[GitHub] [spark] MaxGekk commented on a change in pull request #28643: [SPARK-31762][SQL][FOLLOWUP] Avoid double formatting in legacy fractional formatter

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MaxGekk commented on a change in pull request #28643:
URL: https://github.com/apache/spark/pull/28643#discussion_r431126438



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File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/TimestampFormatter.scala
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@@ -127,16 +127,37 @@ class FractionTimestampFormatter(zoneId: ZoneId)
   override protected lazy val formatter = DateTimeFormatterHelper.fractionFormatter
 
   // The new formatter will omit the trailing 0 in the timestamp string, but the legacy formatter
-  // can't. Here we borrow the code from Spark 2.4 DateTimeUtils.timestampToString to omit the
-  // trailing 0 for the legacy formatter as well.
+  // can't. Here we use the legacy formatter to format the given timestamp up to seconds fractions,
+  // and custom implementation to format the fractional part without trailing zeros.
   override def format(ts: Timestamp): String = {
-    val timestampString = ts.toString
     val formatted = legacyFormatter.format(ts)
-
-    if (timestampString.length > 19 && timestampString.substring(19) != ".0") {
-      formatted + timestampString.substring(19)
-    } else {
+    var nanos = ts.getNanos
+    if (nanos == 0) {
       formatted
+    } else {
+      // Formats non-zero seconds fraction w/o trailing zeros. For example:

Review comment:
       I borrowed the implementation from JDK 11 java.sql.Timestamp.toString:
   https://github.com/AdoptOpenJDK/openjdk-jdk11/blob/master/src/java.sql/share/classes/java/sql/Timestamp.java#L266-L313




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