robinyqiu commented on a change in pull request #12348:
URL: https://github.com/apache/beam/pull/12348#discussion_r466188613



##########
File path: 
sdks/java/extensions/sql/zetasql/src/main/java/org/apache/beam/sdk/extensions/sql/zetasql/translation/ExpressionConverter.java
##########
@@ -850,6 +848,25 @@ private RexNode convertSimpleValueToRexNode(TypeKind kind, 
Value value) {
         // TODO: Doing micro to mills truncation, need to throw exception.
         ret = rexBuilder().makeLiteral(convertTimeValueToTimeString(value), 
timeType, false);
         break;
+      case TYPE_DATETIME:
+        // Cannot simply call makeTimestampWithLocalTimeZoneLiteral() for 
ZetaSQL DATETIME type
+        // because later it will be unparsed to the string representation of 
timestamp (e.g. "SELECT
+        // DATETIME '2008-12-25 15:30:00'" will be unparsed to "SELECT 
TIMESTAMP '2008-12-25
+        // 15:30:00:000000'"). So we create a wrapper function here such that 
we can later recognize
+        // it and customize its unparsing in BeamBigQuerySqlDialect.
+        ret =
+            rexBuilder()
+                .makeCall(
+                    SqlOperators.createZetaSqlFunction(
+                        BeamBigQuerySqlDialect.DATETIME_LITERAL_FUNCTION,

Review comment:
       We do not expect `toString()` to be called. You need to override 
`unparse()`.
   
   You can look at other similar overriding classes in that file and see how it 
works. Like 
https://github.com/apache/beam/blob/6fdde4f4eab72b49b10a8bb1cb3be263c5c416b5/sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/meta/provider/bigquery/BeamSqlUnparseContext.java#L107
 




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