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optimize limit push for join case #4411

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Merged
merged 2 commits into from
Dec 1, 2022
Merged

optimize limit push for join case #4411

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95d9557
optimize limit push for join case
liukun4515 Nov 29, 2022
772b83e
address comments
liukun4515 Nov 29, 2022
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158 changes: 158 additions & 0 deletions datafusion/optimizer/src/limit_push_down.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -39,6 +39,10 @@ impl LimitPushDown {
}
}

fn is_no_join_condition(join: &Join) -> bool {
join.on.is_empty() && join.filter.is_none()
}

fn push_down_join(
join: &Join,
left_limit: Option<usize>,
Expand Down Expand Up @@ -192,6 +196,24 @@ impl OptimizerRule for LimitPushDown {
LogicalPlan::Join(join) => {
let limit = fetch + skip;
let new_join = match join.join_type {
JoinType::Left | JoinType::Right | JoinType::Full
if is_no_join_condition(join) =>
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@alamb alamb Nov 30, 2022 •
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I wonder how often there will be no join conditions 🤔 that effectively then becomes a CROSS JOIN

To be clear I think the optimization is still correct, I just wonder how often it will appear in real queries

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@liukun4515 liukun4515 Dec 1, 2022

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I wonder how often there will be no join conditions 🤔 that effectively then becomes a CROSS JOIN

To be clear I think the optimization is still correct, I just wonder how often it will appear in real queries

Good question for this, in the current framework of optimizer in the datafusion, the join query will be converted to the cross join by other rule.

The limit push down rule just do the best to optimize the plan.

Some SQL are generated by the program, and the join type and on condition are added by the program, it will generate sql like select * from left_table right join right_table.

{
// push left and right
push_down_join(join, Some(limit), Some(limit))
}
JoinType::LeftSemi | JoinType::LeftAnti
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@jackwener jackwener Nov 29, 2022

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Forget RightSemi/RightAnti?

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@liukun4515 liukun4515 Nov 29, 2022

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👍 good catch

if is_no_join_condition(join) =>
{
// push left
push_down_join(join, Some(limit), None)
}
JoinType::RightSemi | JoinType::RightAnti
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@liukun4515 liukun4515 Nov 29, 2022

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add the right semi and right anti
cc @jackwener

if is_no_join_condition(join) =>
{
// push right
push_down_join(join, None, Some(limit))
}
JoinType::Left => push_down_join(join, Some(limit), None),
JoinType::Right => push_down_join(join, None, Some(limit)),
Comment on lines 217 to 218
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@jackwener jackwener Nov 29, 2022

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remove these

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@Dandandan Dandandan Nov 29, 2022

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I think those are the lesser restrictive versions of push down (only to left/right side)

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@liukun4515 liukun4515 Nov 29, 2022

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remove these

JoinType::Left | JoinType::Right | JoinType::Full
                        if is_no_join_condition(join) =>

the on condition is empty, the rule can be applied, but if the on condition is not empty,

     JoinType::Left => push_down_join(join, Some(limit), None),
                    JoinType::Right => push_down_join(join, None, Some(limit)),

can be applied.

cc @jackwener

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@jackwener jackwener Nov 29, 2022

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Understanded it

_ => push_down_join(join, None, None),
Expand Down Expand Up @@ -606,6 +628,142 @@ mod test {
assert_optimized_plan_eq(&outer_query, expected)
}

