[GitHub] [flink] Airblader commented on a change in pull request #17481: [FLINK-24387][table-planner] Support JSON_STRING.

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Airblader commented on a change in pull request #17481:
URL: https://github.com/apache/flink/pull/17481#discussion_r728893668



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File path: flink-table/flink-table-planner/src/main/java/org/apache/flink/table/planner/functions/sql/FlinkSqlOperatorTable.java
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@@ -1145,6 +1145,16 @@ public boolean isDeterministic() {
     public static final SqlFunction JSON_EXISTS = SqlStdOperatorTable.JSON_EXISTS;
     public static final SqlFunction JSON_VALUE = SqlStdOperatorTable.JSON_VALUE;
     public static final SqlFunction JSON_QUERY = SqlStdOperatorTable.JSON_QUERY;
+    public static final SqlFunction JSON_STRING =
+            new SqlFunction(
+                    "JSON_STRING",
+                    SqlKind.OTHER_FUNCTION,
+                    ReturnTypes.cascade(
+                            ReturnTypes.explicit(SqlTypeName.VARCHAR),
+                            SqlTypeTransforms.LEAST_NULLABLE),

Review comment:
       Calcite's `TO_NULLABLE` actually has a (to me) surprising behavior that breaks tests: for structured types – like `ROW` – it also checks the nullability of all of the inner fields. This means that `ROW<r0 STRING> NOT NULL` would be considered nullable. Big "wat?" moment to me, and anxious to have someone explain to me why that makes sense.
   
   In any case, `LEAST_NULLABLE` does not have this behavior, and otherwise does the same thing in our case here.




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