fix: The limit_pushdown physical optimization rule removes limits in some cases leading to incorrect results (#20048)

## Which issue does this PR close?

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## Rationale for this change

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Bug 1: When pushing down limits, we recurse down the physical plan
accumulating limits until we reach a node where we can't push the limit
down further. At this point, we insert another limit executor (or push
it into the current node, if that node supports it). After this, we
continue recursing to try to find more limits to push down. If we do
find another, we remove it, but we don't set the
`GlobalRequirements::satisfied` field back to false, meaning we don't
always re-insert this limit.

Bug 2: When we're pushing down a limit with a skip/offset and no
fetch/limit and we run into a node that supports fetch, we set
`GlobalRequirements::satisfied` to true. This is wrong: the limit is not
satisfied because fetch doesn't support skip/offset. Instead, we should
set `GlobalRequirements::satisfied` to true if skip/offset is 0.

## What changes are included in this PR?

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is sometimes worth providing a summary of the individual changes in this
PR.
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This includes a one-line change to the push down limit logic that fixes
the issue.

## Are these changes tested?

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I added a test that replicates the issue and fails without this change.

## Are there any user-facing changes?

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Author: masonh22

datafusion/core/tests/physical_optimizer/limit_pushdown.rs

@@ -26,14 +26,15 @@ use arrow::compute::SortOptions;
 use arrow::datatypes::{DataType, Field, Schema, SchemaRef};
 use datafusion_common::config::ConfigOptions;
 use datafusion_common::error::Result;
-use datafusion_expr::Operator;
+use datafusion_expr::{JoinType, Operator};
 use datafusion_physical_expr::Partitioning;
 use datafusion_physical_expr::expressions::{BinaryExpr, col, lit};
 use datafusion_physical_expr_common::sort_expr::{LexOrdering, PhysicalSortExpr};
 use datafusion_physical_optimizer::PhysicalOptimizerRule;
 use datafusion_physical_optimizer::limit_pushdown::LimitPushdown;
 use datafusion_physical_plan::empty::EmptyExec;
 use datafusion_physical_plan::filter::FilterExec;
+use datafusion_physical_plan::joins::NestedLoopJoinExec;
 use datafusion_physical_plan::projection::ProjectionExec;
 use datafusion_physical_plan::repartition::RepartitionExec;
 use datafusion_physical_plan::{ExecutionPlan, get_plan_string};
@@ -87,6 +88,16 @@ fn empty_exec(schema: SchemaRef) -> Arc<dyn ExecutionPlan> {
     Arc::new(EmptyExec::new(schema))
 }
 
