Grokbase Groups HBase user April 2013
FAQ
Silly question...
Null support. In a system where a column may or may not exist, how do you support null?

;-)

In terms of a key, it's a primary key and can't be null.


So what am I missing?


Sent from a remote device. Please excuse any typos...

Mike Segel
On Apr 1, 2013, at 10:26 PM, Nick Dimiduk wrote:

Furthermore, is is more important to support null values than squeeze all
representations into minimum size (4-bytes for int32, &c.)?
On Apr 1, 2013 4:41 PM, "Nick Dimiduk" wrote:
On Mon, Apr 1, 2013 at 4:31 PM, James Taylor wrote:

From the SQL perspective, handling null is important.

From your perspective, it is critical to support NULLs, even at the
expense of fixed-width encodings at all or supporting representation of a
full range of values. That is, you'd rather be able to represent NULL than
-2^31?
On 04/01/2013 01:32 PM, Nick Dimiduk wrote:

Thanks for the thoughtful response (and code!).

I'm thinking I will press forward with a base implementation that does
not
support nulls. The idea is to provide an extensible set of interfaces,
so I
think this will not box us into a corner later. That is, a mirroring
package could be implemented that supports null values and accepts
the relevant trade-offs.

Thanks,
Nick

On Mon, Apr 1, 2013 at 12:26 PM, Matt Corgan <mcorgan@hotpads.com>
wrote:

I spent some time this weekend extracting bits of our serialization
code to
a public github repo at http://github.com/hotpads/**data-tools<http://github.com/hotpads/data-tools>
.
Contributions are welcome - i'm sure we all have this stuff laying
around.

You can see I've bumped into the NULL problem in a few places:
*

https://github.com/hotpads/**data-tools/blob/master/src/**
main/java/com/hotpads/data/**primitive/lists/LongArrayList.**java<https://github.com/hotpads/data-tools/blob/master/src/main/java/com/hotpads/data/primitive/lists/LongArrayList.java>
*

https://github.com/hotpads/**data-tools/blob/master/src/**
main/java/com/hotpads/data/**types/floats/DoubleByteTool.**java<https://github.com/hotpads/data-tools/blob/master/src/main/java/com/hotpads/data/types/floats/DoubleByteTool.java>

Looking back, I think my latest opinion on the topic is to reject
nullability as the rule since it can cause unexpected behavior and
confusion. It's cleaner to provide a wrapper class (so both
LongArrayList
plus NullableLongArrayList) that explicitly defines the behavior, and
costs
a little more in performance. If the user can't find a pre-made wrapper
class, it's not very difficult for each user to provide their own
interpretation of null and check for it themselves.

If you reject nullability, the question becomes what to do in situations
where you're implementing existing interfaces that accept nullable
params.
The LongArrayList above implements List<Long> which requires an
add(Long)
method. In the above implementation I chose to swap nulls with
Long.MIN_VALUE, however I'm now thinking it best to force the user to
make
that swap and then throw IllegalArgumentException if they pass null.


On Mon, Apr 1, 2013 at 11:41 AM, Doug Meil <
doug.meil@explorysmedical.com
wrote:
HmmmŠ good question.

I think that fixed width support is important for a great many rowkey
constructs cases, so I'd rather see something like losing MIN_VALUE and
keeping fixed width.




On 4/1/13 2:00 PM, "Nick Dimiduk" wrote:

Heya,
Thinking about data types and serialization. I think null support is
an
important characteristic for the serialized representations,
especially
when considering the compound type. However, doing so in directly
incompatible with fixed-width representations for numerics. For
instance,
if we want to have a fixed-width signed long stored on 8-bytes, where
do
you put null? float and double types can cheat a little by folding
negative
and positive NaN's into a single representation (this isn't strictly
correct!), leaving a place to represent null. In the long example
case,
the
obvious choice is to reduce MAX_VALUE or increase MIN_VALUE by one.
This
will allocate an additional encoding which can be used for null. My
experience working with scientific data, however, makes me wince at
the
idea.

The variable-width encodings have it a little easier. There's already
enough going on that it's simpler to make room.

Remember, the final goal is to support order-preserving serialization.
This
imposes some limitations on our encoding strategies. For instance,
it's
not
enough to simply encode null, it really needs to be encoded as 0x00 so
as
to sort lexicographically earlier than any other value.
What do you think? Any ideas, experiences, etc?

Thanks,
Nick

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