[GitHub] [spark] allisonwang-db commented on a change in pull request #32303: [SPARK-34382][SQL] Support LATERAL subqueries

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allisonwang-db commented on a change in pull request #32303:
URL: https://github.com/apache/spark/pull/32303#discussion_r619415048



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File path: sql/core/src/test/resources/sql-tests/inputs/join-lateral.sql
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@@ -0,0 +1,84 @@
+-- Test cases for lateral join
+
+CREATE VIEW t1(c1, c2) AS VALUES (0, 1), (1, 2);
+CREATE VIEW t2(c1, c2) AS VALUES (0, 2), (0, 3);
+
+-- lateral join with single column select
+SELECT * FROM t1, LATERAL (SELECT c1);

Review comment:
       > Is this compiled into a join query? We cannot compile it into a simple scan query like PostgreSQL?
   > 
   > ```
   > postgres=# explain SELECT * FROM t1, LATERAL (SELECT c1) t2;
   >                           QUERY PLAN                          
   > --------------------------------------------------------------
   >  Values Scan on "*VALUES*"  (cost=0.00..0.03 rows=2 width=12)
   > (1 row)
   > ```
   
   Yes it should be rewritten as a simple scan query`SELECT c1 FROM t1` instead of a join query. I am thinking about adding an optimizer rule for OneRowRelation. This can be applied to correlated subqueries as well.




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