jonahgao commented on issue #9394:
URL:
https://github.com/apache/arrow-datafusion/issues/9394#issuecomment-1970333727
I think this might be caused by join reorder, execute the following two
queries in DataFusion
```sh
DataFusion CLI v36.0.0
❯ create table a1 as values
([1,2,3], 1),
([4,5,6], 2);
❯ create table a2 as values
(1),
(2);
❯ create table b as values (10), (20), (30);
❯ select * from a1,b;
+-----------+---------+---------+
| column1 | column2 | column1 |
+-----------+---------+---------+
| [1, 2, 3] | 1 | 10 |
| [4, 5, 6] | 2 | 10 |
| [1, 2, 3] | 1 | 20 |
| [4, 5, 6] | 2 | 20 |
| [1, 2, 3] | 1 | 30 |
| [4, 5, 6] | 2 | 30 |
+-----------+---------+---------+
6 rows in set. Query took 0.006 seconds.
❯ select * from a2,b;
+---------+---------+
| column1 | column1 |
+---------+---------+
| 1 | 10 |
| 1 | 20 |
| 1 | 30 |
| 2 | 10 |
| 2 | 20 |
| 2 | 30 |
+---------+---------+
6 rows in set. Query took 0.006 seconds.
```
The result of query `select * from a2,b` is the same to DuckDB, but query
`select * from a1,b` is different.
This is because query `select * from a1,b` has been reordered, and we can
confirm it with `explain verbose`.
Differentiate the left and right tables by the number of rows in statistics.
This reordering behavior is triggered by the bytes size of tables.
https://github.com/apache/arrow-datafusion/blob/ca37ce37933f7874d404364cb8c23438baceb46d/datafusion/core/src/physical_optimizer/join_selection.rs#L73-L77
Because we need to load the left side into memory, this behavior should be
reasonable.
Additionally, I've also noticed that there is a TODO here, which might be
helpful.
> // TODO: In PrestoSQL, the optimizer flips join sides only if one side is
much smaller than the other by more than SIZE_DIFFERENCE_THRESHOLD times, by
default is is 8 times.
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