ydb/yql/essentials/docs/en/syntax/select/unique_distinct_hints.md

UNIQUE DISTINCT hints

Directly after SELECT, it is possible to add SQL hints unique or distinct, which declare that this projection generates data containing unique values in the specified set of columns of a row-based or columnar table. This can be used to optimize subsequent subqueries executed on this projection, or for writing to table meta-attributes during INSERT.

• Columns are specified in the hint values, separated by spaces.
• If the set of columns is not specified, uniqueness applies to the entire set of columns in this projection.
• unique - indicates unique or null values. According to the SQL standard, each null is unique: NULL = NULL -> NULL.
• distinct - indicates completely unique values including null: NULL IS DISTINCT FROM NULL -> FALSE.
• Multiple sets of columns can be specified in several hints for a single projection.
• If the hint contains a column that is not in the projection, it will be ignored.