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From "Cardon, Tejay E" <>
Subject RE: EXTERNAL: Re: Iterators and seeking the middle of a row
Date Fri, 14 Sep 2012 14:37:55 GMT
So, if I understand what you're saying correctly, I would need to pass the raw source columns
as my column constraint, and then use some other mechanism to control the columns in my seek
calls.  That way, I ensure that bottom level iterator has the raw columns it needs, but the
intermediate Iterators can then be adjusted with crafted calls to seek() with appropriate
adjustments to the columns argument?  Is that correct?


From: Billie Rinaldi []
Sent: Friday, September 14, 2012 8:09 AM
Subject: Re: EXTERNAL: Re: Iterators and seeking the middle of a row

On Thu, Sep 13, 2012 at 4:44 PM, Cardon, Tejay E <<>>
Excellent, thank you William.  That raises an interesting point for me.  In my case, as with
the IntersectingIterator, the schema of my iterator's topKey and topValue is not the same
as the schema for the underlying source.

In IntersectingIterator, for example, the underlying source has data in the format;

row: shardID, colfam: term, colqual: docID

But the data being returned by the iterator is in the form

row: shardID, colfam: (empty), colqual: docID

Would I expect a seek on that iterator to have a range based on the ColF and ColQ being returned,
or the ones being used on the sources?

It appears from the code of IntersectingIterator that seek is called based on the out-going
schema, and the code then translates the keys in the range into the source schema before seeking
the sources.

Yes, I believe that is correct.  The seek is passed down through the iterator stack, so an
iterator that changes the schema has an opportunity to adjust the range when seeking its sources.
 We recently discovered that the behavior with column filtering is not as intuitive.  When
you fetch columns with a scanner, column filters are created at the system level (so they'll
be sources for user level iterators) and they are passed the set of fetched columns directly,
without giving the user iterators a chance to transform the columns.  This doesn't come up
with the IntersectingIterator because it manages the columns itself (there's no reason to
fetch columns with it), but in general this would be something to watch out for when writing
schema-transforming iterators.



From: William Slacum [<>]
Sent: Thursday, September 13, 2012 4:39 PM
Subject: EXTERNAL: Re: Iterators and seeking the middle of a row

Another thing to keep in mind is that the documentation is actually meant to enforce the notion
that, between returning keys, your iterator could be destroyed and reconstituted. If an iterator
is originally given a range, ("a", "c"), and it returns a key "b", the system *may* deconstruct
the iterator stack and at a later time, reinitialize it with the range ("b", "c"), since "b"
was the last place your iterator stack was known to be at.
On Thu, Sep 13, 2012 at 3:34 PM, William Slacum <<>>
Remember that the range given to an iterator is, at some point in time, user set. If a client
only wants to scan between keys K1 and K2, and each occur in the same row, then the iterator
should not be considering data that is outside of the range supplied to it. Someone can correct
me if I'm wrong, but I also believe that if a client received a key outside of the original
scan range, then that was considered a termination condition and the scan would stop.

Let's say I have a flat record structure for people, where the row is the name of the person,
the column family is some attribute about them, and the column qualifier is the value for
that attribute. Here's a record for Bob:

Bob eyes: blue
Bob hair: brown
Bob height: tall
Bob pants: brown
Bob shirt: white
Bob tie: blue

If you were searching for all attributes that were 'brown', you could do a look up using the
range `new Range("Bob", "Bob")`. Your iterator would be able to see all of Bob and return
to the user his hair and pants color. However, you could just as easily perform your look
up with `new Range(new Key("Bob", "height"), new Key("Bob", "z"))`*. Your iterator would then
be allowed to look at a subset of Bob, starting at his height and continuing until the end
of his record.

* I used "z" because it sorts lexicographically after the other attributes.
On Thu, Sep 13, 2012 at 1:01 PM, Keith Turner <<>>
On Thu, Sep 13, 2012 at 3:50 PM, Cardon, Tejay E
<<>> wrote:
> The javadoc for states:
> "Iterators that examine groups of adjacent key/value pairs (e.g. rows) to
> determine their top key and value should be sure that they properly handle a
> seek to a key in the middle of such a group (e.g. the middle of a row). Even
> if the client always seeks to a range containing an entire group (a,c), the
> tablet server could send back a batch of entries corresponding to (a,b],
> then reseek the iterator to range (b,c) when the scan is continued."
> However, it gives no indication of what proper handling is.  What should an
> iterator that considers and entire row do in this case?  Does it simply
> ignore the row?  Attempt to seek its source iterator to the full row of the
> first range?  I'm struggling to understand the best approach here
org.apache.accumulo.core.iterators.user.RowFilter does what you
suggested.  It seeks to the beggining of a row if the range starts in
the middle of the row.  Look at the javadoc for the row filter, it
discusses the seeking behavior.

> In my specific case, if it matters, I'm largely looking for ColumnQualifiers
> which exist in all Column Families in a given set (intersecting iterator,
> sortof).
> Thanks,
> Tejay

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