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Minor: Extract parent/child limit calculation into a function, improve docs #10501

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Merged
alamb merged 3 commits into from
May 15, 2024
Merged

Minor: Extract parent/child limit calculation into a function, improve docs #10501

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85d0fae
Minor: Extract parent/child limit calculation into a function, improv…
alamb May 14, 2024
0637e2c
Update datafusion/optimizer/src/push_down_limit.rs
alamb May 15, 2024
e397291
Merge remote-tracking branch 'apache/main' into alamb/refactor_limit_…
alamb May 15, 2024
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116 changes: 77 additions & 39 deletions datafusion/optimizer/src/push_down_limit.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@

//! [`PushDownLimit`] pushes `LIMIT` earlier in the query plan

use std::cmp::min;
use std::sync::Arc;

use crate::optimizer::ApplyOrder;
Expand Down Expand Up @@ -56,47 +57,12 @@ impl OptimizerRule for PushDownLimit {

if let LogicalPlan::Limit(child) = &*limit.input {
// Merge the Parent Limit and the Child Limit.

// Case 0: Parent and Child are disjoint. (child_fetch <= skip)
// Before merging:
// |........skip........|---fetch-->| Parent Limit
// |...child_skip...|---child_fetch-->| Child Limit
// After merging:
// |.........(child_skip + skip).........|
// Before merging:
// |...skip...|------------fetch------------>| Parent Limit
// |...child_skip...|-------------child_fetch------------>| Child Limit
// After merging:
// |....(child_skip + skip)....|---(child_fetch - skip)-->|

// Case 1: Parent is beyond the range of Child. (skip < child_fetch <= skip + fetch)
// Before merging:
// |...skip...|------------fetch------------>| Parent Limit
// |...child_skip...|-------------child_fetch------------>| Child Limit
// After merging:
// |....(child_skip + skip)....|---(child_fetch - skip)-->|

// Case 2: Parent is in the range of Child. (skip + fetch < child_fetch)
// Before merging:
// |...skip...|---fetch-->| Parent Limit
// |...child_skip...|-------------child_fetch------------>| Child Limit
// After merging:
// |....(child_skip + skip)....|---fetch-->|
let parent_skip = limit.skip;
let new_fetch = match (limit.fetch, child.fetch) {
(Some(fetch), Some(child_fetch)) => {
Some(min(fetch, child_fetch.saturating_sub(parent_skip)))
}
(Some(fetch), None) => Some(fetch),
(None, Some(child_fetch)) => {
Some(child_fetch.saturating_sub(parent_skip))
}
(None, None) => None,
};
let (skip, fetch) =
combine_limit(limit.skip, limit.fetch, child.skip, child.fetch);
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@alamb alamb May 14, 2024

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this PR just refactors all the existing logic to a function


let plan = LogicalPlan::Limit(Limit {
skip: child.skip + parent_skip,
fetch: new_fetch,
skip,
fetch,
input: Arc::new((*child.input).clone()),
});
return self
Expand Down Expand Up @@ -217,6 +183,78 @@ impl OptimizerRule for PushDownLimit {
}
}

/// Combines two limits into a single
///
/// Returns the combined limit `(skip, fetch)`
///
/// # Case 0: Parent and Child are disjoint. (`child_fetch <= skip`)
///
/// ```text
/// Before merging:
/// |........skip........|---fetch-->| Parent Limit
/// |...child_skip...|---child_fetch-->| Child Limit
/// ```
///
/// After merging:
/// ```text
/// |.........(child_skip + skip).........|
/// ```
///
/// Before merging:
/// ```text
/// |...skip...|------------fetch------------>| Parent Limit
/// |...child_skip...|-------------child_fetch------------>| Child Limit
/// ```
///
/// After merging:
/// ```text
/// |....(child_skip + skip)....|---(child_fetch - skip)-->|
/// ```
///
/// # Case 1: Parent is beyond the range of Child. (`skip < child_fetch <= skip + fetch`)
///
/// Before merging:
/// ```text
/// |...skip...|------------fetch------------>| Parent Limit
/// |...child_skip...|-------------child_fetch------------>| Child Limit
/// ```
///
/// After merging:
/// ```text
/// |....(child_skip + skip)....|---(child_fetch - skip)-->|
/// ```
///
/// # Case 2: Parent is in the range of Child. (`skip + fetch < child_fetch`)
/// Before merging:
/// ```text
/// |...skip...|---fetch-->| Parent Limit
/// |...child_skip...|-------------child_fetch------------>| Child Limit
/// ```
///
/// After merging:
/// ```text
/// |....(child_skip + skip)....|---fetch-->|
/// ```
fn combine_limit(
parent_skip: usize,
parent_fetch: Option<usize>,
child_skip: usize,
child_fetch: Option<usize>,
) -> (usize, Option<usize>) {
let combined_skip = child_skip.saturating_add(parent_skip);

let combined_fetch = match (parent_fetch, child_fetch) {
(Some(parent_fetch), Some(child_fetch)) => {
Some(min(parent_fetch, child_fetch.saturating_sub(parent_skip)))
}
(Some(parent_fetch), None) => Some(parent_fetch),
(None, Some(child_fetch)) => Some(child_fetch.saturating_sub(parent_skip)),
(None, None) => None,
};

(combined_skip, combined_fetch)
}

fn push_down_join(join: &Join, limit: usize) -> Option<Join> {
use JoinType::*;

Expand Down