General framework to decorrelate the subqueries
- Dominant language
- Rust
- Stars
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Description
**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
In the current DataFusion, it has very limited support for correlated subqueries. It can only decorrelate the (NOT) IN/Exists predicate subqueries to Semi/Anti Joins. Even in the simplest IN/Exists cases, if the correlated expressions are not in the Filter/Join conditions, the current decorrelate rules will not support them.
In the paper "Unnesting Arbitrary Queries" by T. Neumann; A. Kemper
(http://www.btw-2015.de/res/proceedings/Hauptband/Wiss/Neumann-Unnesting_Arbitrary_Querie.pdf). It raise a mechanism to unnest arbitrary queries. This was already implemented by the Hyper DB:
For example:
select * from orders
where 1 in (select 1 from part left join (select l_partkey from lineitem where o_orderkey = 2) lineitem on p_partkey = lineitem.l_partkey)
https://hyper-db.de/interface.html#
Both SparkSQL and PostgreSQL do not support decorrelate such kind of queries.
**Describe the solution you'd like**
**Describe alternatives you've considered**
**Additional context**
Contributor guide
Research direction
Start by reading DataFusion's existing correlated-subquery decorrelation behavior and the referenced "Unnesting Arbitrary Queries" paper. Use the SQL example in the issue as a target case, and verify that arbitrary correlated subqueries beyond current IN/EXISTS semi- and anti-join support are handled correctly.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust, sql
- Domain
- databases
- Issue type
- Feature
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Quiet
- Clarity
- Needs clarification
- Newbie friendliness
- 30/100