[GitHub] [arrow-datafusion] liukun4515 commented on a diff in pull request #4411: optimize limit push for join case

[GitBox <gi...@apache.org>]

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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