In this way wouldn't the SP have the same problem that was mentioned by Louis, namely that only the index from the first table in the query would be used?

I did not read it in full details...

But the case of select on views be able to use the index on the first table I believe that's because of using of outer joins, with inner joins that would not be the case...

I am very sorry for my slow response, but thank you for explaining this.

I don't know anything about how Firebird treats indexes in selectable SP, so I have some learning to do in this area.