender": "updates.gender",
      "birthDate": "updates.birthDate",
      "ssn": "updates.ssn",
      "salary": "updates.salary"
    }
  ) \
  .whenNotMatchedInsert(values =
    {
      "id": "updates.id",
      "firstName": "updates.firstName",
      "middleName": "updates.middleName",
      "lastName": "updates.lastName",
      "gender": "updates.gender",
      "birthDate": "updates.birthDate",
      "ssn": "updates.ssn",
      "salary": "updates.salary"
    }
  ) \
  .execute()

See the Delta Lake APIs for Scala, Java, and Python syntax details.

Modify all unmatched rows using merge

Section titled “Modify all unmatched rows using merge”

You can use the WHEN NOT MATCHED BY SOURCE clause to UPDATE or DELETE records in the target table that do not have corresponding records in the source table. We recommend adding an optional conditional clause to avoid fully rewriting the target table.

The following code example shows the basic syntax of using this for deletes, overwriting the target table with the contents of the source table and deleting unmatched records in the target table.

MERGE INTO target
USING source
ON source.key = target.key
WHEN MATCHED
  UPDATE SET *
WHEN NOT MATCHED
  INSERT *
WHEN NOT MATCHED BY SOURCE
  DELETE

The following example adds conditions to the WHEN NOT MATCHED BY SOURCE clause and specifies values to update in unmatched target rows.

MERGE INTO target
USING source
ON source.key = target.key
WHEN MATCHED THEN
  UPDATE SET target.lastSeen = source.timestamp
WHEN NOT MATCHED THEN
  INSERT (key, lastSeen, status) VALUES (source.key,  source.timestamp, 'active')
WHEN NOT MATCHED BY SOURCE AND target.lastSeen >= (current_date() - INTERVAL '5' DAY) THEN
  UPDATE SET target.status = 'inactive'

Operation semantics

Section titled “Operation semantics”

Here is a detailed description of the merge programmatic operation.

  • There can be any number of whenMatched and whenNotMatched clauses.

  • whenMatched clauses are executed when a source row matches a target table row based on the match condition. These clauses have the following semantics.

    • whenMatched clauses can have at most one update and one delete action. The update action in merge only updates the specified columns (similar to the update operation) of the matched target row. The delete action deletes the matched row.

    • Each whenMatched clause can have an optional condition. If this clause condition exists, the update or delete action is executed for any matching source-target row pair only when the clause condition is true.

    • If there are multiple whenMatched clauses, then they are evaluated in the order they are specified. All whenMatched clauses, except the last one, must have conditions.

    • If none of the whenMatched conditions evaluate to true for a source and target row pair that matches the merge condition, then the target row is left unchanged.

    • To update all the columns of the target Delta table with the corresponding columns of the source dataset, use whenMatched(...).updateAll(). This is equivalent to:

      whenMatched(...).updateExpr(Map("col1" -> "source.col1", "col2" -> "source.col2", ...))

      for all the columns of the target Delta table. Therefore, this action assumes that the source table has the same columns as those in the target table, otherwise the query throws an analysis error.

  • whenNotMatched clauses are executed when a source row does not match any target row based on the match condition. These clauses have the following semantics.

    • whenNotMatched clauses can have only the insert action. The new row is generated based on the specified column and corresponding expressions. You do not need to specify all the columns in the target table. For unspecified target columns, NULL is inserted.

    • Each whenNotMatched clause can have an optional condition. If the clause condition is present, a source row is inserted only if that condition is true for that row. Otherwise, the source column is ignored.

    • If there are multiple whenNotMatched clauses, then they are evaluated in the order they are specified. All whenNotMatched clauses, except the last one, must have conditions.

    • To insert all the columns of the target Delta table with the corresponding columns of the source dataset, use whenNotMatched(...).insertAll(). This is equivalent to:

      whenNotMatched(...).insertExpr(Map("col1" -> "source.col1", "col2" -> "source.col2", ...))

      for all the columns of the target Delta table. Therefore, this action assumes that the source table has the same columns as those in the target table, otherwise the query throws an analysis error.

  • whenNotMatchedBySource clauses are executed when a target row does not match any source row based on the merge condition. These clauses have the following semantics.

