There are few ways to look at it:
1) Lets say your row key is a LONG value that keeps incrementing for every
writes, then, you can use, HTable.getRowOrBefore method multiple times to get
hold of latest N entries. For the first call, you can pass Long.MAX_VALUE as
row key, the HBase will return you the row with key just less than
Long.MAX_VALUE, and in this case, it will be the latest row key. Let's call
this "latestRowKey". For next call to "getRowOrBefore", you can pass
"latestRowKey - 1" as row key, and so on.
http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/HTable.html#getRowOrBefore(byte[],
byte[])
2) Another variant of above approach can be wherein your row key is
"<timestamp>". In this case, you can pass the current time to first call to
"getRowOrBefore" and (latestRowKey - 1) to subsequent calls.
3) Use reverse timestamp. This is discussed in Hbase chapter in "Hadoop - The
definitive guide book".
If you use row key as Long.MAX_VALUE - System.currentTimeInMillis(), then,
all the latest entries will show up first if you do a Scan on the table.
This way first N entries will be your latest N entries.
Regards,
Srikanth
-----Original Message-----
From: Harsh J [mailto:[email protected]]
Sent: Tuesday, July 26, 2011 10:25 PM
To: [email protected]
Subject: Re: Retrieving last 100 rows by timestamp
Hello Ajay,
See the recent mailing list discussion on using reverse timestamps in key. It
should help your case, I think.
On 26-Jul-2011, at 8:50 PM, Something Something wrote:
> Hello,
>
> Need to create a report that shows 'last 100 rows by timestamp'. This query
> should return almost instantaneously. Any recommendation regarding the
> design? I was thinking of creating a table with 'sequence #' as a key and
> value would be 'key of another table that contains the master data'.
> Anyway, I will think some more, but if anyone has done something similar, I
> would greatly appreciate it if they would share the knowledge. Thanks.
>
> - Ajay
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