[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #4276: Reimplement `limit_push_down`

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

alamb commented on code in PR #4276:
URL: https://github.com/apache/arrow-datafusion/pull/4276#discussion_r1026854991


##########
datafusion/optimizer/src/limit_push_down.rs:
##########
@@ -635,13 +481,8 @@ mod test {
             .limit(0, Some(10))?
             .build()?;
 
-        // Should push down the smallest limit
-        // Towards table scan
-        // This rule doesn't replace multiple limits
-        let expected = "Limit: skip=0, fetch=10\
-        \n  Limit: skip=0, fetch=10\
-        \n    Limit: skip=10, fetch=10\
-        \n      TableScan: test, fetch=20";
+        let expected = "Limit: skip=10, fetch=10\

Review Comment:
   Those plans look much nicer



##########
datafusion/optimizer/src/limit_push_down.rs:
##########
@@ -576,8 +424,8 @@ mod test {
 
         // Should push the limit down to table provider
         // When it has a select
-        let expected = "Limit: skip=10, fetch=1000\
-        \n  Projection: test.a\
+        let expected = "Projection: test.a\
+        \n  Limit: skip=10, fetch=1000\
         \n    TableScan: test, fetch=1010";

Review Comment:
   https://github.com/apache/arrow-datafusion/blob/8c02485cfc3d2c48bc48a557db58e3e5a0f75777/datafusion/core/src/datasource/datasource.rs#L66-L70
   
   Implies that in fact we still need the `Limit` node even when it has been pushed to the scan as well



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