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From "Keith Turner (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-403) Create general row selection iterator
Date Mon, 12 Mar 2012 15:48:37 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-403?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13227627#comment-13227627
] 

Keith Turner commented on ACCUMULO-403:
---------------------------------------

I will add something to the javadoc.
                
> 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: Keith Turner
>             Fix For: 1.4.0
>
>
> 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();
>       if(!sawBar)
>         return false;
>       row.seek(Range.exact(row, 'foo'));
>       boolean sawFoo = row.hasTop();
>       return sawFoo;
>    }
> }
> {noformat}

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