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From nziebart <...@git.apache.org>
Subject [GitHub] cassandra pull request #170: SliceQueryFilter can fetch more data than neede...
Date Mon, 06 Nov 2017 17:51:28 GMT
GitHub user nziebart opened a pull request:

    https://github.com/apache/cassandra/pull/170

    SliceQueryFilter can fetch more data than needed

    Slice queries currently fetch more data then necessary, when there is only one column
not part of the primary key. Specifically, SliceQueryFilter does not stop reading until it
has seen `limit + 1` live cells, even though in the case we can stop after seeing `limit`
live cells.
    
    We have a use case where we use wide rows to implement versioning, by including a timestamp
as part of the primary key. Every once in a while, we "garbage collect" old versions by deleting
them. This results in a single column containing the latest version, followed by many tombstones.
    
    We use a `LIMIT 1` query to select the latest version (which is the first column in the
row). However, because SliceQueryFilter does not stop until it has seen <limit + 1>
live cells, we have to read all the tombstones following the single live cell. Furthermore,
if these tombstones are covering data in other sstables, we have to read all the corresponding
data when merging the sstable iterators. This can be a massive performance hit, and is unexpectedly
caused by deleting data.
    
    This patch allows the `ColumnCounter` implementation to decide when it has seen enough
cells. For counters that don't require grouping, we can stop immediately after finding the
first cell.


You can merge this pull request into a Git repository by running:

    $ git pull https://github.com/nziebart/cassandra nziebart/limit-1

Alternatively you can review and apply these changes as the patch at:

    https://github.com/apache/cassandra/pull/170.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #170
    
----
commit 52d8a5c0e846b357f90e7e02b8903094447b9945
Author: Nathan Ziebart <nziebart@palantir.com>
Date:   2017-10-06T23:48:03Z

    dont want to see next cell when not grouping

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