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Rename and drop columns with Delta Lake column mapping

  • Artikkeli

Azure Databricks supports column mapping for Delta Lake tables, which enables metadata-only changes to mark columns as deleted or renamed without rewriting data files. It also allows users to name Delta table columns using characters that are not allowed by Parquet, such as spaces, so that users can directly ingest CSV or JSON data into Delta without the need to rename columns due to previous character constraints.

Important

Tables with column mapping enabled can only be read in Databricks Runtime 10.4 LTS and above.

Enabling column mapping on tables might break downstream operations that rely on Delta change data feed. See Change data feed limitations for tables with column mapping enabled.

Enabling column mapping on tables might break streaming reads from the Delta table as a source, including in Delta Live Tables. See Streaming with column mapping and schema changes.

Enable column mapping

Column mapping requires the following Delta protocols:

  • Reader version 2 or above.
  • Writer version 5 or above.

For a Delta table with the required protocol versions, you can enable column mapping by setting delta.columnMapping.mode to name.

You can use the following command to enable column mapping:

  ALTER TABLE <table-name> SET TBLPROPERTIES (
    'delta.columnMapping.mode' = 'name'
  )

See Disable column mapping.

Rename a column

Note

Available in Databricks Runtime 10.4 LTS and above.

When column mapping is enabled for a Delta table, you can rename a column:

ALTER TABLE <table-name> RENAME COLUMN old_col_name TO new_col_name

For more examples, see Update Delta Lake table schema.

Drop columns

Note

Available in Databricks Runtime 11.3 LTS and above.

When column mapping is enabled for a Delta table, you can drop one or more columns:

ALTER TABLE table_name DROP COLUMN col_name
ALTER TABLE table_name DROP COLUMNS (col_name_1, col_name_2, ...)

For more details, see Update Delta Lake table schema.

Supported characters in column names

When column mapping is enabled for a Delta table, you can include spaces and any of these characters in the table’s column names: ,;{}()\n\t=.

Streaming with column mapping and schema changes

Important

This feature is in Public Preview in Databricks Runtime 13.3 LTS and above.

You can provide a schema tracking location to enable streaming from Delta tables with column mapping enabled. This overcomes an issue in which non-additive schema changes could result in broken streams.

Each streaming read against a data source must have its own schemaTrackingLocation specified. The specified schemaTrackingLocation must be contained within the directory specified for the checkpointLocation of the target table for streaming write.

Note

For streaming workloads that combine data from multiple source Delta tables, you need to specify unique directories within the checkpointLocation for each source table.

The option schemaTrackingLocation is used to specify the path for schema tracking, as shown in the following code example:

checkpoint_path = "/path/to/checkpointLocation"

(spark.readStream
  .option("schemaTrackingLocation", checkpoint_path)
  .table("delta_source_table")
  .writeStream
  .option("checkpointLocation", checkpoint_path)
  .toTable("output_table")
)

Disable column mapping

In Databricks Runtime 15.3 and above, you can use the DROP FEATURE command to remove column mapping from a table and downgrade the table protocol.

See Drop Delta table features.