Function extract_subqueries 

pub fn extract_subqueries(
    expr: &Expr,
    outer: &TableScope,
    catalog: &dyn SqlCatalog,
    functions: &FunctionRegistry,
    temporal: TemporalScope,
) -> Result<SubqueryExtraction>

Extract subquery predicates from a WHERE clause.

outer is the scope of the enclosing query, so a correlated reference inside a subquery resolves against the relation that owns it.

Returns the extracted subquery joins and the remaining WHERE expression (with subquery predicates removed). If the entire WHERE is a single subquery predicate, remaining_where is None.