AssHero commented on code in PR #2823:
URL: https://github.com/apache/arrow-datafusion/pull/2823#discussion_r914353805
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datafusion/optimizer/src/eliminate_limit.rs:
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@@ -35,35 +39,98 @@ impl EliminateLimit {
}
}
+/// Ancestor indicates the current ancestor in the LogicalPlan tree
+/// when traversing down related to "eliminate limit".
+enum Ancestor {
+ /// Limit
+ FromLimit { skip: Option<usize> },
+ /// Other nodes that don't affect the adjustment of "Limit"
+ NotRelevant,
+}
+
+/// replaces LIMIT 0 with an [LogicalPlan::EmptyRelation]
+/// replaces LIMIT node whose ancestor LIMIT's skip is greater than or equal
to current's fetch
+/// with an [LogicalPlan::EmptyRelation]
+fn eliminate_limit(
+ _optimizer: &EliminateLimit,
+ ancestor: &Ancestor,
+ plan: &LogicalPlan,
+ _optimizer_config: &OptimizerConfig,
+) -> Result<LogicalPlan> {
+ match plan {
+ LogicalPlan::Limit(Limit {
+ skip, fetch, input, ..
+ }) => {
+ let ancestor_skip = match ancestor {
+ Ancestor::FromLimit { skip, .. } => skip.unwrap_or(0),
+ _ => 0,
+ };
+ // If ancestor's skip is equal or greater than current's fetch,
+ // replaces with an [LogicalPlan::EmptyRelation].
+ // For such query, the inner query(select * from xxx limit 5)
should be optimized as an EmptyRelation:
+ // select * from (select * from xxx limit 5) a limit 2 offset 5;
+ match fetch {
+ Some(fetch) => {
+ if *fetch == 0 || ancestor_skip >= *fetch {
+ return Ok(LogicalPlan::EmptyRelation(EmptyRelation {
+ produce_one_row: false,
+ schema: input.schema().clone(),
+ }));
+ }
+ }
+ None => {}
+ }
+
+ let expr = plan.expressions();
+
+ // apply the optimization to all inputs of the plan
+ let inputs = plan.inputs();
+ let new_inputs = inputs
+ .iter()
+ .map(|plan| {
+ eliminate_limit(
+ _optimizer,
+ &Ancestor::FromLimit { skip: *skip },
+ plan,
+ _optimizer_config,
+ )
+ })
+ .collect::<Result<Vec<_>>>()?;
+
+ from_plan(plan, &expr, &new_inputs)
+ }
+ // Rest: recurse and find possible LIMIT 0/Multi LIMIT OFFSET nodes
+ _ => {
+ // For those plans(projection/sort/..) which do not affect the
output rows of sub-plans, we still use ancestor;
+ // otherwise, use NotRelevant instead.
+ let ancestor = match plan {
+ LogicalPlan::Projection { .. } | LogicalPlan::Sort { .. } =>
ancestor,
+ _ => &Ancestor::NotRelevant,
Review Comment:
> I wonder if `Ancester::Unknown` would make the intent clearer compared to
`Ancester::NotRelevant`?
This is consistent with limit_push_down.
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