feat: change Expr OuterReferenceColumn and Alias to Box type for reducing expr struct size #16771
+348
−323
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
github-actions
bot
added
sql
zhuqi-lucas
changed the title
alamb
reviewed
Jul 14, 2025
Contributor
alamb
left a comment
alamb
left a comment
There was a problem hiding this comment.
Thanks @zhuqi-lucas -- this looks quite nice
I think it is an API change so I will mark the PR as such and I think it is a good improvement
However, given it is an API change I think we should
- Wait until DataFusion 50.0.0 (let's not make any more changes in DataFusion 49 that we are starting to prepare for)
- Update the upgrading guide in https://datafusion.apache.org/library-user-guide/upgrading.html to explain to people hw to change things
| 10)--------------Projection: generate_series(Int64(1), CAST(outer_ref(t1.t1_int) AS Int64)) AS __unnest_placeholder(generate_series(Int64(1),outer_ref(t1.t1_int))) | ||
| 11)----------------EmptyRelation | ||
| physical_plan_error This feature is not implemented: Physical plan does not support logical expression OuterReferenceColumn(UInt32, Column { relation: Some(Bare { table: "t1" }), name: "t1_int" }) | ||
| physical_plan_error This feature is not implemented: Physical plan does not support logical expression OuterReferenceColumn((UInt32, Column { relation: Some(Bare { table: "t1" }), name: "t1_int" })) |
Contributor
There was a problem hiding this comment.
do you know why the plan changes? I don't think the extra () really adds much -- can we restore the original?
Contributor
Author
There was a problem hiding this comment.
I changed it to OuterReference in latest PR, so it will use OuterReference instead of this change now, thanks!
Contributor
Author
There was a problem hiding this comment.
Next step, we may can add debug/fmt for OuterReference to restore the original behaviour.
Contributor
Author
There was a problem hiding this comment.
Restore all slt in latest PR, no diff from main branch.
datafusion/expr/src/expr.rs
Outdated
| // `Box`ing the fields to make `Expr` smaller | ||
| // See https://github.com/apache/datafusion/issues/16199 for details | ||
| assert_eq!(size_of::<Expr>(), 128); | ||
| assert_eq!(size_of::<Expr>(), 112); |
Contributor
There was a problem hiding this comment.
This change saves 16 bytes per Expr. Nice.
I will run some planning benchmarks and see if we can see any difference
Contributor
Author
There was a problem hiding this comment.
Thank you @alamb , i am doing more amazing experiment, try to reduce from 128 to 80, so we can save 48 bytes per Expr!
| pub enum Expr { | ||
| /// An expression with a specific name. | ||
| Alias(Alias), | ||
| Alias(Box<Alias>), |
Contributor
There was a problem hiding this comment.
Another change that might be a bit less impactful would be to box the fields of Alias instead
So Alias {
expr: Box
..
}
It may be just as bad / worse though
datafusion/expr/src/expr.rs
Outdated
| /// A placeholder which holds a reference to a qualified field | ||
| /// in the outer query, used for correlated sub queries. | ||
| OuterReferenceColumn(DataType, Column), | ||
| OuterReferenceColumn(Box<(DataType, Column)>), |
Contributor
There was a problem hiding this comment.
If we are going to make this change anyways, can we also pull this into a named struct rather than a unnamed tuple
like
struct OuterReference {
// fields here
}
enum Expr {
...
OuterReferenceColumn(OuterReference),
...
}
Contributor
Author
There was a problem hiding this comment.
Thank you @alamb for this good suggestion, addressed in latest PR.
Contributor
|
🤖 |
Contributor
|
🤖: Benchmark completed Details
|
Contributor
Author
Thank you @alamb for review and i agree that we can do this for Datafusion 50.0.0, and since we have enough time, i want to do more improvement, try to reduce Expr more, from size 128 to 80, i am in progress now. |
zhuqi-lucas
changed the title
Contributor
Author
|
Updated: Successfully changed the size from 128 to 80 in latest PR. |
Contributor
|
🤖 |
Contributor
|
🤖: Benchmark completed Details
|
Contributor
|
🤖 |
Contributor
|
🤖: Benchmark completed Details
|
Contributor
Author
Thank you @alamb for benchmark, interesting, it seems the performance decrease from 112 size to 80, i will investigate it, if i can't find the root cause, i will revert to 112 bytes first! |
Contributor
Author
|
Strange, i can't reproduce this in my local: with_param_values_many_columns 1.00 129.8±0.74µs ? ?/sec 1.06 137.2±0.79µs |
kosiew
reviewed
Jul 16, 2025
Contributor
kosiew
left a comment
kosiew
left a comment
There was a problem hiding this comment.
👍
Just some nit for your review.
Comment on lines
260
to
261
| Expr::Alias(alias_box) => { | ||
| // alias_box: Box<Alias> | ||
| let Alias { expr, name, .. } = *alias_box; |
Contributor
There was a problem hiding this comment.
nit - use boxed_alias instead, for consistency
Contributor
Author
There was a problem hiding this comment.
Thank you @kosiew , addressed in latest PR.