+fn nested_loop_join_exec(
+    left: Arc<dyn ExecutionPlan>,
+    right: Arc<dyn ExecutionPlan>,
+    join_type: JoinType,
+) -> Result<Arc<dyn ExecutionPlan>> {
+    Ok(Arc::new(NestedLoopJoinExec::try_new(
+        left, right, None, &join_type, None,
+    )?))
+}
+
 #[test]
 fn transforms_streaming_table_exec_into_fetching_version_when_skip_is_zero() -> Result<()>
 {
@@ -343,3 +354,104 @@ fn merges_local_limit_with_global_limit() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn preserves_nested_global_limit() -> Result<()> {
+    // If there are multiple limits in an execution plan, they all need to be
+    // preserved in the optimized plan.
+    //
+    // Plan structure:
+    // GlobalLimitExec: skip=1, fetch=1
+    //   NestedLoopJoinExec (Left)
+    //     EmptyExec (left side)
+    //     GlobalLimitExec: skip=2, fetch=1
+    //       NestedLoopJoinExec (Right)
+    //         EmptyExec (left side)
+    //         EmptyExec (right side)
+    let schema = create_schema();
+
+    // Build inner join: NestedLoopJoin(Empty, Empty)
+    let inner_left = empty_exec(Arc::clone(&schema));
+    let inner_right = empty_exec(Arc::clone(&schema));
+    let inner_join = nested_loop_join_exec(inner_left, inner_right, JoinType::Right)?;
+
+    // Add inner limit: GlobalLimitExec: skip=2, fetch=1
+    let inner_limit = global_limit_exec(inner_join, 2, Some(1));
+
+    // Build outer join: NestedLoopJoin(Empty, GlobalLimit)
+    let outer_left = empty_exec(Arc::clone(&schema));
+    let outer_join = nested_loop_join_exec(outer_left, inner_limit, JoinType::Left)?;
+
+    // Add outer limit: GlobalLimitExec: skip=1, fetch=1
+    let outer_limit = global_limit_exec(outer_join, 1, Some(1));
+
+    let initial = get_plan_string(&outer_limit);
+    let expected_initial = [
+        "GlobalLimitExec: skip=1, fetch=1",
+        "  NestedLoopJoinExec: join_type=Left",
+        "    EmptyExec",
+        "    GlobalLimitExec: skip=2, fetch=1",
+        "      NestedLoopJoinExec: join_type=Right",
+        "        EmptyExec",
+        "        EmptyExec",
+    ];
+    assert_eq!(initial, expected_initial);
+
+    let after_optimize =
+        LimitPushdown::new().optimize(outer_limit, &ConfigOptions::new())?;
+    let expected = [
+        "GlobalLimitExec: skip=1, fetch=1",
+        "  NestedLoopJoinExec: join_type=Left",
+        "    EmptyExec",
+        "    GlobalLimitExec: skip=2, fetch=1",
+        "      NestedLoopJoinExec: join_type=Right",
+        "        EmptyExec",
+        "        EmptyExec",
+    ];
+    assert_eq!(get_plan_string(&after_optimize), expected);
+
+    Ok(())
+}
+
+#[test]
+fn preserves_skip_before_sort() -> Result<()> {
+    // If there's a limit with skip before a node that (1) supports fetch but
+    // (2) does not support limit pushdown, that limit should not be removed.
+    //
+    // Plan structure:
+    // GlobalLimitExec: skip=1, fetch=None
+    //   SortExec: TopK(fetch=4)
+    //     EmptyExec
+    let schema = create_schema();
+
+    let empty = empty_exec(Arc::clone(&schema));
+
+    let ordering = [PhysicalSortExpr {
+        expr: col("c1", &schema)?,
+        options: SortOptions::default(),
+    }];
+    let sort = sort_exec(ordering.into(), empty)
+        .with_fetch(Some(4))
+        .unwrap();
+
+    let outer_limit = global_limit_exec(sort, 1, None);
+
+    let initial = get_plan_string(&outer_limit);
+    let expected_initial = [
+        "GlobalLimitExec: skip=1, fetch=None",
+        "  SortExec: TopK(fetch=4), expr=[c1@0 ASC], preserve_partitioning=[false]",
+        "    EmptyExec",
+    ];
+    assert_eq!(initial, expected_initial);
+
+    let after_optimize =
+        LimitPushdown::new().optimize(outer_limit, &ConfigOptions::new())?;
+    let expected = [
+        "GlobalLimitExec: skip=1, fetch=3",
+        "  SortExec: TopK(fetch=4), expr=[c1@0 ASC], preserve_partitioning=[false]",
+        "    EmptyExec",
+    ];
+    assert_eq!(get_plan_string(&after_optimize), expected);
+
+    Ok(())
+}

datafusion/physical-optimizer/src/limit_pushdown.rs

@@ -155,6 +155,7 @@ pub fn pushdown_limit_helper(
         global_state.skip = skip;
         global_state.fetch = fetch;
         global_state.preserve_order = limit_exec.preserve_order();
+        global_state.satisfied = false;
 