#[test]
fn limit_should_push_down_join_without_condition() -> Result<()> {
let table_scan_1 = test_table_scan()?;
let table_scan_2 = test_table_scan_with_name("test2")?;
let left_keys: Vec<&str> = Vec::new();
let right_keys: Vec<&str> = Vec::new();
let plan = LogicalPlanBuilder::from(table_scan_1.clone())
.join(
&LogicalPlanBuilder::from(table_scan_2.clone()).build()?,
JoinType::Left,
(left_keys.clone(), right_keys.clone()),
None,
)?
.limit(0, Some(1000))?
.build()?;

let expected = "Limit: skip=0, fetch=1000\
\n Left Join: \
\n Limit: skip=0, fetch=1000\
\n TableScan: test, fetch=1000\
\n Limit: skip=0, fetch=1000\
\n TableScan: test2, fetch=1000";

assert_optimized_plan_eq(&plan, expected)?;

let plan = LogicalPlanBuilder::from(table_scan_1.clone())
.join(
&LogicalPlanBuilder::from(table_scan_2.clone()).build()?,
JoinType::Right,
(left_keys.clone(), right_keys.clone()),
None,
)?
.limit(0, Some(1000))?
.build()?;

let expected = "Limit: skip=0, fetch=1000\
\n Right Join: \
\n Limit: skip=0, fetch=1000\
\n TableScan: test, fetch=1000\
\n Limit: skip=0, fetch=1000\
\n TableScan: test2, fetch=1000";

assert_optimized_plan_eq(&plan, expected)?;

let plan = LogicalPlanBuilder::from(table_scan_1.clone())
.join(
&LogicalPlanBuilder::from(table_scan_2.clone()).build()?,
JoinType::Full,
(left_keys.clone(), right_keys.clone()),
None,
)?
.limit(0, Some(1000))?
.build()?;

let expected = "Limit: skip=0, fetch=1000\
\n Full Join: \
\n Limit: skip=0, fetch=1000\
\n TableScan: test, fetch=1000\
\n Limit: skip=0, fetch=1000\
\n TableScan: test2, fetch=1000";

assert_optimized_plan_eq(&plan, expected)?;

let plan = LogicalPlanBuilder::from(table_scan_1.clone())
.join(
&LogicalPlanBuilder::from(table_scan_2.clone()).build()?,
JoinType::LeftSemi,
(left_keys.clone(), right_keys.clone()),
None,
)?
.limit(0, Some(1000))?
.build()?;

let expected = "Limit: skip=0, fetch=1000\
\n LeftSemi Join: \
\n Limit: skip=0, fetch=1000\
\n TableScan: test, fetch=1000\
\n TableScan: test2";

assert_optimized_plan_eq(&plan, expected)?;

let plan = LogicalPlanBuilder::from(table_scan_1.clone())
.join(
&LogicalPlanBuilder::from(table_scan_2.clone()).build()?,
JoinType::LeftAnti,
(left_keys.clone(), right_keys.clone()),
None,
)?
.limit(0, Some(1000))?
.build()?;

let expected = "Limit: skip=0, fetch=1000\
\n LeftAnti Join: \
\n Limit: skip=0, fetch=1000\
\n TableScan: test, fetch=1000\
\n TableScan: test2";

assert_optimized_plan_eq(&plan, expected)?;

let plan = LogicalPlanBuilder::from(table_scan_1.clone())
.join(
&LogicalPlanBuilder::from(table_scan_2.clone()).build()?,
JoinType::RightSemi,
(left_keys.clone(), right_keys.clone()),
None,
)?
.limit(0, Some(1000))?
.build()?;

let expected = "Limit: skip=0, fetch=1000\
\n RightSemi Join: \
\n TableScan: test\
\n Limit: skip=0, fetch=1000\
\n TableScan: test2, fetch=1000";

assert_optimized_plan_eq(&plan, expected)?;

let plan = LogicalPlanBuilder::from(table_scan_1)
.join(
&LogicalPlanBuilder::from(table_scan_2).build()?,
JoinType::RightAnti,
(left_keys, right_keys),
None,
)?
.limit(0, Some(1000))?
.build()?;

let expected = "Limit: skip=0, fetch=1000\
\n RightAnti Join: \
\n TableScan: test\
\n Limit: skip=0, fetch=1000\
\n TableScan: test2, fetch=1000";

assert_optimized_plan_eq(&plan, expected)
}

#[test]
fn limit_should_push_down_left_outer_join() -> Result<()> {
let table_scan_1 = test_table_scan()?;
Expand Down