Constraints can and do help performance. Unique constraints may allow SQL to ignore DISTINCT operations or reduce group by statements to simpler forms or know absolutely for sure how many rows a particular operation will affect. Check constraints can allow SQL to ignore entire predicates (eg WHERE x > 0 when there's a constraint forcing x to be > 0) or if a predicate requires rows that violate constraints (where x IS NULL on a not null column). Similar with foreign keys.
Thanks for your anwser.
I need an example which is a bit more convincing.
Relational constraints were removed because of performance problems.
This should not happen.
I think that this is a design problem. With a 'correct' design and implementation, constraints should not lead to performance problems.
Argument 2: Constraints in general, but relation constraints specifically help the query processor to build better query plans.
But I can not find any convincing examples of this. Without the convincing examples or webreferences the above arguments are weak.
I tried to build some constructions to demonstrate the effect of removing the relational constraints and show that this was bad for performance regrettably I did not succeed.
Again thanks for your anwser and time