[GitHub] [iceberg] rdblue commented on a change in pull request #1127: ORC: fix date metrics to adjust milliseconds to local timezone

[GitBox <gi...@apache.org>]

rdblue commented on a change in pull request #1127:
URL: https://github.com/apache/iceberg/pull/1127#discussion_r446425293



##########
File path: orc/src/main/java/org/apache/iceberg/orc/OrcMetrics.java
##########
@@ -158,8 +158,7 @@ private static Metrics buildOrcMetrics(final long numOfRows, final TypeDescripti
           .orElse(null);
     } else if (columnStats instanceof DateColumnStatistics) {
       min = Optional.ofNullable(((DateColumnStatistics) columnStats).getMinimum())
-          .map(minStats -> DateTimeUtil.daysFromDate(
-              DateTimeUtil.EPOCH.plus(minStats.getTime(), ChronoUnit.MILLIS).toLocalDate()))
+          .map(minStats -> DateTimeUtil.daysFromDate(((Date) minStats).toLocalDate()))

Review comment:
       Javadoc for `Date.toLocalDate` says that the LocalDate returned is "in local time zone". It is implemented like this:
   
   ```java
   return LocalDate.of(getYear() + 1900, getMonth() + 1, getDate());
   ```
   
   And, those getter methods use the current zone. So this cannot be correct because the value is dependent on the local zone. I think your intent was to account for the adjustment made by ORC that you referenced by saying "Getting the min/max stats for Date types in ORC are returned with milliseconds adjusted to the local timezone."
   
   I think the main problem is that this uses a path where the value is modified by ORC based on the JVM environment, which breaks a fundamental rule in Iceberg. I don't think that we should use a path where the value is modified.




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