goldmedal opened a new pull request, #15334:
URL: https://github.com/apache/datafusion/pull/15334
## Which issue does this PR close?
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- Closes #13486
## Rationale for this change
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When working on unparsing the plan optimized by `ScalarSubqueryToJoin`, I
noticed it could generate a LEFT JOIN without the join condition.
```
Projection: customer.c_custkey [c_custkey:Int64]
Filter: customer.c_custkey = __scalar_sq_1.max(orders.o_custkey)
[c_custkey:Int64, c_name:Utf8, max(orders.o_custkey):Int64;N]
Left Join: [c_custkey:Int64, c_name:Utf8, max(orders.o_custkey):Int64;N]
TableScan: customer [c_custkey:Int64, c_name:Utf8]
SubqueryAlias: __scalar_sq_1 [max(orders.o_custkey):Int64;N]
Projection: max(orders.o_custkey) [max(orders.o_custkey):Int64;N]
Aggregate: groupBy=[[]], aggr=[[max(orders.o_custkey)]]
[max(orders.o_custkey):Int64;N]
TableScan: orders [o_orderkey:Int64, o_custkey:Int64,
o_orderstatus:Utf8, o_totalprice:Float64;N]
```
The unparsing result will look like
```sql
SELECT
customer.c_custkey
FROM
customer
LEFT JOIN (
SELECT
max(orders.o_custkey)
FROM
orders
) AS __scalar_sq_1
WHERE
(
customer.c_custkey = __scalar_sq_1."max(orders.o_custkey)"
)
```
Most databases don't allow the join without the condition besides the cross
join.
## DuckDB
```
D select * from c1 join c2;
Parser Error:
syntax error at or near ";"
```
## Postgres
```
test=# select * from t1 join t2;
ERROR: syntax error at or near ";"
LINE 1: select * from t1 join t2;
```
## DataFusion
```
> select * from t1 join t2;
+----+----+
| c1 | c1 |
+----+----+
| 1 | 1 |
+----+----+
1 row(s) fetched.
Elapsed 0.005 seconds.
```
I prefer to align this behavior with others. It makes the logical plan
created by DataFusion reliable.
#13486 also mentioned that `join` without condition isn't a valid SQL.
## What changes are included in this PR?
- Enforce the join to re
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sometimes worth providing a summary of the individual changes in this PR.
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- Enforce JOIN plan to require conditions when built
- ScalarSubqueryToJoin will generate the `LEFT JOIN` plan with the condition
`True`.
## Are these changes tested?
yes
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## Are there any user-facing changes?
The join syntax without condition is not allowed anymore.
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