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From 程磊 <comnetw...@163.com>
Subject It is a SkipScanFilter bug?
Date Tue, 28 Feb 2017 11:31:50 GMT
When I work on PHOENIX-1193 to make skip scan work with reverse scan and read the code of SkipScanFilter.navigate
method, it seems that SkipScanFilter.navigate method has a bug, please help me see if I am
right.


See following simple unit test first,the rowKey is composed of three PInteger columns,and
the slots of SkipScanFilter are:
[[[1 - 4]], [5, 7], [[9 - 10]]],
when SkipScanFilter.filterKeyValue method is invoked on a KeyValue whose rowKey is [2,7,11],
obviously SkipScanFilter.filterKeyValue
returns ReturnCode.SEEK_NEXT_USING_HINT and SkipScanFilter.getNextCellHint returns [3,5,9],
but unfortunately, SkipScanFilter.getNextCellHint actually returns [2,8,5,9] , a very strange
value, following unit tests failed.


@Test public void testNavigate() { RowKeySchemaBuilder builder = new RowKeySchemaBuilder(3);
for(int i=0;i<3;i++) { builder.addField( new PDatum() { @Override public boolean isNullable()
{ return false; } @Override public PDataType getDataType() { return PInteger.INSTANCE; } @Override
public Integer getMaxLength() { return PInteger.INSTANCE.getMaxLength(null); } @Override public
Integer getScale() { return PInteger.INSTANCE.getScale(null); } @Override public SortOrder
getSortOrder() { return SortOrder.getDefault(); } }, false, SortOrder.getDefault()); } List<List<KeyRange>>
rowKeyColumnRangesList=Arrays.asList( Arrays.asList( PInteger.INSTANCE.getKeyRange(PInteger.INSTANCE.toBytes(1),
true, PInteger.INSTANCE.toBytes(4), true)), Arrays.asList( KeyRange.getKeyRange(PInteger.INSTANCE.toBytes(5)),
KeyRange.getKeyRange(PInteger.INSTANCE.toBytes(7))), Arrays.asList( PInteger.INSTANCE.getKeyRange(PInteger.INSTANCE.toBytes(9),
true, PInteger.INSTANCE.toBytes(10), true)) ); SkipScanFilter skipScanFilter=new SkipScanFilter(rowKeyColumnRangesList,
builder.build()); System.out.println(skipScanFilter); byte[] rowKey=ByteUtil.concat( PInteger.INSTANCE.toBytes(2),
PInteger.INSTANCE.toBytes(7), PInteger.INSTANCE.toBytes(11)); KeyValue keyValue=KeyValue.createFirstOnRow(rowKey);
ReturnCode returnCode=skipScanFilter.filterKeyValue(keyValue); assertTrue(returnCode == ReturnCode.SEEK_NEXT_USING_HINT);
Cell nextCellHint=skipScanFilter.getNextCellHint(keyValue); assertTrue(Bytes.toStringBinary(CellUtil.cloneRow(nextCellHint)).equals("\\x80\\x00\\x00\\x03\\x80\\x00\\x00\\x05\\x80\\x00\\x00\\x09"));
}


Let us see what's wrong, first column of rowKey [2,7,11 ] is 2, which is in SkipScanFilter's
first slot range [1-4], so position[0] is 0 and we go to the second column 7, which match
the second range [7] of SkipScanFilter's second slot [5, 7],so position[1] is 1 and we go
to the third column 11, which is bigger than the third slot range [9 - 10],so position[2]
is 0 and we begin to backtrack to second column, because the second range [7] of SkipScanFilter's
second slot is singleKey and there is no more range,so position[1] is 0 and we continue to
backtrack to first column, because the first slot range [1-4] is not singleKey so we stop
backtracking at first column.


Now the problem comes, in following line 448 of SkipScanFilter.navigate method,SkipScanFilter.setStartKey
method is invoked,first copy rowKey columns before ptr to SkipScanFilter.startKey, because
now ptr still point to the third column, so copy the first and second columns to SkipScanFilter.startKey,SkipScanFilter.startKey
is [2,7] after this step , then setStartKey method copy the lower bound SkipScanFilter.slots
from j+1 column, accoring to SkipScanFilter.position array,now j is 0, and both position[1]
and position[2] are 0,so SkipScanFilter.startKey becomes [2,7,5,9], and in following line
457, ByteUtil.nextKey is invoked on [2,7], [2,7] is incremented to [2,8], finally SkipScanFilter.startKey
is [2,8,5,9].


| 448 int currentLength = setStartKey(ptr, minOffset, j+1, nSlots, false); 449 // From here
on, we use startKey as our buffer (resetting minOffset and maxOffset) 450 // We've copied
the part of the current key above that we need into startKey 451 // Reinitialize the iterator
to be positioned at previous slot position 452 minOffset = 0; 453 maxOffset = startKeyLength;
454 schema.iterator(startKey, minOffset, maxOffset, ptr, ScanUtil.getRowKeyPosition(slotSpan,
j)+1); 455 // Do nextKey after setting the accessor b/c otherwise the null byte may have 456
// been incremented causing us not to find it 457 ByteUtil.nextKey(startKey, currentLength);
|


From above analysis, we can see the second column is repeatedly copied to SkipScanFilter.startKey,
copy 7 to SkipScanFilter.startKey is error ,before SkipScanFilter.setStartKey method is invoked
in line 448, we should first move ptr to j+1, that is to say ,following code might be inserted
before line 448:


schema.reposition( ptr, ScanUtil.getRowKeyPosition(slotSpan, i), ScanUtil.getRowKeyPosition(slotSpan,
j+1), minOffset, maxOffset, slotSpan[j+1]);


Another problem is why the existing SkipScanFilter unit tests are ok? That is because in all
existing unit test cases, backtrack column is just before current column which ptr points
to, i.e. i==j+1, and this problem may not affect the final query result , but obviously, it
may lead to the erroneous startKey and cause the Skip Scan inefficient.
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