Github user amansinha100 commented on a diff in the pull request:

    https://github.com/apache/drill/pull/519#discussion_r68673292
  
    --- Diff: 
exec/java-exec/src/main/java/org/apache/drill/exec/planner/logical/partition/PruneScanRule.java
 ---
    @@ -269,13 +283,54 @@ protected void doOnMatch(RelOptRuleCall call, Filter 
filterRel, Project projectR
             int recordCount = 0;
             int qualifiedCount = 0;
     
    -        // Inner loop: within each batch iterate over the 
PartitionLocations
    -        for(PartitionLocation part: partitions){
    -          if(!output.getAccessor().isNull(recordCount) && 
output.getAccessor().get(recordCount) == 1){
    -            newPartitions.add(part);
    -            qualifiedCount++;
    +        if (checkForSingle &&
    +            partitions.get(0).isCompositePartition() /* apply single 
partition check only for composite partitions */) {
    +          // Inner loop: within each batch iterate over the 
PartitionLocations
    +          for (PartitionLocation part : partitions) {
    +            assert part.isCompositePartition();
    +            if(!output.getAccessor().isNull(recordCount) && 
output.getAccessor().get(recordCount) == 1) {
    +              newPartitions.add(part);
    +              if (isSinglePartition) { // only need to do this if we are 
already single partition
    +                // compose the array of partition values for the 
directories that are referenced by filter:
    +                // e.g suppose the dir hierarchy is year/quarter/month and 
the query is:
    +                //     SELECT * FROM T WHERE dir0=2015 AND dir1 = 'Q1',
    +                // then for 2015/Q1/Feb, this will have ['2015', 'Q1', 
null]
    --- End diff --
    
    Good point.  The path [2015, null, Jan]  should *not* qualify for the 
single partition optimization because in the general case there could be 
multiple 'Jan' subdirectories under the dir1 directory.  We would still use the 
metadata cache but at the level of the dir0, so ideally one should get 
cacheFileRoot=/tmp/t2/2015.   I have a fix for this and will update the PR. 


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