    • whenNotMatchedBySource clauses can specify delete and update actions.
    • Each whenNotMatchedBySource clause can have an optional condition. If the clause condition is present, a target row is modified only if that condition is true for that row. Otherwise, the target row is left unchanged.
    • If there are multiple whenNotMatchedBySource clauses, then they are evaluated in the order they are specified. All whenNotMatchedBySource clauses, except the last one, must have conditions.
    • By definition, whenNotMatchedBySource clauses do not have a source row to pull column values from, and so source columns can’t be referenced. For each column to be modified, you can either specify a literal or perform an action on the target column, such as SET target.deleted_count = target.deleted_count + 1.

Schema validation

Section titled “Schema validation”

merge automatically validates that the schema of the data generated by insert and update expressions are compatible with the schema of the table. It uses the following rules to determine whether the merge operation is compatible:

  • For update and insert actions, the specified target columns must exist in the target Delta table.
  • For updateAll and insertAll actions, the source dataset must have all the columns of the target Delta table. The source dataset can have extra columns and they are ignored.

If you do not want the extra columns to be ignored and instead want to update the target table schema to include new columns, see Automatic schema evolution.

  • For all actions, if the data type generated by the expressions producing the target columns are different from the corresponding columns in the target Delta table, merge tries to cast them to the types in the table.

Automatic schema evolution

Section titled “Automatic schema evolution”

Schema evolution allows users to resolve schema mismatches between the target and source table in merge. It handles the following two cases:

  1. A column in the source table is not present in the target table. The new column is added to the target schema, and its values are inserted or updated using the source values.
  2. A column in the target table is not present in the source table. The target schema is left unchanged; the values in the additional target column are either left unchanged (for UPDATE) or set to NULL (for INSERT).

Here are a few examples of the effects of merge operation with and without schema evolution.

ColumnsQuery (in SQL)Behavior without schema evolution (default)Behavior with schema evolution
Target: key, value Source: key, value, new_valuesql MERGE INTO target_table t USING source_table s ON t.key = s.key WHEN MATCHED THEN UPDATE SET * WHEN NOT MATCHED THEN INSERT *The table schema remains unchanged; only columns key, value are updated/inserted.The table schema is changed to (key, value, new_value). Existing records with matches are updated with the value and new_value in the source. New rows are inserted with the schema (key, value, new_value).
Target: key, old_value Source: key, new_valuesql MERGE INTO target_table t USING source_table s ON t.key = s.key WHEN MATCHED THEN UPDATE SET * WHEN NOT MATCHED THEN INSERT *UPDATE and INSERT actions throw an error because the target column old_value is not in the source.The table schema is changed to (key, old_value, new_value). Existing records with matches are updated with the new_value in the source leaving old_value unchanged. New records are inserted with the specified key, new_value, and NULL for the old_value.
Target: key, old_value Source: key, new_valuesql MERGE INTO target_table t USING source_table s ON t.key = s.key WHEN MATCHED THEN UPDATE SET new_value = s.new_valueUPDATE throws an error because column new_value does not exist in the target table.The table schema is changed to (key, old_value, new_value). Existing records with matches are updated with the new_value in the source leaving old_value unchanged, and unmatched records have NULL entered for new_value.
Target: key, old_value Source: key, new_valuesql MERGE INTO target_table t USING source_table s ON t.key = s.key WHEN NOT MATCHED THEN INSERT (key, new_value) VALUES (s.key, s.new_value)INSERTthrows an error because columnnew_valuedoes not exist in the target table.The table schema is changed to(key, old_value, new_value). New records are inserted with the specified key, new_value, and NULLfor theold_value. Existing records have NULLentered fornew_valueleavingold_value unchanged. See note (1).

Special considerations for schemas that contain arrays of structs

Section titled “Special considerations for schemas that contain arrays of structs”

Delta MERGE INTO supports resolving struct fields by name and evolving schemas for arrays of structs. With schema evolution enabled, target table schemas will evolve for arrays of structs, which also works with any nested structs inside of arrays.

Here are a few examples of the effects of merge operations with and without schema evolution for arrays of structs.

