Previously the Operation enum consisted of:
- Operation::Scan
- Operation::Search
- Operation::Subquery
Which was always a dumb hack because what we really are doing is
an Operation::Scan on a "virtual"/"pseudo" table (overloaded names...)
derived from a subquery appearing in the FROM clause.
Hence, refactor the relevant data structures so that the Table enum
now contains a new variant:
Table::FromClauseSubquery
And the Operation enum only consists of Scan and Search.
No functional changes (intended, at least!)