Fix total count for paged specifications with GROUP BY

[robin850]

Hello,

This patch tries to fix using paginated results found through a custom Specification that relies on a GROUP BY clause.

I hope the test is clear enough, but just in case, the problem is that since the getCountQuery method of SimpleJpaRepository takes the exact same query produced by the specification but injects a COUNT in the SELECT, the COUNT is done on each group of records so we end up counting them and not the "root" ones.

Using DISTINCT does the trick here. Actually I can't think of a case where using DISTINCT would be problematic so in my mind the patch could be to totally remove the if/else statement and rely on countDistinct anyway but maybe I'm blinded with my application and that's a bit too rash.

For the record, the if/else statement has been added in a74fadd.

Closes #1296.
Closes #2361.
Closes #1858.

Have a nice day.

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[schauder]

Sorry for being so late to reply to the original issue.
As explained there we will not consider this PR.

[ankitpec72]

@robin850 @schauder We are facing the same issue in Spring-data-jpa-2.7.13 version. Is there any solution or this issue is already fixed in some other version? The count query with group by is returning wrong total count value.

[robin850]

@ankitpec72 : No, as stated in @schauder's comment, the fact that it doesn't work is "normal" because you should only manage WHERE clauses through Specification.

To be honest, we currently have a copy of this PR's SimpleJpaRepository.java file in our application to be able to correctly use GROUP BY. It's far from ideal but I don't know if there is an "elegant" API currently to achieve the same thing.