[GitHub] [spark] gengliangwang commented on a change in pull request #25605: [SPARK-28730][SPARK-28495][SQL][FOLLOW-UP] Revise the doc of option spark.sql.storeAssignmentPolicy

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gengliangwang commented on a change in pull request #25605: [SPARK-28730][SPARK-28495][SQL][FOLLOW-UP] Revise the doc of option spark.sql.storeAssignmentPolicy
URL: https://github.com/apache/spark/pull/25605#discussion_r318464933
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
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 @@ -1643,12 +1643,16 @@ object SQLConf {
   val STORE_ASSIGNMENT_POLICY =
     buildConf("spark.sql.storeAssignmentPolicy")
       .doc("When inserting a value into a column with different data type, Spark will perform " +
-        "type coercion. Currently we support 3 policies for the type coercion rules: ansi, " +
-        "legacy and strict. With ansi policy, Spark performs the type coercion as per ANSI SQL. " +
-        "With legacy policy, Spark allows casting any value to any data type. " +
-        "The legacy policy is the only behavior in Spark 2.x and it is compatible with Hive. " +
+        "type coercion. Currently, we support 3 policies for the type coercion rules: ANSI, " +
+        "legacy and strict. With ANSI policy, Spark performs the type coercion as per ANSI SQL. " +
+        "In practice, it follows PostgreSQL. It disallows certain unreasonable type conversions " +
 
 Review comment:
   This is because the ANSI store assignment rules are high level specified.
   Take the `string` -> `int` as an example, it doesn't explicit specifying whether string columns can be converted to integer columns. I tried Mysql/Oracle and the conversion is allowed, while PostgreSQL disallows it. 
   As we are porting tests from PostgreSQL (SPARK-27763), Spark tends to follow PostgreSQL in behaviors. On the table insertion, I think the rules of PostgreSQL are stricter and more reasonable comparing to MySQL/Oracle. For more details see the docs summarized by @maropu  https://docs.google.com/spreadsheets/u/1/d/1I4pl1TIe8u0G4sGSc7OCItySfvK0pOTkU4IPw5Z0Qck/edit?usp=sharing .
   
   So I think we should point out that we are following PostgreSQL particularly. 
   

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