pub fn required_columns_by_table(ast: &Ast) -> BTreeMap<Box<str>, ReqCols>Expand description
The columns each base table in ast’s footprint needs synced (§5.2). A pure function of
the AST (column names, resolved to indices by the caller against the table schema).
Mirrors table_tree’s slot walk + prune (exists_noSync slots are never synced, so
they contribute nothing). The result drives the projected wire schema and project-at-emit.