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From "Adam Kocoloski (JIRA)" <j...@apache.org>
Subject [jira] Commented: (COUCHDB-623) File format for views is space and time inefficient - use a better one
Date Wed, 13 Jan 2010 19:34:54 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-623?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12799896#action_12799896
] 

Adam Kocoloski commented on COUCHDB-623:
----------------------------------------

I believe by "consistency guarantees" Chris meant that a view request uses a single snapshot
of the view index for the entire response.  Even if documents are changed in the interim,
and even if someone else has triggered a view update, your response will still accurately
reflect the state of the DB at a single moment in time.

> File format for views is space and time inefficient - use a better one
> ----------------------------------------------------------------------
>
>                 Key: COUCHDB-623
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-623
>             Project: CouchDB
>          Issue Type: Improvement
>          Components: Database Core
>    Affects Versions: 0.10
>            Reporter: Roger Binns
>
> This was discussed on the dev mailing list over the last few days and noted here so it
isn't forgotten.
> The main database file format is optimised for data integrity - not losing or mangling
documents - and rightly so.
> That same append-only format is also used for views where it is a poor fit.  The more
random the ordering of data supplied, the larger the btree.  The larger the keys (in bytes)
the larger the btree.  As an example my 2GB of raw JSON data turns into a 3.9GB CouchDB database
but a 27GB view file (before compacting to 900MB).  Since views are not replicated, this requires
a disproportionate amount of disk space on each receiving server (not to mention I/O load).
 The format also affects view generation performance.  By loading my documents into CouchDB
in an order by the most emitted value in views I was able to reduce load time from 75 minutes
to 40 minutes with the view file size being 15GB instead of 27GB, but still very distant from
the 900MB post compaction.
> Views are a performance enhancement.  They save you from having to visit every document
when doing some queries.  The data within in a view is generated and hence the only consequence
of losing view data is a performance one and the view can be regenerated anyway.  Consequently
the file format should be one that is optimised for performance and size.  The only integrity
feature needed is the ability to tell that the view is potentially corrupt (eg the power failed
while it was being generated/updated).

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