mwkang commented on code in PR #6900:
URL: https://github.com/apache/hbase/pull/6900#discussion_r2039655382
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hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/StoreScanner.java:
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@@ -832,7 +845,7 @@ private void updateMetricsStore(boolean memstoreRead) {
* @return null is the top cell doesn't change. Otherwise, the NextState to
return
*/
private NextState needToReturn(List<? super ExtendedCell> outResult) {
- if (!outResult.isEmpty() && topChanged) {
+ if (topChanged) {
Review Comment:
Let’s assume the data stored in HBase is as follows:
(1) row0/family2:qf1/DeleteColumn
(2) row0/family2:qf1/Put/value2
(3) row1/family1:qf1/Put/value2
(4) row1/family2:qf1/Put/value2
Now, suppose a user starts scanning from **row0**.
In
[`RegionScannerImpl#nextInternal`](https://github.com/apache/hbase/blob/0b3c17302843d1f4d6f3c6b458f837cb9c274510/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RegionScannerImpl.java#L415),
when the [current
cell](https://github.com/apache/hbase/blob/0b3c17302843d1f4d6f3c6b458f837cb9c274510/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RegionScannerImpl.java#L446)’s
row is `row0`, after reading entry (2) in `StoreScanner`, if a flush happens,
a `topChanged` occurs ([`Storescanner.peek() is changed where before
...`](https://github.com/apache/hbase/blob/0b3c17302843d1f4d6f3c6b458f837cb9c274510/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/StoreScanner.java#L1074)),
and the value of `StoreScanner`’s `heap.peek()` becomes (4)
`row1/family2:qf1/Put/value2`.
Since it is the next row, `StoreScanner` *should* return at that point — but
it fails to recognize that it has moved to the next row because
[outResult](https://github.com/apache/hbase/blob/0b3c17302843d1f4d6f3c6b458f837cb9c274510/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/StoreScanner.java#L835)
is empty, and ends up including the new row in the result.
Then, in `RegionScannerImpl`, it sees that `nextKv`’s row is different from
the current cell’s row, and returns (since it has moved to a different row).
As a result, even though (3) and (4) belong to the same row (`row1`), they
are returned to the client as if they were from different rows.
(3) and (4) should be combined into a single
[Result](https://github.com/apache/hbase/blob/0b3c17302843d1f4d6f3c6b458f837cb9c274510/hbase-client/src/main/java/org/apache/hadoop/hbase/client/Result.java),
but they end up being returned as separate Result instances.
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