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
(currently not supported for columnar tables).
- 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 ornull
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.