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Merged
andygrove merged 9 commits into from
May 20, 2022
Merged

Support limit push down for offset_plan #2566

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35bbdd5
support offset push down
Ted-Jiang May 18, 2022
a6aaed5
change the limit and offset order
Ted-Jiang May 18, 2022
302fd4b
add test for join and subquery
Ted-Jiang May 18, 2022
27879cc
fmt
Ted-Jiang May 18, 2022
365dbf0
Apply suggestions from code review
Ted-Jiang May 19, 2022
20366bd
add sql test in planner.rs
Ted-Jiang May 20, 2022
aa87825
Merge remote-tracking branch 'origin/issue_2550' into issue_2550
Ted-Jiang May 20, 2022
59a1813
add sql test in planner.rs
Ted-Jiang May 20, 2022
613de25
fix clippy
Ted-Jiang May 20, 2022
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208 changes: 203 additions & 5 deletions datafusion/core/src/optimizer/limit_push_down.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,7 @@ use crate::logical_plan::plan::Projection;
use crate::logical_plan::{Limit, TableScan};
use crate::logical_plan::{LogicalPlan, Union};
use crate::optimizer::optimizer::OptimizerRule;
use datafusion_expr::logical_plan::Offset;
use std::sync::Arc;

/// Optimization rule that tries pushes down LIMIT n
Expand All @@ -43,18 +44,24 @@ fn limit_push_down(
upper_limit: Option<usize>,
plan: &LogicalPlan,
_execution_props: &ExecutionProps,
is_offset: bool,
) -> Result<LogicalPlan> {
match (plan, upper_limit) {
(LogicalPlan::Limit(Limit { n, input }), upper_limit) => {
let smallest = upper_limit.map(|x| std::cmp::min(x, *n)).unwrap_or(*n);
let new_limit: usize = if is_offset {
*n
} else {
upper_limit.map(|x| std::cmp::min(x, *n)).unwrap_or(*n)
};
Ok(LogicalPlan::Limit(Limit {
n: smallest,
n: new_limit,
// push down limit to plan (minimum of upper limit and current limit)
input: Arc::new(limit_push_down(
_optimizer,
Some(smallest),
Some(new_limit),
input.as_ref(),
_execution_props,
false,
)?),
}))
}
Expand Down Expand Up @@ -95,6 +102,7 @@ fn limit_push_down(
upper_limit,
input.as_ref(),
_execution_props,
false,
)?),
schema: schema.clone(),
alias: alias.clone(),
Expand All @@ -119,6 +127,7 @@ fn limit_push_down(
Some(upper_limit),
x,
_execution_props,
false,
)?),
}))
})
Expand All @@ -129,6 +138,21 @@ fn limit_push_down(
schema: schema.clone(),
}))
}
// offset 5 limit 10 then push limit 15 (5 + 10)
// Limit should always be Offset's input
(LogicalPlan::Offset(Offset { offset, input }), Some(upper_limit)) => {
let new_limit = offset + upper_limit;
Ok(LogicalPlan::Offset(Offset {
offset: *offset,
input: Arc::new(limit_push_down(
_optimizer,
Some(new_limit),
input.as_ref(),
_execution_props,
true,
)?),
}))
}
// For other nodes we can't push down the limit
// But try to recurse and find other limit nodes to push down
_ => {
Expand All @@ -138,7 +162,9 @@ fn limit_push_down(
let inputs = plan.inputs();
let new_inputs = inputs
.iter()
.map(|plan| limit_push_down(_optimizer, None, plan, _execution_props))
.map(|plan| {
limit_push_down(_optimizer, None, plan, _execution_props, false)
})
.collect::<Result<Vec<_>>>()?;

utils::from_plan(plan, &expr, &new_inputs)
Expand All @@ -152,7 +178,7 @@ impl OptimizerRule for LimitPushDown {
plan: &LogicalPlan,
execution_props: &ExecutionProps,
) -> Result<LogicalPlan> {
limit_push_down(self, None, plan, execution_props)
limit_push_down(self, None, plan, execution_props, false)
}

fn name(&self) -> &str {
Expand All @@ -167,6 +193,8 @@ mod test {
logical_plan::{col, max, LogicalPlan, LogicalPlanBuilder},
test::*,
};
use datafusion_expr::exists;
use datafusion_expr::logical_plan::JoinType;

fn assert_optimized_plan_eq(plan: &LogicalPlan, expected: &str) {
let rule = LimitPushDown::new();
Expand Down Expand Up @@ -278,4 +306,174 @@ mod test {

Ok(())
}

#[test]
fn limit_pushdown_with_offset_projection_table_provider() -> Result<()> {
let table_scan = test_table_scan()?;

let plan = LogicalPlanBuilder::from(table_scan)
.project(vec![col("a")])?
.offset(10)?
.limit(1000)?
.build()?;

// Should push the limit down to table provider
// When it has a select
let expected = "Limit: 1000\
\n Offset: 10\
\n Projection: #test.a\
\n TableScan: test projection=None, limit=1010";

assert_optimized_plan_eq(&plan, expected);

Ok(())
}

#[test]
fn limit_pushdown_with_offset_after_limit() -> Result<()> {
let table_scan = test_table_scan()?;

let plan = LogicalPlanBuilder::from(table_scan)
.project(vec![col("a")])?
.limit(1000)?
.offset(10)?
.build()?;

// Not push the limit down to table provider
// When offset after limit
let expected = "Offset: 10\
\n Limit: 1000\
\n Projection: #test.a\
\n TableScan: test projection=None, limit=1000";

assert_optimized_plan_eq(&plan, expected);

Ok(())
}

#[test]
fn limit_push_down_with_offset_take_smaller_limit() -> Result<()> {
let table_scan = test_table_scan()?;

let plan = LogicalPlanBuilder::from(table_scan)
.offset(10)?
.limit(1000)?
.limit(10)?
.build()?;

