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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1784) [hbase] delete
Date Thu, 06 Sep 2007 17:52:32 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1784?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12525481

stack commented on HADOOP-1784:

After study, 1. and 2. in the above are straight-forward.  3. as described, as a means of
deleting all behind a particular timestamp is expensive.  Here's why:

On every get, I need to check the future.  That is, I need to read rows in front of the currently
specified row/column/timestamp combination to look see if there is a delete record with the
same row/column but a timestamp ahead of the stipulated one.  If one is found, then matching
records should not be returned because they have been 'deleted'.

Instead, lets build the delete-all-versions-behind-a-specified-timestamp using the basic delete
mechanism, the facility whereby a cell is X-d out by the presence of a delete cell of the
exact same row/column/timestamp appearing ahead of the non-null cell.  The API currently has
a delete that takes a column name and on commit you specify the timestamp.  This deletes a
single cell value.  To facilitate bulk delete operations, to the API we'll add a deleteAll.
 Internally, this will find all cells that match the row/column specified and delete all cells
of equal-to or older timestamps.

Later folks might want to do things like only delete the X oldest revisions or only delete
all of exactly the same timestamp but I'll wait until its asked for before attempting an implementation.

> [hbase] delete
> --------------
>                 Key: HADOOP-1784
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1784
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: contrib/hbase
>            Reporter: stack
>            Assignee: stack
> Delete is incomplete in hbase.  Whats there is inconsistent.  Deleted records currently
persist and are never cleaned up.  This issue is about making delete behavior coherent across
gets, scans and compaction.
> Below is from a bit of back and forth between Jim and myself where Jim takes a stab at
outlining a model for delete taking inspiration from how Digital's versioned file system used
> {code}
> Let's say you have 5 versions with timestamps T1, T2, ..., T5 where
> timestamps are increasing from T1 to T5 (so T5 is the newest).
> Before any deletes occur, if you don't specify a timestamp and request N
> versions, you should get T5 first, then T4, T3, ... until you have
> reached N or you run out of versions.
> Now add deletes:
> (In the following, timestamp refers to the timestamp associated with
> the delete operation)
> 1. If no timestamp is specified we are deleting the latest version.
>    If a get or scanner specifies that it wants N versions, then it 
>    should get T4, T3, ..., until we have N versions or we run out of
>    older versions. After compaction, the deletion record and T5 should
>    be elided from the HStore.
> 2. If a timestamp is specified and it exactly matches a version (say
>    T4) and a get or scanner requests N versions, then the client
>    receives T5, T3, T2, ... until we satisfy N or run out of versions.
>    After a compaction, the deletion record and T4 should be elided
>    from the HStore.
> 3. If a timestamp is specified and does not exactly match a version,
>    it means delete every version older than this timestamp. If the
>    timestamp is greater than T5 all versions are considered to be
>    deleted and a get or a scanner will return no results even if 
>    the get or scanner specify an older time. This is consistent
>    with the concept of delete all versions older than timestamp.
>    After a compaction, the delete record and all the values should
>    be elided.
>    If the specified timestamp falls between two older versions (say
>    T4 and T3) then T3, T2 and T1 are considered to be deleted (again
>    this is all versions older than timestamp). A get or scanner
>    that specifies no time but requests N versions can only get T5
>    and T4. A get or scanner that requests a time of T3 or earlier
>    will get no results because those versions are deleted. After
>    a compaction, the deletion record and the deleted versions
>    are elided from the HStore.
> {code}

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