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Data skipping for Delta Lake

  • Artikkeli

Note

In Databricks Runtime 13.3 and above, Databricks recommends using liquid clustering for Delta table layout. Clustering is not compatible with Z-ordering. See Use liquid clustering for Delta tables.

Data skipping information is collected automatically when you write data into a Delta table. Delta Lake on Azure Databricks takes advantage of this information (minimum and maximum values, null counts, and total records per file) at query time to provide faster queries.

You must have statistics collected for columns that are used in ZORDER statements. See What is Z-ordering?.

Specify Delta statistics columns

By default, Delta Lake collects statistics on the first 32 columns defined in your table schema. When predictive optimization is enabled, file-skipping statistics are chosen intelligently and are not limited to the first 32 columns. Predictive optimization automatically runs ANALYZE, a command for collecting statistics, on Unity Catalog managed tables. Databricks recommends enabling predictive optimization for all Unity Catalog managed tables to simplify data maintenance and reduce storage costs. See Predictive optimization for Unity Catalog managed tables.

Important

Predictive optimization with ANALYZE is in Public Preview. It includes intelligent statistics collection during writes. Use this form to sign up for the Public Preview.

If you are not using predictive optimization, you can modify the behavior that limits statistics collections to 32 columns by setting one of the following table properties:

Table property Databricks Runtime supported Description
delta.dataSkippingNumIndexedCols All supported Databricks Runtime versions Increase or decrease the number of columns on which Delta collects statistics. Depends on column order.
delta.dataSkippingStatsColumns Databricks Runtime 13.3 LTS and above Specify a list of column names for which Delta Lake collects statistics. Supersedes dataSkippingNumIndexedCols.

Table properties can be set at table creation or with ALTER TABLE statements. See Delta table properties reference.

Updating these properties does not automatically recompute statistics for existing data. Rather, it impacts the behavior of future statistics collection when adding or updating data in the table. Delta Lake does not leverage statistics for columns not included in the current list of statistics columns.

In Databricks Runtime 14.3 LTS and above, if you have altered the table properties or changed the specified columns for statistics, you can manually trigger the recomputation of statistics for a Delta table using the following command:

ANALYZE TABLE table_name COMPUTE DELTA STATISTICS

Note

Long strings are truncated during statistics collection. You might choose to exclude long string columns from statistics collection, especially if the columns aren’t used frequently for filtering queries.

What is Z-ordering?

Note

Databricks recommends using liquid clustering for all new Delta tables. You cannot use ZORDER in combination with liquid clustering.

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

OPTIMIZE events
WHERE date >= current_timestamp() - INTERVAL 1 day
ZORDER BY (eventType)

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.

Note

  • Z-ordering is not idempotent but aims to be an incremental operation. The time it takes for Z-ordering is not guaranteed to reduce over multiple runs. However, if no new data was added to a partition that was just Z-ordered, another Z-ordering of that partition will not have any effect.

  • 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.