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From Dave Brosius <dbros...@mebigfatguy.com>
Subject Re: understanding of native indexes: limitations, potential side effects,...
Date Wed, 16 May 2012 19:03:49 GMT
Each index you define on the source CF is created using an internal CF 
that has as its key the value of the column it's indexing, and as its 
columns, all the keys of all the rows in the source CF that have that 
value. So if all your rows in your source CF have the same value, then 
your index cf will have one row with N columns for each N rows in the 
original CF.



On 05/16/2012 02:58 PM, David Vanderfeesten wrote:
> Txs Jeremiah,
> But I am not sure I am following " number of columns could be equal to 
> number of rows ".  Is native index implemented as one cf shared over 
> all the indexes (one row in the idx CF corresponding to one index) or  
> is there an internal index cf per index?. My (potential wrong) mindset 
> was the latter. In that case if you would index a column with a very 
> high cardinality like for example serialNbr,  this corresponding 
> internal idx cf will just lead to almost the same nbr of rows as the 
> original cf containing the serialnbr. I can''t match that what you are 
> explaining...
>
> - David
>
> On Wed, May 16, 2012 at 6:23 PM, Jeremiah Jordan 
> <JEREMIAH.JORDAN@morningstar.com 
> <mailto:JEREMIAH.JORDAN@morningstar.com>> wrote:
>
>     The limitation is because number of columns could be equal to
>     number of rows.  If number of rows is large this can become an issue.
>
>     -Jeremiah
>
>     ------------------------------------------------------------------------
>     *From:* David Vanderfeesten [feestend@gmail.com
>     <mailto:feestend@gmail.com>]
>     *Sent:* Wednesday, May 16, 2012 6:58 AM
>     *To:* user@cassandra.apache.org <mailto:user@cassandra.apache.org>
>     *Subject:* understanding of native indexes: limitations, potential
>     side effects,...
>
>     Hi
>
>     I like to better understand the limitations of native indexes,
>     potential side effects and scenarios where they are required.
>
>     My understanding so far :
>     - Is that indexes on each node are storing indexes for data
>     locally on the node itself.
>     - Indexes do not return values in a sorted way (hashes of the
>     indexed row keys are defining the order)
>     - Given by the design referred in the first bullet, a coordinator
>     node receiving a read of a native index, needs to spawn a read to
>     multiple nodes(set of nodes together covering at least the
>     complete key space + potentially more to assure read consistency
>     level).
>     - Each write to an indexed column leads to an additional local
>     read of the index to update the index (kind of obvious but easily
>     forgotten when tuning your system for write-only workload)
>     - When using a where clause in CQL you need at least to specify an
>     equal condition on a native indexed column. Additional conditions
>     in the where clause are filtered out by the coordinator node
>     receiving the CQL query.
>     - native indexes do not support very well columns with high number
>     of discrete values throughout the entire CF.
>
>     Is upper understanding correct and complete?
>     Some doubts:
>     - about the limitation of indexing columns with high number of
>     discrete values:
>     I assume native indexes  are implemented with an internally
>     managed CF per index. With high cardinality values, in worst case,
>     the number of rows in the index are identical to the number of
>     rows of the indexed CF. Or are there other reasons for the
>     limitation, and if that's the case, is there a guideline on the
>     max. nbr of cardinality that is still reasonable?
>     -Are column updates and the update of the indexes (read + write
>     action) atomic and isolated from concurrent updates?
>
>     Txs!
>
>     David
>
>
>
>
>


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