Command like SQL LIMIT in HBase

Question

Does HBase have any command that works like SQL LIMIT query?

I can do it by setStart and setEnd, but I do not want to iterate all

Answer 1

If one uses HBase Shell, the following command could be used to limit the query results:The "LIMIT" must be enclosed in single quotes.

scan 'table-name', {'LIMIT' => 10}

Answer 2

In HBase 1.2, the Scan.setMaxResultSize(N) may not act as a parameter of row number limitation. The maxResultSize limit the maximum result size in bytes (cached in the client side). I found the ResultScanner.next(int nbRows) can limit the row numbers during the iteration.

Answer 3

From the HBase shell you can use LIMIT:

hbase> scan 'test-table', {'LIMIT' => 5}

From the Java API you can use Scan.setMaxResultSize(N) or scan.setMaxResultsPerColumnFamily(N).

• HBase API docs - Scan.setMaxResultSize
• HBase API docs - Scan.setMaxResultsPerColumnFamily

Answer 4

There is a filter called PageFilter. Its meant for this purpose.

Scan scan = new Scan(Bytes.toBytes("smith-"));
scan.addColumn(Bytes.toBytes("personal"), Bytes.toBytes("givenName"));
scan.addColumn(Bytes.toBytes("contactinfo"), Bytes.toBytes("email"));
scan.setFilter(new PageFilter(25));
ResultScanner scanner = table.getScanner(scan);
for (Result result : scanner) {
    // ...
}

http://java.dzone.com/articles/handling-big-data-hbase-part-4

Answer 5

A guaranteed way is to do the limiting on the client side, inside the iterator loop. This is the approach taken in the HBase Ruby Shell. From table.rb ($HBASE_HOME/hbase-shell/src/main/ruby/hbase/table.rb): Line 467:

  # Start the scanner
  scanner = @table.getScanner(_hash_to_scan(args))
  iter = scanner.iterator

  # Iterate results
  while iter.hasNext
    if limit > 0 && count >= limit
      break
    end

    row = iter.next
    ...
 end

It can be made a bit more efficient by adding scan.setFilter(new PageFilter(limit)) and scan.setCaching(limit), and then table.getScanner(scan). The page filter will ensure that each region server will return at most limit rows, the scan caching limit will ensure that each region server will read ahead and cache at most 'limit' rows, and then the client loop limit checking can break the loop after getting the first 'limit' rows in the order received by the client.