Source schemaTarget schemaBehavior without schema evolution (default)Behavior with schema evolution
array<struct<b: string, a: string>>array<struct<a: int, b: int>>The table schema remains unchanged. Columns will be resolved by name and updated or inserted.The table schema remains unchanged. Columns will be resolved by name and updated or inserted.
array<struct<a: int, c: string, d: string>>array<struct<a: string, b: string>>update and insert throw errors because c and d do not exist in the target table.The table schema is changed to array<struct<a: string, b: string, c: string, d: string>>. c and d are inserted as NULL for existing entries in the target table. update and insert fill entries in the source table with a casted to string and b as NULL.
array<struct<a: string, b: struct<c: string, d: string>>>array<struct<a: string, b: struct<c: string>>>update and insert throw errors because d does not exist in the target table.The target table schema is changed to array<struct<a: string, b: struct<c: string, d: string>>>. d is inserted as NULL for existing entries in the target table.

Performance tuning

Section titled “Performance tuning”

You can reduce the time taken by merge using the following approaches:

  • Reduce the search space for matches: By default, the merge operation searches the entire Delta table to find matches in the source table. One way to speed up merge is to reduce the search space by adding known constraints in the match condition. For example, suppose you have a table that is partitioned by country and date and you want to use merge to update information for the last day and a specific country. Adding the condition

    events.date = current_date() AND events.country = 'USA'

    will make the query faster as it looks for matches only in the relevant partitions. Furthermore, it will also reduce the chances of conflicts with other concurrent operations. See Concurrency control for more details.

  • Compact files: If the data is stored in many small files, reading the data to search for matches can become slow. You can compact small files into larger files to improve read throughput. See Compact files for details.

  • Control the shuffle partitions for writes: The merge operation shuffles data multiple times to compute and write the updated data. The number of tasks used to shuffle is controlled by the Spark session configuration spark.sql.shuffle.partitions. Setting this parameter not only controls the parallelism but also determines the number of output files. Increasing the value increases parallelism but also generates a larger number of smaller data files.

  • Repartition output data before write: For partitioned tables, merge can produce a much larger number of small files than the number of shuffle partitions. This is because every shuffle task can write multiple files in multiple partitions, and can become a performance bottleneck. In many cases, it helps to repartition the output data by the table’s partition columns before writing it. You enable this by setting the Spark session configuration spark.databricks.delta.merge.repartitionBeforeWrite.enabled to true.

Merge examples

Section titled “Merge examples”

Here are a few examples on how to use merge in different scenarios.

Data deduplication when writing into Delta tables

Section titled “Data deduplication when writing into Delta tables”

A common ETL use case is to collect logs into Delta table by appending them to a table. However, often the sources can generate duplicate log records and downstream deduplication steps are needed to take care of them. With merge, you can avoid inserting the duplicate records.

MERGE INTO logs
USING newDedupedLogs
ON logs.uniqueId = newDedupedLogs.uniqueId AND logs.date > current_date() - INTERVAL 7 DAYS
WHEN NOT MATCHED AND newDedupedLogs.date > current_date() - INTERVAL 7 DAYS
  THEN INSERT *

If you know that you may get duplicate records only for a few days, you can optimized your query further by partitioning the table by date, and then specifying the date range of the target table to match on.

MERGE INTO logs
USING newDedupedLogs
ON logs.uniqueId = newDedupedLogs.uniqueId AND logs.date > current_date() - INTERVAL 7 DAYS
WHEN NOT MATCHED AND newDedupedLogs.date > current_date() - INTERVAL 7 DAYS
  THEN INSERT *

This is more efficient than the previous command as it looks for duplicates only in the last 7 days of logs, not the entire table. Furthermore, you can use this insert-only merge with Structured Streaming to perform continuous deduplication of the logs.

  • In a streaming query, you can use merge operation in foreachBatch to continuously write any streaming data to a Delta table with deduplication. See the following streaming example for more information on foreachBatch.
  • In another streaming query, you can continuously read deduplicated data from this Delta table. This is possible because an insert-only merge only appends new data to the Delta table.

Slowly changing data (SCD) Type 2 operation into Delta tables

Section titled “Slowly changing data (SCD) Type 2 operation into Delta tables”

Another common operation is SCD Type 2, which maintains history of all changes made to each key in a dimensional table. Such operations require updating existing rows to mark previous values of keys as old, and the inserting the new rows as the latest values. Given a source table with updates and the target table with the dimensional data, SCD Type 2 can be expressed with merge.

