[GitHub] [spark] viirya commented on a change in pull request #29950: [SPARK-32945][SQL] Avoid collapsing projects if reaching max allowed common exprs

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

viirya commented on a change in pull request #29950:
URL: https://github.com/apache/spark/pull/29950#discussion_r522519012



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File path: sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
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@@ -1963,6 +1963,27 @@ object SQLConf {
     .booleanConf
     .createWithDefault(true)
 
+  val MAX_COMMON_EXPRS_IN_COLLAPSE_PROJECT =
+    buildConf("spark.sql.optimizer.maxCommonExprsInCollapseProject")
+      .doc("An integer number indicates the maximum allowed number of common input expression " +
+        "from lower Project when being collapsed into upper Project by optimizer rule " +
+        "`CollapseProject`. Normally `CollapseProject` will collapse adjacent Project " +
+        "and merge expressions. But in some edge cases, expensive expressions might be " +
+        "duplicated many times in merged Project by this optimization. This config sets " +
+        "a maximum number. Once an expression is duplicated more than this number " +
+        "if merging two Project, Spark SQL will skip the merging. Note that normally " +
+        "in whole-stage codegen Project operator will de-duplicate expressions internally, " +
+        "but in edge cases Spark cannot do whole-stage codegen and fallback to interpreted " +
+        "mode. In such cases, users can use this config to avoid duplicate expressions. " +
+        "Note that even users exclude `CollapseProject` rule using " +
+        "`spark.sql.optimizer.excludedRules`, at physical planning phase Spark will still " +
+        "collapse projections. This config is also effective on collapsing projections in " +
+        "the physical planning.")
+      .version("3.1.0")
+      .intConf
+      .checkValue(_ > 0, "The value of maxCommonExprsInCollapseProject must be larger than zero.")
+      .createWithDefault(20)

Review comment:
       Sure. It is safer.




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