Section 2: 12.Advanced Delta Lake Features

Time Travel

• Allows auditing of data changes for improved transparency and tracking.
• Use the DESCRIBE HISTORY command to view historical changes in a table.
• Query older versions of data using a timestamp:
  • Example: SELECT * FROM my_table TIMESTAMP AS OF "2019-01-01"
• Query specific versions using version numbers:
  • Example: SELECT * FROM my_table VERSION AS OF 36
  • Alternative syntax: SELECT * FROM my_table@v36
• Rollback to previous versions using the RESTORE TABLE command:
  • Restore to a specific timestamp: RESTORE TABLE my_table TO TIMESTAMP AS OF "2019-01-01"
  • Restore to a specific version: RESTORE TABLE my_table TO VERSION AS OF 36

Compaction

• Involves reducing the number of small files to enhance performance.
• Utilize the OPTIMIZE command for compacting:
  • Example: OPTIMIZE my_table

Indexing

• Improves query performance by co-locating column information.
• Use the OPTIMIZE command:
  • Example: OPTIMIZE my_table ZORDER BY column_name
• ZORDER BY organizes data in a way that optimizes retrieval based on a specific column.

Vacuum a Delta Table

• Cleans up unused data files, including uncommitted files and those no longer in the latest state of the table.
• The VACUUM command is used to remove these files:
  • Syntax: VACUUM table_name [retention period]
  • Default retention period is 7 days, after which files can be permanently removed.
• Note that executing a vacuum operation disables time travel for the cleaned-up files.

Time Travel Capabilities

• Delta Lake automatically versioned tables provide a full audit trail of changes.
• Use the DESCRIBE HISTORY command in SQL to view table history.
• Query older table versions using timestamps with TIMESTAMP AS OF or version numbers with VERSION AS OF or @v.
• Rollbacks are facilitated by the RESTORE TABLE command to a specific timestamp or version in case of errors.

Compacting Small Files

• Compaction of small files into larger ones enhances read query performance in Delta Lake.
• Trigger compaction using the OPTIMIZE command to consolidate many small files.
• Z-Order indexing can be applied during compaction with ZORDER BY, optimizing data organization within files.

Z-Order Indexing

• Z-Order indexing co-locates and rearranges column information, leading to efficient data skipping.
• For example, if applying Z Order on a numerical column like ID, values are organized into files by ranges, allowing Delta to quickly locate the necessary file during queries.

Cleaning Up Unused Data Files

• Delta Lake facilitates garbage collection of unused files with the VACUUM command.
• Specify a retention period threshold, with a default of 7 days, to delete files older than this threshold.
• Be cautious: vacuuming removes the ability to time travel to versions older than the retention period, as associated data files are deleted.

Explore the advanced features of Delta Lake in this quiz, including time travel, compacting small files, indexing, and vacuuming. Learn how to audit data changes and query older versions of your data using timestamps. This is a crucial part of the Databricks Certified Data Engineer Associate preparation.