_delta_log is a hidden directory within a Delta table that contains metadata about the table, including a log of all the transactions that have been performed on the table. Each time a transaction is performed on the table, such as an insert, update, or delete, a new entry is added to the _delta_log directory.
The _delta_log directory is used by Delta Lake to maintain the consistency and integrity of the data in the table. It ensures that all transactions are ACID-compliant and that data is versioned correctly. By storing this information in the _delta_log directory, Delta Lake is able to provide features such as time travel queries, which allow you to query the table as it existed at a specific point in time.
Uses of _delta_log
The _delta_log directory in a Delta table has several important uses in Delta Lake. Here are some of the key uses:
- Transaction history: The _delta_log directory maintains a log of all transactions that have been performed on the table. This transaction history is critical for ensuring data consistency, ACID compliance, and data versioning.
- Data versioning: Because the _delta_log directory maintains a record of all transactions, it enables Delta Lake to support data versioning. This means that you can access and query the table as it existed at any point in the past.
- Disaster recovery: The transaction log maintained in the _delta_log directory can also be used for disaster recovery. If your Delta table becomes corrupted or deleted, you can use the transaction log to recover the table to a previous state.
- Time travel queries: Delta Lake allows you to run time travel queries on Delta tables, which means you can query the table as it existed at a specific point in time. This functionality is made possible by the transaction history stored in the _delta_log directory.
- Optimization: The _delta_log directory also plays a role in optimizing Delta Lake’s performance. It contains information that Delta Lake uses to optimize query performance and improve the efficiency of data reads and writes.
DeltaTable and DeltaLog
DeltaTable: A DeltaTable is a table in Delta Lake that is designed for handling large-scale, structured, and semi-structured data. DeltaTables provide several key features, including ACID transactions, scalable metadata handling, and data versioning. DeltaTables also support a variety of data formats, including Parquet, ORC, and JSON.
DeltaLog: The DeltaLog is a hidden directory within a DeltaTable that contains metadata about the table, including a log of all the transactions that have been performed on the table. Each time a transaction is performed on the table, such as an insert, update, or delete, a new entry is added to the DeltaLog directory. The DeltaLog is used by Delta Lake to maintain the consistency and integrity of the data in the table, ensure that all transactions are ACID-compliant, and support features such as time travel queries.