Struct mz_transform::demand::Demand

pub struct Demand {
    recursion_guard: RecursionGuard,
}

Drive demand from the root through operators.

This transform alerts operators to their columns that influence the ultimate output of the expression, and gives them permission to swap other columns for dummy values. As part of this, operators should not actually use any of these dummy values, lest they run-time error.

This transformation primarily informs the Join operator, which can simplify its intermediate state when it knows that certain columns are not observed in its output. Internal arrangements need not maintain columns that are no longer required in the join pipeline, which are those columns not required by the output nor any further equalities.

Nowadays, this transform is mostly obsoleted by ProjectionPushdown. However, I know of one thing that it still does that ProjectionPushdown doesn’t do (there might be more such things): if you have something like

    Project (#0, #1)
      Join on=(#0 = #1)

then this is turned into

    Project (#0, #0)
      Join on=(#0 = #1)

This can be beneficial for projecting out some columns earlier inside a complex join (by the LIR planning), and then recovering them after the join (if needed) by duplicating existing columns.

After the last run of Demand, there should always be a ProjectionPushdown, so that dummies are eliminated from plans.

Fields

recursion_guard: RecursionGuard

Implementations

impl Demand

fn action( &self, relation: &mut MirRelationExpr, columns: BTreeSet<usize>, gets: &mut BTreeMap<Id, BTreeSet<usize>>, ) -> Result<(), TransformError>

Columns to be produced.

Trait Implementations

impl CheckedRecursion for Demand

fn recursion_guard(&self) -> &RecursionGuard

Extracts a reference to the recursion guard embedded within the type.

fn checked_recur<F, T, E>(&self, f: F) -> Result<T, E>

where F: FnOnce(&Self) -> Result<T, E>, E: From<RecursionLimitError>,

Checks whether it is safe to recur and calls f if so.

fn checked_recur_mut<F, T, E>(&mut self, f: F) -> Result<T, E>

where F: FnOnce(&mut Self) -> Result<T, E>, E: From<RecursionLimitError>,

Like CheckedRecursion::checked_recur, but operates on a mutable reference to Self.

impl Debug for Demand

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for Demand

fn default() -> Demand

Returns the “default value” for a type.

impl Transform for Demand

fn name(&self) -> &'static str

A short string naming the transform, as it will be reported in metrics.

fn actually_perform_transform( &self, relation: &mut MirRelationExpr, _: &mut TransformCtx<'_>, ) -> Result<(), TransformError>

Transform a relation into a functionally equivalent relation.

fn transform( &self, relation: &mut MirRelationExpr, args: &mut TransformCtx<'_>, ) -> Result<(), TransformError>

Transforms a relation into a functionally equivalent relation.

fn debug(&self) -> String

A string describing the transform.