DISTINCT removal in aggregate functions

Basics

One can use DISTINCT keyword to de-duplicate the arguments of an aggregate function. For example:

SELECT COUNT(DISTINCT col1) FROM tbl1;

In order to compute this, the optimizer has to de-duplicate the function arguments. This may be computationally expensive.

After fix for MDEV-30660 (available from MariaDB 10.5.25, 10.6.18, 10.11.8, 11.0.6, 11.1.5, 11.2.4, 11.4.2) , the optimizer is able to detect that set of aggregate function arguments is already distinct and so no de-duplication is necessary.

When one can skip de-duplication

The most basic example: if we're doing a select from one table, then the values of Attachment 'primary_key' not found are already distinct:

SELECT aggregate_func(DISTINCT tbl.primary_key, ...) FROM tbl;
<<sql>>

If the SELECT has other constant tables, that's also ok, as they will not create duplicates.

The next step: a part of the primary key can be "bound" by the GROUP BY clause.  Consider a query:
<<sql>>
SELECT aggregate_func(DISTINCT t1.pk1, ...) FROM t1 GROUP BY t1.pk2;

Suppose the table has PRIMARY KEY(pk1, pk2). GROUP BY pk2 fixes the value of pk2 within each. Then, the values of pk1 must be unique within each group, and de-duplication is not necessary.

Observability

EXPLAIN or EXPLAIN FORMAT=JSON do not show any details about how aggregate functions are computed. One has to look at the Optimizer Trace. Search for aggregator_type:

When de-duplication is necessary, it will show:

          {
            "prepare_sum_aggregators": {
              "function": "count(distinct t1.col1)",
              "aggregator_type": "distinct"
            }
          }

When de-duplication is not necessary, it will show:

          {
            "prepare_sum_aggregators": {
              "function": "count(distinct t1.pk1)",
              "aggregator_type": "simple"
            }
          }