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https://issues.apache.org/jira/browse/CASSANDRA-226?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12720933#action_12720933
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Jun Rao commented on CASSANDRA-226:
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For time-sorted CFs, within an SSTable, there shouldn't be duplicated columns.
Otherwise, older versions of a column never get garbage collected.
The two separate maps within Memtable are convenient for incrementally
maintaining columns in timestamp order as well as doing efficient updates by
column names. I am not sure how you can reduce it to one map without
sacrificing performance.
> Make time-sorted CFs behave consistently
> ----------------------------------------
>
> Key: CASSANDRA-226
> URL: https://issues.apache.org/jira/browse/CASSANDRA-226
> Project: Cassandra
> Issue Type: Bug
> Reporter: Jonathan Ellis
>
> Time-sorted CFs are indexed and sorted by time in the SSTable. So lookup by
> name is inefficient, and compaction sucks, because we combine by name even
> though we sort by time. (CASSANDRA-16)
> The right fix is, for time-sorted columns we need to recognize that the right
> "key" as it were is (time, name) not name. We should allow slice by time and
> lookup by time or time,name but not just name. (Similarly, we should not
> allow lookup by time on name-sorted CFs.) This means that name will not
> necessarily be unique in time-sorted CFs but that is the right behavior!
> Time-based CFs are more like appending to a list than putting to a map.
> (As part of this we could track the most recent append in time-sorted CFs to
> prevent the bug in CASSANDRA-223. I am not sure that is the "right" way to
> go but I do mention it as a possibility that this change enables.)
> In the code, this will allow more regular treatment of CFs and less special
> casing. (It will also allow using only a single map for memtable columns,
> reducing memory usage -- see CASSANDRA-51.)
> Implementation notes: we may be able to represent both types of CF with a
> pluggable Comparator + Indexer combination, which would solve CASSANDRA-185
> and CASSANDRA-189 at the same time.
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