Comment on lines
140
to
142
| if let Expr::Alias(alias_box) = expr { | ||
| // alias_box: &Box<Alias>, so alias_box.as_ref() is &Alias | ||
| let alias: &Alias = alias_box.as_ref(); |
Contributor
There was a problem hiding this comment.
same as above - boxed_alias
datafusion/expr/src/expr_schema.rs
Outdated
Comment on lines
249
to
248
| Expr::Alias(alias_box) => { | ||
| let Alias { expr, .. } = alias_box.as_ref(); |
datafusion/expr/src/utils.rs
Outdated
| stack.push(*left); | ||
| } | ||
| Expr::Alias(Alias { expr, .. }) => stack.push(*expr), | ||
| Expr::Alias(alias_box) => stack.push(*alias_box.expr), |
Contributor
Author
Thank you @kosiew for review and suggestion, addressed in latest PR. |
Member
|
After opening the DF50.0.0 release issue, you can add it to the list |
Contributor
I will rerun |
Contributor
|
🤖 |
zhuqi-lucas
pushed a commit
to zhuqi-lucas/arrow-datafusion
that referenced
this pull request
Feb 10, 2026
Box three large variants in the Expr enum to reduce its memory footprint: - Alias(Alias) -> Alias(Box<Alias>) - Column(Column) -> Column(Box<Column>) - OuterReferenceColumn(FieldRef, Column) -> OuterReferenceColumn(Box<OuterReference>) This reduces the Expr enum size from 112 bytes to 80 bytes (28% reduction), which improves cache locality and reduces memory usage across the query engine since Expr is one of the most frequently used types. Introduces a new OuterReference named struct to replace the tuple variant, improving readability and maintainability. Based on apache#16771, rebased and updated against the latest master. https://claude.ai/code/session_018FV5mepXRLnBVzzWv8ihgb
zhuqi-lucas
force-pushed
the
reduce_expr_size
branch
from
569079b to
b5716d8
Compare
Contributor
There was a problem hiding this comment.
Pull request overview
This PR reduces the size of the Expr enum from 128 bytes to 80 bytes by boxing the Alias and OuterReferenceColumn variants. This is a continuation of ongoing efforts to optimize memory usage in DataFusion's expression system.
Changes:
- Changed
Expr::Alias(Alias)toExpr::Alias(Box<Alias>) - Changed
Expr::OuterReferenceColumn(FieldRef, Column)toExpr::OuterReferenceColumn(Box<OuterReference>)whereOuterReferenceis a new struct containing the field and column - Updated all pattern matching and construction sites throughout the codebase to work with the boxed types
- Updated test expectations for error messages that display
OuterReferenceColumn - Updated the size test to verify the new 80-byte size
Reviewed changes
Copilot reviewed 58 out of 58 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| datafusion/expr/src/expr.rs | Core type changes: boxed Alias and OuterReferenceColumn, added OuterReference struct, updated size test |
| datafusion/expr/src/lib.rs | Exported OuterReference type |
| datafusion/expr/src/*.rs | Updated pattern matching and expression construction for boxed types |
| datafusion/optimizer/src/*.rs | Updated optimizer rules to work with boxed types |
| datafusion/sql/src/*.rs | Updated SQL planning to construct boxed expressions |
| datafusion/substrait/src/*.rs | Updated Substrait conversion to handle boxed types |
| datafusion/proto/src/*.rs | Updated protobuf serialization/deserialization |
| datafusion/physical-expr/src/*.rs | Updated physical expression planning |
| datafusion/core/src/*.rs | Updated core dataframe and execution code |
| datafusion/functions*/src/*.rs | Updated function implementations |
| datafusion/**/tests/*.rs | Updated test code and test expectations |
| datafusion/sqllogictest/test_files/*.slt | Updated expected error messages for OuterReferenceColumn display format |
💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.
Contributor
Author
|
run sql_planner |
Contributor
Author
|
run benchmark sql_planner |
|
🤖 |
Contributor
Author
|
I revive this PR by claude help. cc @alamb And let's see if benchmark will show improvement. |
|
🤖: Benchmark completed Details
|
Contributor
Author
|
Seems still no improvement in latest benchmark. |
zhuqi-lucas
force-pushed
the
reduce_expr_size
branch
from
edf1eaf to
3941aba
Compare
github-actions
bot
added
common
Contributor
Author
|
run benchmark sql_planner |
Contributor
Author
|
run benchmark sql_planner |
Contributor
|
show benchmark queue |
|
🤖 Hi @alamb, you asked to view the benchmark queue (#16771 (comment)).
|
Contributor
Author
|
Thanks @alamb, i see, there are already 2 in queue.
|
Contributor
👍 -- I just wanted to double check that the runner hadn't gotten stuck again (it sometimes does break / get restarted and needs manual intervention) |
|
🤖 |
|
🤖: Benchmark completed Details
|
|
🤖 |
|
🤖: Benchmark completed Details
|
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
5 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Which issue does this PR close?
Continue to reduce the Expr struct size.
Rationale for this change
Continue to reduce the Expr struct size.
What changes are included in this PR?
Continue to reduce the Expr struct size.
Are these changes tested?
Yes
The size reduce from 128 to 112
Updated:
Continue reduce to 80 now!
Are there any user-facing changes?
No