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https://issues.apache.org/jira/browse/HBASE-18059?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16443467#comment-16443467
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Jingyun Tian commented on HBASE-18059:
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[~stack] Because the scanner order is used when the kvComparator cannot
determine which is bigger between 2 cells, that means the 2 cells have same key
and same seqId. But for memstore, all cells have its own seqld, thus it will
nerver reach the compare of scanner order. [~appy] explain this very clear:
{quote} - memstore scanners vs storefile(SF) scanner:
-- if cell in SF has seqId: Its seqId should be less than that of memstore's
cell. Memstore scanner will win.
-- if cell in SF does NOT have seqId: SF'cell [defaults to
seqId=0|https://github.com/apache/hbase/blob/e65d8653e566bbbae03578a1f9ad858cabcb48bc/hbase-common/src/main/java/org/apache/hadoop/hbase/Cell.java#L142],
memstore scanner will win. (Note that seqId of cells in hfiles are removed on
major compaction if older than certain time, default is 5 days)
- memstore vs bulk loaded file(BLF)
-- memstore cell's will have higher seqId, so memstore scanner will win.{quote}
> The scanner order for memstore scanners are wrong
> -------------------------------------------------
>
> Key: HBASE-18059
> URL: https://issues.apache.org/jira/browse/HBASE-18059
> Project: HBase
> Issue Type: Bug
> Components: regionserver, scan, Scanners
> Affects Versions: 2.0.0
> Reporter: Duo Zhang
> Assignee: Jingyun Tian
> Priority: Critical
> Fix For: 2.0.0
>
> Attachments: HBASE-18059.master.001.patch
>
>
> This is comments for KeyValueScanner.getScannerOrder
> {code:title=KeyValueScanner.java}
> /**
> * Get the order of this KeyValueScanner. This is only relevant for
> StoreFileScanners and
> * MemStoreScanners (other scanners simply return 0). This is required for
> comparing multiple
> * files to find out which one has the latest data. StoreFileScanners are
> ordered from 0
> * (oldest) to newest in increasing order. MemStoreScanner gets LONG.max
> since it always
> * contains freshest data.
> */
> long getScannerOrder();
> {code}
> As now we may have multiple memstore scanners, I think the right way to
> select scanner order for memstore scanner is to ordered from Long.MAX_VALUE
> in decreasing order.
> But in CompactingMemStore and DefaultMemStore, the scanner order for memstore
> scanner is also start from 0, which will be messed up with StoreFileScanners.
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