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From keith-turner <...@git.apache.org>
Subject [GitHub] accumulo pull request #275: ACCUMULO-4667 Reworked the LocalityGroupIterator...
Date Fri, 30 Jun 2017 19:13:51 GMT
Github user keith-turner commented on a diff in the pull request:

    https://github.com/apache/accumulo/pull/275#discussion_r125107098
  
    --- Diff: core/src/main/java/org/apache/accumulo/core/iterators/system/LocalityGroupIterator.java
---
    @@ -97,75 +133,116 @@ public static final int seek(HeapIterator hiter, LocalityGroup[]
groups, Set<Byt
         else
           cfSet = Collections.emptySet();
     
    -    for (LocalityGroup lgr : groups) {
    -      // when include is set to true it means this locality groups contains
    -      // wanted column families
    -      boolean include = false;
    +    // determine the set of groups to use
    +    Collection<LocalityGroup> groupsToUse = Collections.EMPTY_LIST;
     
    -      if (cfSet.size() == 0) {
    -        include = !inclusive;
    -      } else if (lgr.isDefaultLocalityGroup && lgr.columnFamilies == null) {
    -        // do not know what column families are in the default locality group,
    -        // only know what column families are not in it
    +    // if no column families specified, then include all groups unless !inclusive
    +    if (cfSet.size() == 0) {
    +      if (!inclusive) {
    +        groupsToUse = groups.groups;
    +      }
    +    } else {
    +      groupsToUse = new HashSet<LocalityGroup>();
     
    +      // do not know what column families are in the default locality group,
    +      // only know what column families are not in it
    +      if (groups.defaultGroup != null) {
             if (inclusive) {
    -          if (!nonDefaultColumnFamilies.containsAll(cfSet)) {
    +          if (!groups.groupByCf.keySet().containsAll(cfSet)) {
                 // default LG may contain wanted and unwanted column families
    -            include = true;
    +            groupsToUse.add(groups.defaultGroup);
               }// else - everything wanted is in other locality groups, so nothing to do
             } else {
    -          // must include, if all excluded column families are in other locality groups
    -          // then there are not unwanted column families in default LG
    -          include = true;
    +          // must include the default group as it may include cfs not in our cfSet
    +          groupsToUse.add(groups.defaultGroup);
    +        }
    +      }
    +
    +      /*
    +       * Need to consider the following cases for inclusive and exclusive (lgcf:locality
group column family set, cf:column family set) lgcf and cf are disjoint
    +       * lgcf and cf are the same cf contains lgcf lgcf contains cf lgccf and cf intersect
but neither is a subset of the other
    +       */
    +      if (!inclusive) {
    +        for (Entry<ByteSequence,LocalityGroup> entry : groups.groupByCf.entrySet())
{
    +          if (!cfSet.contains(entry.getKey())) {
    +            groupsToUse.add(entry.getValue());
    +          }
    +        }
    +      } else if (groups.groupByCf.size() <= cfSet.size()) {
    +        for (Entry<ByteSequence,LocalityGroup> entry : groups.groupByCf.entrySet())
{
    +          if (cfSet.contains(entry.getKey())) {
    +            groupsToUse.add(entry.getValue());
    +          }
             }
           } else {
    -        /*
    -         * Need to consider the following cases for inclusive and exclusive (lgcf:locality
group column family set, cf:column family set) lgcf and cf are
    -         * disjoint lgcf and cf are the same cf contains lgcf lgcf contains cf lgccf
and cf intersect but neither is a subset of the other
    -         */
    -
    -        for (Entry<ByteSequence,MutableLong> entry : lgr.columnFamilies.entrySet())
    -          if (entry.getValue().longValue() > 0)
    -            if (cfSet.contains(entry.getKey())) {
    -              if (inclusive)
    -                include = true;
    -            } else if (!inclusive) {
    -              include = true;
    -            }
    +        for (ByteSequence cf : cfSet) {
    +          LocalityGroup group = groups.groupByCf.get(cf);
    +          if (group != null) {
    +            groupsToUse.add(group);
    +          }
    +        }
           }
    +    }
     
    -      if (include) {
    -        lgr.getIterator().seek(range, EMPTY_CF_SET, false);
    -        hiter.addSource(lgr.getIterator());
    -        numLGSeeked++;
    -      }// every column family is excluded, zero count, or not present
    +    for (LocalityGroup lgr : groupsToUse) {
    +      lgr.getIterator().seek(range, EMPTY_CF_SET, false);
    +      hiter.addSource(lgr.getIterator());
    +      numLGSeeked++;
    +    }
    +
    +    if (used != null) {
    +      used.addAll(groupsToUse);
         }
     
         return numLGSeeked;
       }
     
       @Override
       public void seek(Range range, Collection<ByteSequence> columnFamilies, boolean
inclusive) throws IOException {
    -    seek(this, groups, nonDefaultColumnFamilies, range, columnFamilies, inclusive);
    +    Set<ByteSequence> cfSet;
    +    if (columnFamilies.size() > 0)
    +      cfSet = new HashSet<>(columnFamilies);
    --- End diff --
    
    Always doing this copy is annoying. I wonder if the cost of this copy is made up by the
cost savings of checking against the last families.  I suspect it may be, but not sure.  
I think using the same set of columns for each seek is common, so its nice to use the same
decision.  In the use case were seek is repeatedly being called with a different set of families,
this copy and later equality check would be a lot of overhead with no gain. However I think
this an exotic use case??
    
    I like Guava's `cfSet = ImmutableSet.of(columnFamilies)` because if the input is already
an ImmutableSet it just returns it.   This would be beneficial if code calling seek passed
in ImmutableSet.  We could avoid the copy without worrying about correctness.  Also another
nice thing about using immutable set is the equality check below will be super quick (because
its the same instance it can check for reference equality).  Comparing two different hash
set instances that have the same content requires examining the content (I looked and it calls
`containsAll()`).
    
    I am thinking we should change this to  `cfSet = ImmutableSet.of(columnFamilies)` and
then have a follow on issue to make Accumulo code that constructs these sets in the tablet
server use `ImmutableSet`


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