Re: how to get the row before this one?

[y_...@tsmc.com]

Hi,
I am very curious about what kind of project you are going on.

Fleming Chiu(邱宏明)
707-2260
y_823910@tsmc.com
週一無肉日吃素救地球(Meat Free Monday Taiwan)




|---------+------------------------------->
|         |           Oliver Meyn         |
|         |           <oliver.meyn@zerofoo|
|         |           tprint.net>         |
|         |                               |
|         |           2010/05/31 09:28 PM |
|         |           Please respond to   |
|         |           user                |
|         |                               |
|---------+------------------------------->
  >-------------------------------------------------------------------------------------------------------------|
  |                                                                                                             |
  |       To:      user@hbase.apache.org                                                                        |
  |       cc:      (bcc: Y_823910/TSMC)                                                                         |
  |       Subject: Re: how to get the row before this one?                                                      |
  >-------------------------------------------------------------------------------------------------------------|




Nice!  I thought of this approach but couldn't figure out how to get
Hbase to sort in reverse, and so gave up.  But I was clearly too much
"inside the box" :)

Thanks,
Oliver

On 30-May-10, at 5:49 PM, Igor Ranitovic wrote:

> One thing that I can think of is to sort measurements from the
> newest to oldest:
>
> DevA,12pm,150W
> DevA,9am,100W
>
> Now instead of scanning from-to date, start with the to date and
> keep scanning until you find a row with a date that is older than
> the from date.
> To use your example, start the scan from 1pm and stop when you hit
> 9am.
>
> To sort row from newest to oldest use MAXINT - timestamp, where
> timestamp is seconds since epoch.
>
> Hope this helps.
> i.
>
> Oliver Meyn wrote:
>> Hi all,
>> I have a table "device_power" which has as its row key
>> "device_id.timestamp", and a single column, "power".  A new row is
>> written whenever the power consumption of a device changes, which
>> could be every second, or could be as much as days.
>> When I do a query like "what was the average power of device X
>> between Date 1 and Date 2" I can get all keys where device_id
>> matches X (using PrefixFilter) and timestamp > Date1 and < Date2
>> (using either RowFilter or setTimeRange).  But I also need the last
>> row before my time range to know what state the device was in at
>> the start of my query period.  Maybe an example will help make it
>> more obvious (my timestamps are long integers, but this is easier
>> to read):
>> DevA.9am, 100W
>> DevB.10am, 200W
>> DevB.11am, 150W
>> DevA.12pm, 150W
>> And now the query "What was the average power of DevA btw 10am and
>> 1pm".  Is there a clever way to "get the first row before 10am that
>> matches PrefixFilter("DevA") ?
>> Thanks,
>> Oliver
>

--
Oliver Meyn
Software Architect
Zerofootprint Software, Inc.
oliver.meyn@zerofootprint.net
(416) 365-7557 x144









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