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https://issues.apache.org/jira/browse/COUCHDB-69?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12746585#action_12746585
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Jason Davies commented on COUCHDB-69:
-------------------------------------
Thanks for the excellent feedback Damien.
Firstly, I thought that adding the old revinfos to the by_seq index would be a
relatively cheap operation, as we are simply taking them from
#doc_info{revs=Revs}, and this is already done for conflicts and deleted revs.
So I don't understand why my patch involves *reading* meta data for every old
rev from the disk when updating the by_seq index i.e. in this line:
HistoryRevInfos = if HistoryEnabled -> [{Rev, Seq, Bp} ||
#rev_info{rev=Rev,seq=Seq,historical=true,body_sp=Bp} <- Revs];
true -> [] end,
But yes, it does mean that we are *storing* potentially 1000s of revs in this
index now, so I'm sure that would impact performance when updating the index.
With your second point in mind, I'm wondering whether it would be more
efficient to never delete old entries from the by_seq index (unless we purge),
thus saving us having to store all the old revs each time.
In response to your last point about multiple copies of attachments, this could
be solved if we kept an index of attachments by MD5 hash. I believe rnewson is
keen to get something like this in, so that if you upload the same attachment
to multiple docs, it only needs to store it once.
> Allow selective retaining of older revisions to a document
> ----------------------------------------------------------
>
> Key: COUCHDB-69
> URL: https://issues.apache.org/jira/browse/COUCHDB-69
> Project: CouchDB
> Issue Type: Improvement
> Components: Database Core
> Environment: All
> Reporter: Jan Lehnardt
> Assignee: Paul Joseph Davis
> Priority: Minor
> Fix For: 0.10
>
> Attachments: history_revs.2.patch, history_revs.3.patch,
> history_revs.4.patch, history_revs.5.patch, history_revs.patch
>
>
> At the moment, compaction gets rid of all old revisions of a document. Also,
> replication also deals with the latest revision. It would be nice if it would
> be possible to specify a list of revisions to keep around that do not get
> compacted away and replicated.
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