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From Sylvain Lebresne <sylv...@datastax.com>
Subject Re: Document storage
Date Fri, 30 Mar 2012 17:23:16 GMT
On Fri, Mar 30, 2012 at 6:01 PM, Daniel Doubleday
<daniel.doubleday@gmx.net> wrote:
> But decomposing into columns will lead to more of that:
>
> - Total amount of serialized data is (in most cases a lot) larger than protobuffed /
compressed version

At least with sstable compression, I would expect the difference to
not be too big in practice.

> - If you do selective updates the document will be scattered over multiple ssts plus
if you do sliced reads you can't optimize reads as opposed to the single column version that
when updated is automatically superseding older versions so most reads will hit only one sst

But if you need to do selective updates, then a blob just doesn't work
so that comparison is moot.

Now I don't think anyone pretended that you should never use blobs
(whether that's protobuffed, jsoned, ...). If you don't need selected
updates and having something as compact as possible on disk make a
important difference for you, sure, do use blobs. The only argument is
that you can already do that without any change to the core. What we
are saying is that for the case where you care more about schema
flexibility (being able to do selective updates, to index on some
subpart, etc...) then we think that something like the map and list
idea of CASSANDRA-3647 will probably be a more natural fit to the
current CQL API.

--
Sylvain

>
> All these reads make the hot dataset. If it fits the page cache your fine. If it doesn't
you need to buy more iron.
>
> Really could not resist because your statement seems to be contrary to all our tests
/ learnings.
>
> Cheers,
> Daniel
>
> From dev list:
>
> Re: Document storage
> On Thu, Mar 29, 2012 at 1:11 PM, Drew Kutcharian <drew@venarc.com> wrote:
>>> I think this is a much better approach because that gives you the
>>> ability to update or retrieve just parts of objects efficiently,
>>> rather than making column values just blobs with a bunch of special
>>> case logic to introspect them.  Which feels like a big step backwards
>>> to me.
>>
>> Unless your access pattern involves reading/writing the whole document each time.
In
> that case you're better off serializing the whole document and storing it in a column
as a
> byte[] without incurring the overhead of column indexes. Right?
>
> Hmm, not sure what you're thinking of there.
>
> If you mean the "index" that's part of the row header for random
> access within a row, then no, serializing to byte[] doesn't save you
> anything.
>
> If you mean secondary indexes, don't declare any if you don't want any. :)
>
> Just telling C* to store a byte[] *will* be slightly lighter-weight
> than giving it named columns, but we're talking negligible compared to
> the overhead of actually moving the data on or off disk in the first
> place.  Not even close to being worth giving up being able to deal
> with your data from standard tools like cqlsh, IMO.
>
> --
> Jonathan Ellis
> Project Chair, Apache Cassandra
> co-founder of DataStax, the source for professional Cassandra support
> http://www.datastax.com
>

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