This commit introduces a transformation for SQL queries that contain multiple distinct aggregate functions. When the number of distinct values processed by these functions is greater than 1, they are converted into multi_distinct functions for more efficient handling.
Example:
```
SELECT COUNT(DISTINCT c1), SUM(DISTINCT c2) FROM tbl GROUP BY c3
-- Transformed to
SELECT MULTI_DISTINCT_COUNT(c1), MULTI_DISTINCT_SUM(c2) FROM tbl GROUP BY c3
```
The following functions can be transformed:
- COUNT
- SUM
- AVG
- GROUP_CONCAT
If any unsupported functions are encountered, an error is now reported during the optimization phase.
To ensure the absence of such cases, a final check has been implemented after the rewriting phase.
68bd4a1a96