DDC-2076 Remove useless join over target table of ManyToMany relationship

[plfort]

Quite an old issue : #2758

I noticed this additional "join" while trying to optimize a query. As mentioned in the issue, I don't see a case when this join is useful.
In my case (query optimization), this PR provides a significant reduction of the query cost given by Postgresql

[SenseException]

Looks good to me. Maybe SqlWalker need some integration tests, but I'm not sure if this should be done in the scope of this PR.

[beberlei]

@SenseException The SelectSqlGenerationTest is the integration test for SqlWalker, so this is fine.

I think the patch is good, thanks.

unrelated however I realized this must provide wrong results on composite keys or not? It seems weird to iterate the columns and add multiple in conditions, maybe I am wrong.

found a bug

[beberlei]

@plfort i am a bit hesitant to merge a cosmetic PR into a branch in bugfix mode, the old code works, so this is an improvement and changing the SQL is a bit more risky, so I would like you to rebase this on 2.9 and we can merge it there.

[plfort]

I agree with you about composite keys, it requires further tests. I thought about it but for this change I wanted to keep the behaviour.

[plfort]

Rebase on 2.9 done BTW

[rvanlaak]

if this PR did fix #2758, that issue can get closed now