liukun4515 commented on code in PR #4411:
URL: https://github.com/apache/arrow-datafusion/pull/4411#discussion_r1036635219
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datafusion/optimizer/src/limit_push_down.rs:
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@@ -192,6 +196,24 @@ impl OptimizerRule for LimitPushDown {
LogicalPlan::Join(join) => {
let limit = fetch + skip;
let new_join = match join.join_type {
+ JoinType::Left | JoinType::Right | JoinType::Full
+ if is_no_join_condition(join) =>
Review Comment:
> I wonder how often there will be no join conditions 🤔 that effectively
then becomes a CROSS JOIN
>
> To be clear I think the optimization is still correct, I just wonder how
often it will appear in real queries
Good question for this, in the current framework of optimizer in the
datafusion, the `join` query will be converted to the `cross join` by other
rule.
The `limit push down` rule just do the best to optimize the plan.
Some SQL are generated by the program, and the `join type` and `on
condition` are added by the program, it will generate sql like ` select * from
left_table right join right_table`.
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