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From Michal Augustýn <augustyn.mic...@gmail.com>
Subject Re: skip + limit support in GetSlice
Date Mon, 06 Sep 2010 08:26:27 GMT
Hi Mike,

yes, I read the PDF to the finish. Twice. As I wrote, my application is not
accessed by users, it's accessed by other applications that can access pages
randomly.

So when some application wants to get page 51235 (so skip is 5123500, limit
is 100) then I have to:

1) GetSlice(from: "", to: "", limit: 5123500)
2) Read only the last column name.
3) GetSlice(from: point2value, to: "", limit: 100)

The problem is in 1) - Cassandra has to read 5123500 columns, serialize
them, send them using  Thrift protocol and deserialize them. Finally, I
throw 5123499 of columns away. It doesn't seem to be very efficient.

So I'm looking for another solution for this scenario. I know the right way
for pagination in Cassandra and I'm using them if I can...

So if this kind of pagination cannot be added to standard Cassandra Thrift
API then I should create some separate Thrift API that will handle my
scenario (and avoid high network traffic). Am I right?

Thanks!

Augi


2010/9/5 Mike Peters <cassandra@softwareprojects.com>

>  Hi Michal,
>
> Did you read the PDF Stu sent over, start to finish?  There are several
> different approaches described there.
>
> With Cassandra, what we found works best for pagination:
> * Keep a separate 'total_records' count and increment/decrement it on every
> insert/delete
> * When getting slices, pass 'last seen' as the 'from' and keep the 'to'
> empty.  Pass the number of records you want to show per page in the 'count'.
> * Avoid letting user skip to page X, using Next/Prev/First/Last only (same
> way GMail does it)
>
>
>
> Michal Augustýn wrote:
>
> I know that "Prev/Next" is good solution for web applications. But when I
> want to access data from another application or when I want to access pages
> randomly...
>
>  I don't know the internal structure of memtables etc., so I don't know if
> columns in row are indexable. If now, then I just want to transfer my
> workaround to server (to avoid huge network traffic)...
>
> 2010/9/5 Stu Hood <stu.hood@rackspace.com>
>
>> Cassandra supports the recommended approach from:
>> http://www.percona.com/ppc2009/PPC2009_mysql_pagination.pdf
>>
>> For large numbers of items, skip + limit is extremely inefficent.
>>
>> -----Original Message-----
>> From: "Michal Augustýn" <augustyn.michal@gmail.com>
>> Sent: Sunday, September 5, 2010 5:39am
>> To: user@cassandra.apache.org
>> Subject: skip + limit support in GetSlice
>>
>> Hello,
>>
>> probably this is feature request. Simply, I would like to have support for
>> standard pagination (skip + limit) in GetSlice Thrift method. Is this
>> feature on the road map?
>>
>> Now, I have to perform GetSlice call, that starts on "" and "limit" is set
>> to "skip" value. Then I read the last column name returned and
>> subsequently
>> perform the final GetSlice call - I use the last column name as "start"
>> and
>> set "limit" to "limit" value.
>>
>> This workaround is not very efficient when I need to skip a lot of columns
>> (so "skip" is high) - then a lot of data must be transferred via network.
>> So
>> I think that support for Skip in GetSlice would be very useful (to avoid
>> high network traffic).
>>
>> The implementation could be very straightforward (same as the workaround)
>> or
>> maybe it could be more efficient - I think that whole row (so all columns)
>> must fit into memory so if we have all columns in memory...
>>
>> Thank you!
>>
>> Augi
>>
>>
>>
>
>

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