Since you can index expressions I wonder if that's because you essentially emhave to store the value in the index anyway and that wouldn't be expected for a virtual column?
I don't really understand why this restriction would be necessary. Wouldn't the computed value just get stored in the index btree keys, exactly the same as an indexed expression?
For comparison, MySQL permits indexing of virtual columns, and indexes on expressions share the same underlying implementation to support this. In other words, in MySQL an index on an expression is literally just an index on an internally-hidden virtual column.
