openLooKeng supports statistics based optimizations for queries. For a query to take advantage of these optimizations, openLooKeng must have statistical information for the tables in that query.
Table statistics are provided to the query planner by connectors. Currently, the only connector that supports statistics is the hive connector.
Statistics are exposed to the query planner by a table layout. A table layout represents a subset of a table’s data and contains information about the organizational properties of that data (like sort order and bucketing).
The number of table layouts available for a table and the details of those table layouts are specific to each connector. Using the Hive connector as an example:
- Non-partitioned tables have just one table layout representing all data in the table
- Partitioned tables have a family of table layouts. Each set of partitions to be scanned represents one table layout. openLooKeng will try to pick a table layout consisting of the smallest number of partitions based on filtering predicates from the query.
The following statistics are available in openLooKeng:
- For a table:
- row count: the total number of rows in the table layout
- For each column in a table:
- data size: the size of the data that needs to be read
- nulls fraction: the fraction of null values
- distinct value count: the number of distinct values
- low value: the smallest value in the column
- high value: the largest value in the column
The set of statistics available for a particular query depends on the connector being used and can also vary by table or even by table layout. For example, the Hive connector does not currently provide statistics on data size.
Table statistics can be displayed via the openLooKeng SQL interface using the SHOW STATS command. For the Hive connector, refer to the Hive Connector documentation to learn how to update table statistics.