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From Ian Varley <ivar...@salesforce.com>
Subject Re: Scanning the last N rows
Date Fri, 02 Mar 2012 21:49:56 GMT
Yes, you do have to worry about efficiency. If your rows aren't ordered in the table (by rowkey)
according to the update date, the server will be having to scan the entire table. Your filter
will enable it to not send all of those results to the client, but it's still having to read
them from disk and merge them with the rows in memory. It will likely not even be possible
for a big table (and, if it's not a *big* table, it probably shouldn't be in HBase).

The fundamental thing to note here is that there's no "magic": HBase stores records sorted
in exactly one order; if what you want isn't able to be efficiently found according to that
ordering, then you'll be scanning the whole table. Relational DBs do that too, but they also
have indexes that let you get at things quickly in some other sort order.

Ian

On Mar 2, 2012, at 3:42 PM, Peter Wolf wrote:


Ah ha!  So the row key orders the results, I just do an unbounded Scan,
and stop after N iterations.

Like this...

       Scan scan = new Scan();
       Filter filter = new SingleColumnValueFilter(...);
       scan.setFilter(filter);
       ResultScanner scanner = hTable.getScanner(scan);
       Iterator<Result> it = scanner.iterator();
       for ( int i=0; i<1000 && it.hasNext(); i++) {
           Result result = it.next();
           ... do stuff with result...
       }

Do I have to worry about efficiency?  Is the Server madly retrieving
rows, in the background, that the Client will never use?

Thanks
P



On 3/2/12 4:31 PM, Doug Meil wrote:
Hi there-

Take a look at this section of the book...

http://hbase.apache.org/book.html#reverse.timestamp




On 3/2/12 4:02 PM, "Peter Wolf"<opus111@gmail.com<mailto:opus111@gmail.com>> 
wrote:

Hello all,

I want to retrieve the most recent N rows from a table, with some column
qualifiers.

I can't find a Filter, or anything obvious in my books, or via Google.

What is the idiom for doing this?

Thanks
Peter





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