Firstly thank you for moving this further. I have few observations, not necessarily fatal errors, might create issues later. For now just one thing:

I was able to update my project from ORM 4 to 5, finally getting my complex query to show correct results. However, the performance drop is significant. Probably due to all the HAVING clauses and ON clause filters. Even when using only simple "any" aggregations that worked in version 4.

$this->model->books->findBy([
	ICollection::OR,
	['id' => 1],
	['tags->id' => 2],
]);

ORM 4:

SELECT "books".* 
FROM "books" AS "books" 
LEFT JOIN "books_x_tags" AS "books_x_tags" ON ("books"."id" = "books_x_tags"."book_id") 
LEFT JOIN "tags" AS "tags" ON ("books_x_tags"."tag_id" = "tags"."id") 
WHERE (("books"."id" = 3)) OR (("tags"."id" = 1)) 
GROUP BY "books"."id"

ORM 5:

SELECT "books".* 
FROM "books" AS "books" 
LEFT JOIN "books_x_tags" AS "books_x_tags_any" ON ("books"."id" = "books_x_tags_any"."book_id") 
LEFT JOIN "tags" AS "tags_any" ON (("books_x_tags_any"."tag_id" = "tags_any"."id") AND "tags_any"."id" = 1) 
GROUP BY "books"."id", "books"."title" 
HAVING ((("books"."id" = 3)) OR ((COUNT("tags_any"."id") > 0)))

This is just a simple query, but in complex queries, the drop is from miliseconds to seconds. The results are correct in both. While I am eager to use new aggregation features, I also need my current queries to work fast without change. I don't know how hard and whether even achievable it would be to optimize this. Not to delay release further, my suggestion is a "legacy mode" that would allow user to use old ORM 4 queries (possibly by overriding AnyAggregator::aggregateExpression to always just return $expression;).

Originally posted by @stepapo in #666

Describe the bug
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1. Code which you run
2. Definition of entities
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4. Which SQL queries are executed

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Versions::

• Database: [e.g. MySQL 5.7]
• Orm: [e.g. Orm 3.0]
• Dbal: [e.g. Dbal 3.0]