For the last week I was making some investigations about implementing my task. Here are my thoughts about how it can be done. I want to find all virtual columns that are used in most outer select. Then try to match expressions in where clauses with vcols and rewrite all matching ones, including ones in subselects. Then run new Item processor. When this processor encounters Item_subselect it finds all vcolumns again, but this time only that belongs to subselect. Then the process continues recursively. I currently trying to implement it. Code is available here https://github.com/ZeroICQ/server/compare/10.4...ZeroICQ:MDEV-6017 Selects with joins and unions would require some modifications.
I had most troubles with getting all where clauses from subselects. I am not sure if "if ((*vf)->get_possible_keys().to_ulonglong())" is the right way to check if there are any suitable indexes for given virtual column. And I have some troubles understanding structure of LEX variable after parsing complex queries with joins, subselects and unions. >
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