Creating and Querying Wide Tables
When you create wide tables in Dremio from data in connected data sources and then make them available for queries, keep in mind these limits and best practices.
As of v23.0, Dremio has established two limits on the use of wide tables. These limits are for preventing queries from using too many resources in Dremio clusters. There are two limits:
The first is on the number of leaf columns that a table can include.
The second is on the number of leaf columns that a query can scan in a single table.
The term leaf columns refers to all columns in a table and all leaf-level fields that are used in complex data types in the table.
Maximum Number of Leaf Columns in a Table
Tables in Dremio are allowed a maximum of 6,400 leaf columns. For example, if a table has 1,000 columns and the data type of one column is a STRUCT that has three fields, the count of leaf columns is 1,003.
In previous releases, this maximum was 800, though Dremio made that configurable with the support key
store.plugin.max_metadata_leaf_columns. If you used this support key and have upgraded to v23.0, reset the key so that you can use the maximum of 6,400.
Maximum Number of Leaf Columns Scanned Per Table in a Query
The maximum number of leaf columns that a query is allowed to scan per table or per reflection is 800. (In earlier releases, this limit did not exist.) If a query attempts to scan more than this limit, then you receive this error message:
At most, <limit> columns including leaf level fields of complex type are allowed to be scanned, but the query is scanning <number>. Please include the columns you want to be returned from the query and try again.
If you are trying to use a
SELECT * query to view all columns in a table, you can avoid scanning too many columns by using the
DESCRIBE TABLE command, which returns the name and other details about all columns in the table.
A query is allowed to scan at most <limit> columns, including leaf-level fields in complex data types. However, the query is scanning <number>. Reduce the number of columns scanned by the query and try to run it again.
Best Practices for Querying Wide Tables
Dremio recommends these two best practices:
Create views for querying subsets of the leaf columns in wide tables.
Users who run ad hoc queries directly against wide tables might inadvertently create queries that need to scan more than 800 columns per table. To help users avoid failed queries, create views on subsets of the leaf columns in the wide table and give users access to those views.
Create reflections to satisfy queries on wide tables.
It is best to restrict the leaf columns included in reflections to those that are most frequently queried. Doing so conserves the resources that are required for storing and refreshing reflections.