Function mz_sql::plan::query::plan_view_select

fn plan_view_select(
    qcx: &QueryContext<'_>,
    s: Select<Aug>,
    order_by_exprs: Vec<OrderByExpr<Aug>>
) -> Result<SelectPlan, PlanError>

Plans a SELECT query. The SELECT query may contain an intrusive ORDER BY clause.

Normally, the ORDER BY clause occurs after the columns specified in the SELECT list have been projected. In a query like

CREATE TABLE (a int, b int) (SELECT a FROM t) UNION (SELECT a FROM t) ORDER BY a

it is valid to refer to a, because it is explicitly selected, but it would not be valid to refer to unselected column b.

But PostgreSQL extends the standard to permit queries like

SELECT a FROM t ORDER BY b

where expressions in the ORDER BY clause can refer to both input columns and output columns.