DISTINCT removal in aggregate functions

How the optimizer removes redundant DISTINCT from aggregate-function arguments.

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, MariaDB has to collect the values of col1 and remove the duplicates. This may be computationally expensive.

After the fix for MDEV-30660 (available from MariaDB 10.5.25, MariaDB 10.6.18, MariaDB 10.11.8, MariaDB 11.0.6, MariaDB 11.1.5, MariaDB 11.2.4, MariaDB 11.4.2), the optimizer can detect certain cases when argument of aggregate function will not have duplicates and so de-duplication can be skipped.

When one can skip de-duplication

A basic example: if we're doing a select from one table, then the values of primary_key are already distinct:

SELECT aggregate_func(DISTINCT tbl.primary_key, ...) FROM tbl;

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:

SELECT aggregate_func(DISTINCT t1.pk1, ...) FROM t1 GROUP BY t1.pk2;

Suppose the table has PRIMARY KEY(pk1, pk2). Grouping by pk2 fixes the value of pk2 within each group. 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"
            }
          }

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