Function try_plan_scalar_subquery

nodedb-sql/src/planner/subquery.rs:365–390

Plan a scalar subquery (e.g., `(SELECT AVG(amount) FROM orders)`). Rewrites `col > (SELECT AVG(amount) FROM orders)` as: cross-join with the aggregate result (1 row), then filter `col > result_col`. The cross-join produces a cartesian product, but since the aggregate returns exactly 1 row, every outer row gets paired with that single result row.

(
    subquery: &ast::Query,
    catalog: &dyn SqlCatalog,
    functions: &FunctionRegistry,
    temporal: crate::TemporalScope,
)

/// The cross-join produces a cartesian product, but since the aggregate returns
/// exactly 1 row, every outer row gets paired with that single result row.
fn try_plan_scalar_subquery(
    subquery: &ast::Query,
    catalog: &dyn SqlCatalog,
    functions: &FunctionRegistry,
    temporal: crate::TemporalScope,
) -> Result<Option<ScalarSubqueryResult>> {
    let inner_plan = super::select::plan_query(subquery, catalog, functions, temporal)?;

    // Extract the result column name from the subquery's SELECT list.
    let result_col = match extract_scalar_column(subquery) {
        Some(col) => col,
        None => return Ok(None),
    };

    let replacement = Expr::Identifier(ast::Ident::new(&result_col));

    Ok(Some(ScalarSubqueryResult {
        join: SubqueryJoin {
            outer_column: String::new(),
            inner_plan,
            inner_column: String::new(),
            join_type: JoinType::Cross,
        },
        replacement_expr: replacement,
    }))
}

/// Extract the projected column name from a scalar subquery.
///