Precise calculation micros per month

Author: MaxGekk

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

@@ -21,17 +21,24 @@ import org.apache.spark.sql.types.Decimal
 import org.apache.spark.unsafe.types.CalendarInterval
 
 object IntervalUtils {
-  val MONTHS_PER_YEAR: Int = 12
-  val MONTHS_PER_QUARTER: Byte = 3
-  val YEARS_PER_MILLENNIUM: Int = 1000
-  val YEARS_PER_CENTURY: Int = 100
-  val YEARS_PER_DECADE: Int = 10
-  val MICROS_PER_HOUR: Long = DateTimeUtils.MILLIS_PER_HOUR * DateTimeUtils.MICROS_PER_MILLIS
-  val MICROS_PER_MINUTE: Long = DateTimeUtils.MILLIS_PER_MINUTE * DateTimeUtils.MICROS_PER_MILLIS
-  val DAYS_PER_MONTH: Byte = 30
-  val MICROS_PER_MONTH: Long = DAYS_PER_MONTH * DateTimeUtils.SECONDS_PER_DAY
-  /* 365.25 days per year assumes leap year every four years */
-  val MICROS_PER_YEAR: Long = (36525L * DateTimeUtils.MICROS_PER_DAY) / 100
+  final val MONTHS_PER_YEAR: Int = 12
+  final val MONTHS_PER_QUARTER: Byte = 3
+  final val YEARS_PER_MILLENNIUM: Int = 1000
+  final val YEARS_PER_CENTURY: Int = 100
+  final val YEARS_PER_DECADE: Int = 10
+  final val MICROS_PER_HOUR: Long = DateTimeUtils.MILLIS_PER_HOUR * DateTimeUtils.MICROS_PER_MILLIS
+  final val MICROS_PER_MINUTE: Long = {
+    DateTimeUtils.MILLIS_PER_MINUTE * DateTimeUtils.MICROS_PER_MILLIS
+  }
+  // The average year of the Gregorian calendar 365.2425 days long, see
+  // https://en.wikipedia.org/wiki/Gregorian_calendar
+  // Leap year occurs every 4 years, except for years that are divisible by 100
+  // and not divisible by 400. So, the mean length of of the Gregorian calendar year is:
+  //  1 mean year = (365 + 1/4 - 1/100 + 1/400) days = 365.2425 days
+  // The mean year length in seconds is:
+  //  60 * 60 * 24 * 365.2425 = 31556952.0 = 12 * 2629746
+  final val SECONDS_PER_MONTH: Int = 2629746
+  final val MICROS_PER_MONTH: Long = SECONDS_PER_MONTH * DateTimeUtils.MICROS_PER_SECOND
 
   def getYears(interval: CalendarInterval): Int = {
     interval.months / MONTHS_PER_YEAR
@@ -83,9 +90,8 @@ object IntervalUtils {
 
   // Returns total number of seconds with microseconds fractional part in the given interval.
   def getEpoch(interval: CalendarInterval): Decimal = {
-    var result = interval.microseconds
-    result += MICROS_PER_YEAR * (interval.months / MONTHS_PER_YEAR)
-    result += MICROS_PER_MONTH * (interval.months % MONTHS_PER_YEAR)
+    val monthsDurationUs = Math.multiplyExact(interval.months, MICROS_PER_MONTH)
+    val result = Math.addExact(interval.microseconds, monthsDurationUs)
     Decimal(result, 18, 6)
   }
 }

sql/core/src/test/resources/sql-tests/results/date_part.sql.out

@@ -857,7 +857,7 @@ select date_part('epoch', c) from t2
 -- !query 106 schema
 struct<date_part('epoch', t2.`c`):decimal(18,6)>
 -- !query 106 output
-31873892788.332003
+31896905999.004003
 
 
 -- !query 107