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From "Roger Binns (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 20:37:54 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-623?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12799916#action_12799916
] 

Roger Binns commented on COUCHDB-623:
-------------------------------------

Not again Damien :-)

Simple criteria - the size of the view file should be proportionate to the data in a view
on initial generation.  If you want raw numbers, the view file should be no larger than double
the sum of JSON encoded key, value and _id for each row.

The current multiplier is 15 to 27 times as much which is ludicrous.  Even post compactation
the file is a little on the large side.  And because the view results are not replicated,
the overhead has to be incurred on every machine that replication happens to.

Or put another way, if people are planning on deploying CouchDB how much space would you advise
them to provision?  

When I started, the answer for 10million documents/2.5GB of raw JSON is 72GB:

  23GB for DB, another 21GB for the compacted version, 27+GB for view file, another 1+GB for
compacted view file

By shortening ids to 4 bytes instead of 16 we get:

  4GB for DB, another 4GB for compacted, 27GB for view file, another 1GB for compacted view
file

By being able to sort my documents to be ordered by the most commonly emitted view key:
 
  4GB for DB, another 4GB for compacted, 15GB for view file, another 1GB for compacted view
file

Since the view/DB coexists at the same time as the compaction you need space for both simultaneously.
10 million documents/2GB of data is not something that makes any existing database system
sweat.

> 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
>            Assignee: Damien Katz
>
> 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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