For Merge engine joined tables are removed in analysis stage (see references in #18368 (comment)). But in TreeRewriterResult in aggregation there are references to columns from joined tables, that leads to error. I'm trying to remove such expression from aggregation too to fix original issue.

Cases when aggregation contains columns from both tables not considered yet and don't work on current version on PR.

Because we remove GROUP_BY from query to Merge engine, we can clean aggregation from TreeRewriterResult. But for WHERE we still need accurate removing expressions containing identifiers from second table. Before that we removed WHERE completely that leads to unused primary or partition keys for left table with Merge engine.