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Posted to github@datafusion.apache.org by "simonvandel (via GitHub)" <gi...@apache.org> on 2026/07/14 12:47:41 UTC
[PR] feat: eliminate LEFT JOINs whose right side is unused [datafusion]
simonvandel opened a new pull request, #23566:
URL: https://github.com/apache/datafusion/pull/23566
## Which issue does this PR close?
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- Closes #.
## Rationale for this change
Join elimination is useful in e.g. generated queries or views, where joined columns can end up not being used.
An explanation of when the optimization is valid, can be seen in the docs update for `EliminateJoin`.
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## What changes are included in this PR?
Builds upon the analysis that https://github.com/apache/datafusion/pull/22652 introduced for the `EliminateJoin` optimization pass. We use it to detect when the right-side of a left-join can be removed.
First commit adds test cases.
Second commit adds the optimization change, and shows how it affects the added tests.
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## Are these changes tested?
Yes, SLT additions that test the feature end-to-end.
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## Are there any user-facing changes?
Yes, faster queries!
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Re: [PR] feat: eliminate LEFT JOINs whose right side is unused [datafusion]
Posted by "simonvandel (via GitHub)" <gi...@apache.org>.
simonvandel commented on code in PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#discussion_r3647781648
##########
datafusion/sqllogictest/test_files/joins.slt:
##########
@@ -5608,3 +5608,316 @@ set datafusion.execution.target_partitions = 4;
statement ok
reset datafusion.execution.batch_size;
+
+##########
+# Eliminate unused LEFT JOINs (`EliminateJoin` rule)
Review Comment:
Added some tests in https://github.com/apache/datafusion/pull/23566/commits/342134be61e91045e7534becc29c642e574d17d1
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Re: [PR] feat: eliminate LEFT/RIGHT JOINs with redundant sides [datafusion]
Posted by "simonvandel (via GitHub)" <gi...@apache.org>.
simonvandel commented on PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#issuecomment-5169523040
@neilconway I have resolved the conflicts and decided to keep the tests as-is (coverage in SLT)
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Re: [PR] feat: eliminate LEFT/RIGHT JOINs with redundant sides [datafusion]
Posted by "neilconway (via GitHub)" <gi...@apache.org>.
neilconway merged PR #23566:
URL: https://github.com/apache/datafusion/pull/23566
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Re: [PR] feat: eliminate LEFT JOINs whose right side is unused [datafusion]
Posted by "simonvandel (via GitHub)" <gi...@apache.org>.
simonvandel commented on code in PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#discussion_r3647777324
##########
datafusion/optimizer/src/eliminate_join.rs:
##########
@@ -394,6 +406,30 @@ fn rewrite_join(
let (visible_left, visible_right) = split_join_output_columns(&join, live);
+ // A LEFT JOIN preserves every left row, so when nothing above the join
+ // references the right side's columns and the right side cannot multiply
+ // left rows (the ancestors are duplicate-insensitive, or the right side
+ // is unique on the join keys), the join has no observable effect and can
+ // be replaced by its left input. A join filter cannot prevent this: it
+ // only decides whether a left row is matched or null-padded, and either
+ // way the row is emitted.
+ if join.join_type == JoinType::Left
+ && visible_right.is_empty()
+ && (duplicate_insensitive
+ || side_unique_on_join(
+ join.right.schema(),
+ join.on.iter().map(|(_, right)| right),
+ join.null_equality,
+ ))
Review Comment:
Good idea, I ended up doing a slight refactor instead of your suggestion: https://github.com/apache/datafusion/pull/23566/commits/87a7037cab9d5c274bb40ae9396f1c0dcc5fac0b
so basically pushed the left-join elimination inside `rewritten_join_type`
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Re: [PR] feat: eliminate LEFT/RIGHT JOINs with redundant sides [datafusion]
Posted by "neilconway (via GitHub)" <gi...@apache.org>.
neilconway commented on PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#issuecomment-5147991332
@simonvandel Sorry for the delay in responding here!
> It's true that eliminate_join.rs has no unit tests added in this PR, so in that sense there is a gap. But I have added SLT tests for the optimization which show positive/negative cases. Do you still want unit tests as well, even if that means we now test the code redundantly?
It's probably fine to skip, although the existing `EliminateJoin` codepaths have both unit and SLT tests, so there is precedent. I don't feel super strongly about it though.
If you can you fix the merge conflict and make a call on if you'd like to add unit tests, and I think we're good to land this.
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Re: [PR] feat: eliminate LEFT JOINs whose right side is unused [datafusion]
Posted by "neilconway (via GitHub)" <gi...@apache.org>.
neilconway commented on code in PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#discussion_r3596758392
##########
datafusion/optimizer/src/eliminate_join.rs:
##########
@@ -394,6 +406,30 @@ fn rewrite_join(
let (visible_left, visible_right) = split_join_output_columns(&join, live);
+ // A LEFT JOIN preserves every left row, so when nothing above the join
+ // references the right side's columns and the right side cannot multiply
+ // left rows (the ancestors are duplicate-insensitive, or the right side
+ // is unique on the join keys), the join has no observable effect and can
+ // be replaced by its left input. A join filter cannot prevent this: it
+ // only decides whether a left row is matched or null-padded, and either
+ // way the row is emitted.
+ if join.join_type == JoinType::Left
+ && visible_right.is_empty()
+ && (duplicate_insensitive
+ || side_unique_on_join(
+ join.right.schema(),
+ join.on.iter().map(|(_, right)| right),
+ join.null_equality,
+ ))
Review Comment:
Consider refactoring this into a helper, so we can use it here and also in `rewritten_join_type`?