// Should push down the smallest limit
// Towards table scan
// This rule doesn't replace multiple limits
let expected = "Limit: 10\
\n Limit: 10\
\n Offset: 10\
\n TableScan: test projection=None, limit=20";

assert_optimized_plan_eq(&plan, expected);

Ok(())
}

#[test]
fn limit_doesnt_push_down_with_offset_aggregation() -> Result<()> {
let table_scan = test_table_scan()?;

let plan = LogicalPlanBuilder::from(table_scan)
.aggregate(vec![col("a")], vec![max(col("b"))])?
.offset(10)?
.limit(1000)?
.build()?;

// Limit should *not* push down aggregate node
let expected = "Limit: 1000\
\n Offset: 10\
\n Aggregate: groupBy=[[#test.a]], aggr=[[MAX(#test.b)]]\
\n TableScan: test projection=None";

assert_optimized_plan_eq(&plan, expected);

Ok(())
}

#[test]
fn limit_should_push_down_with_offset_union() -> Result<()> {
Comment thread
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let table_scan = test_table_scan()?;

let plan = LogicalPlanBuilder::from(table_scan.clone())
.union(LogicalPlanBuilder::from(table_scan).build()?)?
.offset(10)?
.limit(1000)?
.build()?;

// Limit should push down through union
let expected = "Limit: 1000\
\n Offset: 10\
\n Union\
\n Limit: 1010\
\n TableScan: test projection=None, limit=1010\
\n Limit: 1010\
\n TableScan: test projection=None, limit=1010";

assert_optimized_plan_eq(&plan, expected);

Ok(())
}

#[test]
fn limit_should_push_down_with_offset_join() -> Result<()> {
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let table_scan_1 = test_table_scan()?;
let table_scan_2 = test_table_scan_with_name("test2")?;

let plan = LogicalPlanBuilder::from(table_scan_1)
.join(
&LogicalPlanBuilder::from(table_scan_2).build()?,
JoinType::Left,
(vec!["a"], vec!["a"]),
)?
.limit(1000)?
.offset(10)?
.build()?;

// Limit pushdown Not supported in Join
let expected = "Offset: 10\
\n Limit: 1000\
\n Left Join: #test.a = #test2.a\
\n TableScan: test projection=None\
\n TableScan: test2 projection=None";

assert_optimized_plan_eq(&plan, expected);

Ok(())
}

#[test]
fn limit_should_push_down_with_offset_sub_query() -> Result<()> {
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let table_scan_1 = test_table_scan_with_name("test1")?;
let table_scan_2 = test_table_scan_with_name("test2")?;

let subquery = LogicalPlanBuilder::from(table_scan_1)
.project(vec![col("a")])?
.filter(col("a").eq(col("test1.a")))?
.build()?;

let outer_query = LogicalPlanBuilder::from(table_scan_2)
.project(vec![col("a")])?
.filter(exists(Arc::new(subquery)))?
.limit(100)?
.offset(10)?
.build()?;

// Limit pushdown Not supported in sub_query
let expected = "Offset: 10\
\n Limit: 100\
\n Filter: EXISTS (Subquery: Filter: #test1.a = #test1.a\
\n Projection: #test1.a\
\n TableScan: test1 projection=None)\
\n Projection: #test2.a\
\n TableScan: test2 projection=None";

assert_optimized_plan_eq(&outer_query, expected);

Ok(())
}
}
9 changes: 5 additions & 4 deletions datafusion/core/src/sql/planner.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -296,9 +296,10 @@ impl<'a, S: ContextProvider> SqlToRel<'a, S> {

let plan = self.order_by(plan, query.order_by)?;

let plan: LogicalPlan = self.offset(plan, query.offset)?;
let plan: LogicalPlan = self.limit(plan, query.limit)?;

self.limit(plan, query.limit)
//make limit as offset's input will enable limit push down simply
self.offset(plan, query.offset)

@Ted-Jiang Ted-Jiang May 18, 2022

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If there is an offset in sql, we will put it as root node in plan tree. So it will easily push new_limit to limit operator.

@andygrove andygrove May 19, 2022

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Don't we need to apply the offset before the limit here?

It looks like we don't currently have a test with both a non-zero offset and a limit. Could you add one here so we can see what the plan looks like?

Per https://www.postgresql.org/docs/current/queries-limit.html If both OFFSET and LIMIT appear, then OFFSET rows are skipped before starting to count the LIMIT rows that are returned. and I am not sure this change will ensure that

@Ted-Jiang Ted-Jiang May 20, 2022 •
edited
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form the explain from pg

postgres=# explain analyze  select * from users limit 5 offset 7;
                                                 QUERY PLAN
-------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.17..0.29 rows=5 width=80) (actual time=0.117..0.226 rows=5 loops=1)
   ->  Seq Scan on users  (cost=0.00..239.02 rows=10002 width=80) (actual time=0.013..0.082 rows=12 loops=1)
 Planning Time: 0.060 ms
 Execution Time: 0.350 ms
(4 rows)

I think in PG, the limit operator has a param with offset， in DF we separate it. So we need apply offset after limit a+b

add test #2566 (comment)

}

fn set_expr_to_plan(
Expand Down Expand Up @@ -4825,8 +4826,8 @@ mod tests {
#[test]
fn test_offset_with_limit() {
let sql = "select id from person where person.id > 100 LIMIT 5 OFFSET 0;";
let expected = "Limit: 5\
\n Offset: 0\
let expected = "Offset: 0\
\n Limit: 5\
\n Projection: #person.id\
\n Filter: #person.id > Int64(100)\
\n TableScan: person projection=None";
Expand Down