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From Sylvain Lebresne <sylv...@datastax.com>
Subject Re: Do supercolumns have a purpose?
Date Thu, 03 Feb 2011 12:00:47 GMT
> Is there any advantage to using supercolumns
> (columnFamilyName[superColumnName[columnName[val]]]) instead of regular
> columns with concatenated keys
> (columnFamilyName[superColumnName@columnName[val]])?
>
> When I designed my data model, I used supercolumns wherever I needed two
> levels of key depth - just because they were there, and I figured that
they
> must be there for a reason.
>
> Now I see that in 0.7 secondary indexes don't work on supercolumns or
> subcolumns (is that right?), which seems to me like a very serious
> limitation of supercolumn families.
>
> It raises the question: Is there anything that supercolumn families are
good
> for?

There is a bunch of queries that you cannot do (or less conveniently) if you
encode super columns using regular columns with concatenated keys:

1) If you use regular columns with concatenated keys, the count argument
count simple columns. With super columns it counts super columns. It means
that you can't do "give me the 10 first super columns of this row".

2) If you need to get x super columns by name, you'll have to issue x
get_slice query (one of each super column). On the client side it sucks.
Internally in Cassandra we could do it reasonably well though.

3) You cannot remove entire super columns since there is no support for
range
deletions.

Moreover, the encoding with concatenated keys uses more disk space (and less
disk used for the same information means less things to read so it may have
a slight impact on read performance too -- it's probably really slight on
most
usage but nevertheless).

> And here's a related question: Why can't Cassandra implement supercolumn
> families as regular column families, internally, and give you that
> functionality?

For the 1) and 2) above, we could deal with those internally fairly easily I
think and rather well (which means it wouldn't be much worse
performance-wise
than with the actual implementaion of super columns, not that it would be
better). For 3), range deletes are harder and would require more significant
changes (that doesn't mean that Cassandra will never have it). Even without
that, there would be the disk space lost.

--
Sylvain

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