##########
datafusion/optimizer/src/eliminate_join.rs:
##########
@@ -394,6 +406,30 @@ fn rewrite_join(
let (visible_left, visible_right) = split_join_output_columns(&join, live);
+ // A LEFT JOIN preserves every left row, so when nothing above the join
+ // references the right side's columns and the right side cannot multiply
+ // left rows (the ancestors are duplicate-insensitive, or the right side
+ // is unique on the join keys), the join has no observable effect and can
+ // be replaced by its left input. A join filter cannot prevent this: it
+ // only decides whether a left row is matched or null-padded, and either
+ // way the row is emitted.
+ if join.join_type == JoinType::Left
Review Comment:
What about supporting `RIGHT JOIN`s as well?
##########
datafusion/sqllogictest/test_files/joins.slt:
##########
@@ -5608,3 +5608,316 @@ set datafusion.execution.target_partitions = 4;
statement ok
reset datafusion.execution.batch_size;
+
+##########
+# Eliminate unused LEFT JOINs (`EliminateJoin` rule)
Review Comment:
Can we add test cases for `LIMIT` and `LEFT JOIN LATERAL`?
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Re: [PR] feat: eliminate LEFT JOINs whose right side is unused [datafusion]
Posted by "neilconway (via GitHub)" <gi...@apache.org>.
neilconway commented on code in PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#discussion_r3603579321
##########
datafusion/optimizer/src/eliminate_join.rs:
##########
@@ -394,6 +406,30 @@ fn rewrite_join(
let (visible_left, visible_right) = split_join_output_columns(&join, live);
+ // A LEFT JOIN preserves every left row, so when nothing above the join
+ // references the right side's columns and the right side cannot multiply
+ // left rows (the ancestors are duplicate-insensitive, or the right side
+ // is unique on the join keys), the join has no observable effect and can
+ // be replaced by its left input. A join filter cannot prevent this: it
+ // only decides whether a left row is matched or null-padded, and either
+ // way the row is emitted.
+ if join.join_type == JoinType::Left
Review Comment:
I'm not sure there's a rush to land left-join support super fast 😊 But sure, if you'd prefer to split this into two PRs, that's fine with me.
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Re: [PR] feat: eliminate LEFT JOINs whose right side is unused [datafusion]
Posted by "simonvandel (via GitHub)" <gi...@apache.org>.
simonvandel commented on code in PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#discussion_r3647780311
##########
datafusion/optimizer/src/eliminate_join.rs:
##########
@@ -394,6 +406,30 @@ fn rewrite_join(
let (visible_left, visible_right) = split_join_output_columns(&join, live);
+ // A LEFT JOIN preserves every left row, so when nothing above the join
+ // references the right side's columns and the right side cannot multiply
+ // left rows (the ancestors are duplicate-insensitive, or the right side
+ // is unique on the join keys), the join has no observable effect and can
+ // be replaced by its left input. A join filter cannot prevent this: it
+ // only decides whether a left row is matched or null-padded, and either
+ // way the row is emitted.
+ if join.join_type == JoinType::Left
Review Comment:
I have now implemented right-join support in https://github.com/apache/datafusion/pull/23566/commits/c88044402221d5649dc284a27185cc24f8039579
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Re: [PR] feat: eliminate LEFT/RIGHT JOINs with redundant sides [datafusion]
Posted by "codecov-commenter (via GitHub)" <gi...@apache.org>.
codecov-commenter commented on PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#issuecomment-5074022297
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- Coverage 80.71% 80.71% -0.01%
==========================================
Files 1090 1090
Lines 370384 370395 +11
Branches 370384 370395 +11
==========================================
+ Hits 298961 298965 +4
- Misses 53603 53604 +1
- Partials 17820 17826 +6
```
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Re: [PR] feat: eliminate LEFT/RIGHT JOINs with redundant sides [datafusion]
Posted by "simonvandel (via GitHub)" <gi...@apache.org>.
simonvandel commented on PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#issuecomment-5073845129
Thank you for your review @neilconway. I have responded to your comments, and adjusted the PR description to not mention individual commits.
> The existing analysis has both unit tests and SLTs; can you take a look at whether any of the existing unit tests should be extended to cover outer joins?
It's true that eliminate_join.rs has no unit tests added in this PR, so in that sense there is a gap. But I have added SLT tests for the optimization which show positive/negative cases. Do you still want unit tests as well, even if that means we now test the code redundantly?
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Re: [PR] feat: eliminate LEFT JOINs whose right side is unused [datafusion]
Posted by "simonvandel (via GitHub)" <gi...@apache.org>.
simonvandel commented on code in PR #23566:
URL: https://github.com/apache/datafusion/pull/23566#discussion_r3597299766
##########
datafusion/optimizer/src/eliminate_join.rs:
##########
@@ -394,6 +406,30 @@ fn rewrite_join(
let (visible_left, visible_right) = split_join_output_columns(&join, live);
+ // A LEFT JOIN preserves every left row, so when nothing above the join
+ // references the right side's columns and the right side cannot multiply
+ // left rows (the ancestors are duplicate-insensitive, or the right side
+ // is unique on the join keys), the join has no observable effect and can
+ // be replaced by its left input. A join filter cannot prevent this: it
+ // only decides whether a left row is matched or null-padded, and either
+ // way the row is emitted.
+ if join.join_type == JoinType::Left
Review Comment:
Sure, would you be okay with this being a follow-up task? Then we could land the left-join support faster
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