Here is a concrete example of maintaining the history of addresses for a customer along with the active date range of each address. When a customer’s address needs to be updated, you have to mark the previous address as not the current one, update its active date range, and add the new address as the current one.

customersTable = ...  # DeltaTable with schema (customerId, address, current, effectiveDate, endDate)


updatesDF = ...       # DataFrame with schema (customerId, address, effectiveDate)


# Rows to INSERT new addresses of existing customers
newAddressesToInsert = updatesDF \
  .alias("updates") \
  .join(customersTable.toDF().alias("customers"), "customerid") \
  .where("customers.current = true AND updates.address <> customers.address")


# Stage the update by unioning two sets of rows
# 1. Rows that will be inserted in the whenNotMatched clause
# 2. Rows that will either update the current addresses of existing customers or insert the new addresses of new customers
stagedUpdates = (
  newAddressesToInsert
  .selectExpr("NULL as mergeKey", "updates.*")   # Rows for 1
  .union(updatesDF.selectExpr("updates.customerId as mergeKey", "*"))  # Rows for 2.
)


# Apply SCD Type 2 operation using merge
customersTable.alias("customers").merge(
  stagedUpdates.alias("staged_updates"),
  "customers.customerId = mergeKey") \
.whenMatchedUpdate(
  condition = "customers.current = true AND customers.address <> staged_updates.address",
  set = {                                      # Set current to false and endDate to source's effective date.
    "current": "false",
    "endDate": "staged_updates.effectiveDate"
  }
).whenNotMatchedInsert(
  values = {
    "customerid": "staged_updates.customerId",
    "address": "staged_updates.address",
    "current": "true",
    "effectiveDate": "staged_updates.effectiveDate",  # Set current to true along with the new address and its effective date.
    "endDate": "null"
  }
).execute()

Write change data into a Delta table

Section titled “Write change data into a Delta table”

Similar to SCD, another common use case, often called change data capture (CDC), is to apply all data changes generated from an external database into a Delta table. In other words, a set of updates, deletes, and inserts applied to an external table needs to be applied to a Delta table. You can do this using merge as follows.

deltaTable = ... # DeltaTable with schema (key, value)


# DataFrame with changes having following columns
# - key: key of the change
# - time: time of change for ordering between changes (can replaced by other ordering id)
# - newValue: updated or inserted value if key was not deleted
# - deleted: true if the key was deleted, false if the key was inserted or updated
changesDF = spark.table("changes")


# Find the latest change for each key based on the timestamp
# Note: For nested structs, max on struct is computed as
# max on first struct field, if equal fall back to second fields, and so on.
latestChangeForEachKey = changesDF \
  .selectExpr("key", "struct(time, newValue, deleted) as otherCols") \
  .groupBy("key") \
  .agg(max("otherCols").alias("latest")) \
  .select("key", "latest.*") \


deltaTable.alias("t").merge(
    latestChangeForEachKey.alias("s"),
    "s.key = t.key") \
  .whenMatchedDelete(condition = "s.deleted = true") \
  .whenMatchedUpdate(set = {
    "key": "s.key",
    "value": "s.newValue"
  }) \
  .whenNotMatchedInsert(
    condition = "s.deleted = false",
    values = {
      "key": "s.key",
      "value": "s.newValue"
    }
  ).execute()

Upsert from streaming queries using foreachBatch

Section titled “Upsert from streaming queries using foreachBatch”

You can use a combination of merge and foreachBatch (see foreachbatch for more information) to write complex upserts from a streaming query into a Delta table. For example:

  • Write streaming aggregates in Update Mode: This is much more efficient than Complete Mode.

    from delta.tables import *
    

    deltaTable = DeltaTable.forPath(spark, "/data/aggregates")
    

    # Function to upsert microBatchOutputDF into Delta table using merge
    def upsertToDelta(microBatchOutputDF, batchId):
      deltaTable.alias("t").merge(
          microBatchOutputDF.alias("s"),
          "s.key = t.key") \
        .whenMatchedUpdateAll() \
        .whenNotMatchedInsertAll() \
        .execute()
    }
    

    # Write the output of a streaming aggregation query into Delta table
    streamingAggregatesDF.writeStream \
      .format("delta") \
      .foreachBatch(upsertToDelta) \
      .outputMode("update") \
      .start()

  • Write a stream of database changes into a Delta table: The merge query for writing change data can be used in foreachBatch to continuously apply a stream of changes to a Delta table.

  • Write a stream data into Delta table with deduplication: The insert-only merge query for deduplication can be used in foreachBatch to continuously write data (with duplicates) to a Delta table with automatic deduplication.