>> Let table A be inherited by A1, A2, A3.
>> How to select from A records where actual relations are A1, A2 ?
>Why not just select directly from the child tables? I can't get excited
>about optimizing the case you propose.
Because "WHERE concrete_class IN (a,b,c)" is much more convenient and
flexible way of forming select then manually split request into many unions.
Also, this query runs on top of "abstract class", so inheritance really
assists me here.