[SPARK-29888][SQL] new interval string parser shall handle numeric with only fractional part

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/IntervalUtils.scala

@@ -496,6 +496,17 @@ object IntervalUtils {
           state = TRIM_BEFORE_SIGN
         case TRIM_BEFORE_SIGN => trimToNextState(b, SIGN)
         case SIGN =>
+          currentValue = 0
+          fraction = 0
+          // We preset next state from SIGN to TRIM_BEFORE_VALUE. If we meet '.' in the SIGN state,
+          // it means that the interval value we deal with here is a numeric with only fractional
+          // part, such as '.11 second', which can be parsed to 0.11 seconds. In this case, we need
+          // to reset next state to `VALUE_FRACTIONAL_PART` to go parse the fraction part of the
+          // interval value.
+          state = TRIM_BEFORE_VALUE
+          // We preset the scale to an invalid value to track fraction presence in the UNIT_BEGIN
+          // state. If we meet '.', the scale become valid for the VALUE_FRACTIONAL_PART state.
+          fractionScale = -1
           b match {
             case '-' =>
               isNegative = true
@@ -505,14 +516,13 @@ object IntervalUtils {
               i += 1
             case _ if '0' <= b && b <= '9' =>
               isNegative = false
+            case '.' =>
+              isNegative = false
+              fractionScale = (NANOS_PER_SECOND / 10).toInt
+              i += 1
+              state = VALUE_FRACTIONAL_PART
             case _ => return null
           }
-          currentValue = 0
-          fraction = 0
-          // Sets the scale to an invalid value to track fraction presence
-          // in the BEGIN_UNIT_NAME state
-          fractionScale = -1
-          state = TRIM_BEFORE_VALUE
         case TRIM_BEFORE_VALUE => trimToNextState(b, VALUE)
         case VALUE =>
           b match {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/util/IntervalUtilsSuite.scala

@@ -106,6 +106,7 @@ class IntervalUtilsSuite extends SparkFunSuite {
     checkFromString("-1.5 seconds", new CalendarInterval(0, 0, -1500000))
     // truncate nanoseconds to microseconds
     checkFromString("0.999999999 seconds", new CalendarInterval(0, 0, 999999))
+    checkFromString(".999999999 seconds", new CalendarInterval(0, 0, 999999))
     checkFromInvalidString("0.123456789123 seconds", "Error parsing interval string")
   }