Optimizations

Delta Lake provides optimizations that accelerate data lake operations.

Optimize performance with file management

To improve query speed, Delta Lake supports the ability to optimize the layout of data in storage. There are various ways to optimize the layout.

Compaction (bin-packing)

Note

This feature is available in Delta Lake 1.2.0 and above.

Delta Lake can improve the speed of read queries from a table by coalescing small files into larger ones.

OPTIMIZE '/path/to/delta/table' -- Optimizes the path-based Delta Lake table

OPTIMIZE delta_table_name;

OPTIMIZE delta.`/path/to/delta/table`;

-- If you have a large amount of data and only want to optimize a subset of it, you can specify an optional partition predicate using `WHERE`:
OPTIMIZE delta_table_name WHERE date >= '2017-01-01'

from delta.tables import *

deltaTable = DeltaTable.forPath(spark, pathToTable)  # For path-based tables
# For Hive metastore-based tables: deltaTable = DeltaTable.forName(spark, tableName)

deltaTable.optimize().executeCompaction()

# If you have a large amount of data and only want to optimize a subset of it, you can specify an optional partition predicate using `where`
deltaTable.optimize().where("date='2021-11-18'").executeCompaction()

import io.delta.tables._

val deltaTable = DeltaTable.forPath(spark, pathToTable)  // For path-based tables
// For Hive metastore-based tables: val deltaTable = DeltaTable.forName(spark, tableName)

deltaTable.optimize().executeCompaction()

// If you have a large amount of data and only want to optimize a subset of it, you can specify an optional partition predicate using `where`
deltaTable.optimize().where("date='2021-11-18'").executeCompaction()

For Scala, Java, and Python API syntax details, see the Delta Lake APIs.

Note

  • Bin-packing optimization is idempotent, meaning that if it is run twice on the same dataset, the second run has no effect.

  • Bin-packing aims to produce evenly-balanced data files with respect to their size on disk, but not necessarily number of tuples per file. However, the two measures are most often correlated.

  • Python and Scala APIs for executing OPTIMIZE operation are available from Delta Lake 2.0 and above.

  • Set Spark session configuration spark.databricks.delta.optimize.repartition.enabled=true to use repartition(1) instead of coalesce(1) for better performance when compacting many small files.

Readers of Delta tables use snapshot isolation, which means that they are not interrupted when OPTIMIZE removes unnecessary files from the transaction log. OPTIMIZE makes no data related changes to the table, so a read before and after an OPTIMIZE has the same results. Performing OPTIMIZE on a table that is a streaming source does not affect any current or future streams that treat this table as a source. OPTIMIZE returns the file statistics (min, max, total, and so on) for the files removed and the files added by the operation. Optimize stats also contains the number of batches, and partitions optimized.

Data skipping

Note

This feature is available in Delta Lake 1.2.0 and above.

Data skipping information is collected automatically when you write data into a Delta Lake table. Delta Lake takes advantage of this information (minimum and maximum values for each column) at query time to provide faster queries. You do not need to configure data skipping; the feature is activated whenever applicable. However, its effectiveness depends on the layout of your data. For best results, apply Z-Ordering.

Collecting statistics on a column containing long values such as string or binary is an expensive operation. To avoid collecting statistics on such columns you can configure the table property delta.dataSkippingNumIndexedCols. This property indicates the position index of a column in the table’s schema. All columns with a position index less than the delta.dataSkippingNumIndexedCols property will have statistics collected. For the purposes of collecting statistics, each field within a nested column is considered as an individual column. To avoid collecting statistics on columns containing long values, either set the delta.dataSkippingNumIndexedCols property so that the long value columns are after this index in the table’s schema, or move columns containing long strings to an index position greater than the delta.dataSkippingNumIndexedCols property by using ALTER TABLE ALTER COLUMN.

Z-Ordering (multi-dimensional clustering)

Note

This feature is available in Delta Lake 2.0.0 and above.

Z-Ordering is a technique to colocate related information in the same set of files. This co-locality is automatically used by Delta Lake in data-skipping algorithms. This behavior dramatically reduces the amount of data that Delta Lake on Apache Spark needs to read. To Z-Order data, you specify the columns to order on in the ZORDER BY clause:

OPTIMIZE events ZORDER BY (eventType)

-- If you have a large amount of data and only want to optimize a subset of it, you can specify an optional partition predicate by using "where".
OPTIMIZE events WHERE date = '2021-11-18' ZORDER BY (eventType)

from delta.tables import *

deltaTable = DeltaTable.forPath(spark, pathToTable)  # path-based table
# For Hive metastore-based tables: deltaTable = DeltaTable.forName(spark, tableName)

deltaTable.optimize().executeZOrderBy(eventType)

# If you have a large amount of data and only want to optimize a subset of it, you can specify an optional partition predicate using `where`
deltaTable.optimize().where("date='2021-11-18'").executeZOrderBy(eventType)

import io.delta.tables._

val deltaTable = DeltaTable.forPath(spark, pathToTable)  // path-based table
// For Hive metastore-based tables: val deltaTable = DeltaTable.forName(spark, tableName)

deltaTable.optimize().executeZOrderBy(eventType)

// If you have a large amount of data and only want to optimize a subset of it, you can specify an optional partition predicate by using "where".
deltaTable.optimize().where("date='2021-11-18'").executeZOrderBy(eventType)

For Scala, Java, and Python API syntax details, see the Delta Lake APIs

If you expect a column to be commonly used in query predicates and if that column has high cardinality (that is, a large number of distinct values), then use ZORDER BY.

You can specify multiple columns for ZORDER BY as a comma-separated list. However, the effectiveness of the locality drops with each extra column. Z-Ordering on columns that do not have statistics collected on them would be ineffective and a waste of resources. This is because data skipping requires column-local stats such as min, max, and count. You can configure statistics collection on certain columns by reordering columns in the schema, or you can increase the number of columns to collect statistics on. See Data skipping.

Note

  • Z-Ordering is not idempotent. Everytime the Z-Ordering is executed it will try to create a new clustering of data in all files (new and existing files that were part of previous Z-Ordering) in a partition.

  • Z-Ordering aims to produce evenly-balanced data files with respect to the number of tuples, but not necessarily data size on disk. The two measures are most often correlated, but there can be situations when that is not the case, leading to skew in optimize task times.

    For example, if you ZORDER BY date and your most recent records are all much wider (for example longer arrays or string values) than the ones in the past, it is expected that the OPTIMIZE job’s task durations will be skewed, as well as the resulting file sizes. This is, however, only a problem for the OPTIMIZE command itself; it should not have any negative impact on subsequent queries.

Multi-part checkpointing

Note

This feature is available in Delta Lake 2.0.0 and above. This feature is in experimental support mode.

Delta Lake table periodically and automatically compacts all the incremental updates to the Delta log into a Parquet file. This “checkpointing” allows read queries to quickly reconstruct the current state of the table (that is, which files to process, what is the current schema) without reading too many files having incremental updates.

Delta Lake protocol allows splitting the checkpoint into multiple Parquet files. This parallelizes and speeds up writing the checkpoint. In Delta Lake, by default each checkpoint is written as a single Parquet file. To to use this feature, set the SQL configuration spark.databricks.delta.checkpoint.partSize=<n>, where n is the limit of number of actions (such as AddFile) at which Delta Lake on Apache Spark will start parallelizing the checkpoint and attempt to write a maximum of this many actions per checkpoint file.

Note

This feature requires no reader side configuration changes. The existing reader already supports reading a checkpoint with multiple files.