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From "ramkrishna.s.vasudevan (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-14221) Reduce the number of time row comparison is done in a Scan
Date Fri, 14 Aug 2015 05:29:46 GMT
ramkrishna.s.vasudevan created HBASE-14221:

             Summary: Reduce the number of time row comparison is done in a Scan
                 Key: HBASE-14221
                 URL: https://issues.apache.org/jira/browse/HBASE-14221
             Project: HBase
          Issue Type: Sub-task
            Reporter: ramkrishna.s.vasudevan
            Assignee: ramkrishna.s.vasudevan
             Fix For: 2.0.0

When we tried to do some profiling with the PE tool found this.
Currently we do row comparisons in 3 places in a simple Scan case.
1) ScanQueryMatcher
       int ret = this.rowComparator.compareRows(curCell, cell);
    if (!this.isReversed) {
      if (ret <= -1) {
        return MatchCode.DONE;
      } else if (ret >= 1) {
        // could optimize this, if necessary?
        // Could also be called SEEK_TO_CURRENT_ROW, but this
        // should be rare/never happens.
        return MatchCode.SEEK_NEXT_ROW;
    } else {
      if (ret <= -1) {
        return MatchCode.SEEK_NEXT_ROW;
      } else if (ret >= 1) {
        return MatchCode.DONE;
2) In StoreScanner next() while starting to scan the row
    if (!scannerContext.hasAnyLimit(LimitScope.BETWEEN_CELLS) || matcher.curCell == null ||
        isNewRow || !CellUtil.matchingRow(peeked, matcher.curCell)) {
      this.countPerRow = 0;
Particularly to see if we are in a new row.
3) In HRegion
          heap.next(results, scannerContext);

          nextKv = heap.peek();
moreCellsInRow = moreCellsInRow(nextKv, currentRowCell);
Here again there are cases where we need to careful for a MultiCF case.  Was trying to solve
this for the MultiCF case but is having lot of cases to solve. But atleast for a single CF
case I think these comparison can be reduced.
So for a single CF case in the SQM we are able to find if we have crossed a row using the
code pasted above in SQM. That comparison is definitely needed.
Now in case of a single CF the HRegion is going to have only one element in the heap and so
the 3rd comparison can surely be avoided if the StoreScanner.next() was over due to MatchCode.DONE
caused by SQM.

Coming to the 2nd compareRows that we do in StoreScanner. next() - even that can be avoided
if we know that the previous next() call was over due to a new row. Doing all this I found
that the compareRows in the profiler which was 19% got reduced to 13%. Initially we can solve
for single CF case which can be extended to MultiCF cases.

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