Home | About | Sematext search-lucene.com search-hadoop.com
 Search Hadoop and all its subprojects:

Switch to Threaded View
HBase >> mail # user >> Scanning the last N rows


Copy link to this message
-
Re: Scanning the last N rows
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"<[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>>  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