Create general row selection iterator
-------------------------------------
Key: ACCUMULO-403
URL: https://issues.apache.org/jira/browse/ACCUMULO-403
Project: Accumulo
Issue Type: New Feature
Components: client, tserver
Reporter: Keith Turner
Assignee: Billie Rinaldi
Fix For: 1.5.0
The WholeRowIterator support filtering rows that meet a certain criteria.
However it reads the entire row into memory. It is possible to efficiently
select rows w/o reading them into memory by using two iterators. One iterator
for selection, one for reading. When its determined that a row is not needed
using the selection iterator, then seek the read iterator over the row.
This pattern could be made into an easy to use iterator that users extend. The
iterator could have an abstract method that user implement to decide if they
want to select or filter a row. Could look something like the following.
{noformat}
class RowSelectionIterator extends WrappingIterator {
public abstract boolean selectRow(SortedKeyValueIterator row);
}
{noformat}
Below is a simple example of a row selection iterator that returns rows that
have the columns foo and bar.
{noformat}
class FooBarRowSelector extends RowSelectionIterator {
public boolean selectRow(SortedKeyValueIterator row){
Text row = row.getTopKey().getRow();
//seek instead of scanning, this more efficient for large rows w/ lots of
columns...
//if the row only has a few columns scanning is probably faster... also
seeking the
//columns in sorted order is more efficient.
row.seek(Range.exact(row, 'bar');
boolean sawBar = row.hasTop();
row.seek(Range.exact(row, 'foo'));
boolean sawFoo = row.hasTop();
return sawBar && sawFoo;
}
}
{noformat}
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