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From "Duo Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-17125) Inconsistent result when use filter to read data
Date Fri, 21 Apr 2017 09:17:04 GMT

    [ https://issues.apache.org/jira/browse/HBASE-17125?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15978373#comment-15978373
] 

Duo Zhang commented on HBASE-17125:
-----------------------------------

If not use filter, get up to the specified number of versions of each column.
<p>
If use filter, it means the maximum versions of each column will be checked by filter. So
the scan may return less than the value you set here as the filter may filter out some cells.
If you want to get a specific number of version for each column after filtering, please call
setMaxVersions() and use SpecifiedNumVersionsColumnFilter. Notice that the SpecifiedNumVersionsColumnFilter
should be placed at the last position in FilterList to make sure it will be checked at last.

Better? And seems get also has this method so you need to change the wording from scan to
get if possible?

Thanks.

> Inconsistent result when use filter to read data
> ------------------------------------------------
>
>                 Key: HBASE-17125
>                 URL: https://issues.apache.org/jira/browse/HBASE-17125
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Guanghao Zhang
>            Assignee: Guanghao Zhang
>            Priority: Critical
>             Fix For: 2.0.0
>
>         Attachments: example.diff, HBASE-17125.master.001.patch, HBASE-17125.master.002.patch,
HBASE-17125.master.002.patch, HBASE-17125.master.003.patch, HBASE-17125.master.004.patch,
HBASE-17125.master.005.patch, HBASE-17125.master.006.patch, HBASE-17125.master.007.patch,
HBASE-17125.master.008.patch, HBASE-17125.master.009.patch
>
>
> Assume a cloumn's max versions is 3, then we write 4 versions of this column. The oldest
version doesn't remove immediately. But from the user view, the oldest version has gone. When
user use a filter to query, if the filter skip a new version, then the oldest version will
be seen again. But after compact the region, then the oldest version will never been seen.
So it is weird for user. The query will get inconsistent result before and after region compaction.
> The reason is matchColumn method of UserScanQueryMatcher. It first check the cell by
filter, then check the number of versions needed. So if the filter skip the new version, then
the oldest version will be seen again when it is not removed.
> Have a discussion offline with [~Apache9] and [~fenghh], now we have two solution for
this problem. The first idea is check the number of versions first, then check the cell by
filter. As the comment of setFilter, the filter is called after all tests for ttl, column
match, deletes and max versions have been run.
> {code}
>   /**
>    * Apply the specified server-side filter when performing the Query.
>    * Only {@link Filter#filterKeyValue(Cell)} is called AFTER all tests
>    * for ttl, column match, deletes and max versions have been run.
>    * @param filter filter to run on the server
>    * @return this for invocation chaining
>    */
>   public Query setFilter(Filter filter) {
>     this.filter = filter;
>     return this;
>   }
> {code}
> But this idea has another problem, if a column's max version is 5 and the user query
only need 3 versions. It first check the version's number, then check the cell by filter.
So the cells number of the result may less than 3. But there are 2 versions which don't read
anymore.
> So the second idea has three steps.
> 1. check by the max versions of this column
> 2. check the kv by filter
> 3. check the versions which user need.
> But this will lead the ScanQueryMatcher more complicated. And this will break the javadoc
of Query.setFilter.
> Now we don't have a final solution for this problem. Suggestions are welcomed.



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