         // Now the global state has the most recent information, we can remove
         // the `LimitExec` plan. We will decide later if we should add it again
@@ -172,7 +173,7 @@ pub fn pushdown_limit_helper(
     // If we have a non-limit operator with fetch capability, update global
     // state as necessary:
     if pushdown_plan.fetch().is_some() {
-        if global_state.fetch.is_none() {
+        if global_state.skip == 0 {
             global_state.satisfied = true;
         }
         (global_state.skip, global_state.fetch) = combine_limit(

datafusion/sqllogictest/test_files/limit.slt

@@ -706,8 +706,8 @@ ON t1.b = t2.b
 ORDER BY t1.b desc, c desc, c2 desc
 OFFSET 3 LIMIT 2;
 ----
-3 99 82
-3 99 79
+3 98 79
+3 97 96
 
 statement ok
 drop table ordered_table;

datafusion/sqllogictest/test_files/union.slt

@@ -494,22 +494,25 @@ physical_plan
 01)CoalescePartitionsExec: fetch=3
 02)--UnionExec
 03)----ProjectionExec: expr=[count(Int64(1))@0 as cnt]
-04)------AggregateExec: mode=Final, gby=[], aggr=[count(Int64(1))]
-05)--------CoalescePartitionsExec
-06)----------AggregateExec: mode=Partial, gby=[], aggr=[count(Int64(1))]
-07)------------ProjectionExec: expr=[]
-08)--------------AggregateExec: mode=FinalPartitioned, gby=[c1@0 as c1], aggr=[]
-09)----------------RepartitionExec: partitioning=Hash([c1@0], 4), input_partitions=4
-10)------------------AggregateExec: mode=Partial, gby=[c1@0 as c1], aggr=[]
-11)--------------------FilterExec: c13@1 != C2GT5KVyOPZpgKVl110TyZO0NcJ434, projection=[c1@0]
-12)----------------------RepartitionExec: partitioning=RoundRobinBatch(4), input_partitions=1
-13)------------------------DataSourceExec: file_groups={1 group: [[WORKSPACE_ROOT/testing/data/csv/aggregate_test_100.csv]]}, projection=[c1, c13], file_type=csv, has_header=true
-14)----ProjectionExec: expr=[1 as cnt]
-15)------PlaceholderRowExec
-16)----ProjectionExec: expr=[lead(b.c1,Int64(1)) ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING@1 as cnt]
-17)------BoundedWindowAggExec: wdw=[lead(b.c1,Int64(1)) ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING: Field { "lead(b.c1,Int64(1)) ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING": nullable Int64 }, frame: ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING], mode=[Sorted]
-18)--------ProjectionExec: expr=[1 as c1]
-19)----------PlaceholderRowExec
+04)------GlobalLimitExec: skip=0, fetch=3
+05)--------AggregateExec: mode=Final, gby=[], aggr=[count(Int64(1))]
+06)----------CoalescePartitionsExec
+07)------------AggregateExec: mode=Partial, gby=[], aggr=[count(Int64(1))]
+08)--------------ProjectionExec: expr=[]
+09)----------------AggregateExec: mode=FinalPartitioned, gby=[c1@0 as c1], aggr=[]
+10)------------------RepartitionExec: partitioning=Hash([c1@0], 4), input_partitions=4
+11)--------------------AggregateExec: mode=Partial, gby=[c1@0 as c1], aggr=[]
+12)----------------------FilterExec: c13@1 != C2GT5KVyOPZpgKVl110TyZO0NcJ434, projection=[c1@0]
+13)------------------------RepartitionExec: partitioning=RoundRobinBatch(4), input_partitions=1
+14)--------------------------DataSourceExec: file_groups={1 group: [[WORKSPACE_ROOT/testing/data/csv/aggregate_test_100.csv]]}, projection=[c1, c13], file_type=csv, has_header=true
+15)----ProjectionExec: expr=[1 as cnt]
+16)------GlobalLimitExec: skip=0, fetch=3
+17)--------PlaceholderRowExec
+18)----ProjectionExec: expr=[lead(b.c1,Int64(1)) ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING@1 as cnt]
+19)------GlobalLimitExec: skip=0, fetch=3
+20)--------BoundedWindowAggExec: wdw=[lead(b.c1,Int64(1)) ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING: Field { "lead(b.c1,Int64(1)) ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING": nullable Int64 }, frame: ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING], mode=[Sorted]
+21)----------ProjectionExec: expr=[1 as c1]
+22)------------PlaceholderRowExec
 
 
 ########