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RE: Table Wrapper
Peter Marron 2013-06-28, 08:52
Yes indeed. Thank you for prompting me to post again.
It is always helpful to get a response, any response.
There is nothing worse than posting and getting absolutely
nothing in reply.

As you say I have now got quite a few suggestions and even if they
don't all pan out I will have learnt a lot. And I feel happier now with
my ugly hack knowing that at least I haven't missed anything too obvious :)

Thanks again.

Z

From: Stephen Sprague [mailto:[EMAIL PROTECTED]]
Sent: 27 June 2013 16:40
To: [EMAIL PROTECTED]
Subject: Re: Table Wrapper

Well.  You got a few suggestions there Peter.  That in itself is reason to celebrate!

And that was a good description and i fault you not for going into some detail.  The part about keeping it simple is always a challenge I know. :)
I get your point but i don't have anything more really to offer. if running an MR job is a blocker i'm not in a position to suggest anything.

On Thu, Jun 27, 2013 at 3:14 AM, Peter Marron <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:
Hi,

If you're suggesting that I use something like

SELECT * FROM data WHERE MyUdf(data. BLOCK__OFFSET__INSIDE__FILE);

rather than
SELECT * FROM data JOIN small ON data.BLOCK__OFFSET__INSIDE__FILE = small.offset;
then, yes, I have thought of that. However the fact is that reading the billions of records
and filtering them is too slow compared to doing the seeks.

Partitioning would help, but I can't assume that the big data is partitioned in a way that suits
this particular query.

Z

From: Jan Dolinár [mailto:[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>]
Sent: 27 June 2013 10:59
To: user
Subject: Re: Table Wrapper

Slightly less "hackish" way to do this without joins is to write custom UDF that will take data.BLOCK__OFFSET__INSIDE__FILE as input parameter and return the corresponding data from the small file. If you mark it "deterministic" using @UDFType(deterministic = true), the performance should be quite good.

To avoid the full table scan, partitioning is IMHO the best way to speed things up.

Best regards,
J. Dolinar

On Thu, Jun 27, 2013 at 11:18 AM, Peter Marron <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:
Hi,

I have thought about a map-only join, but as I understand it this is still going
to do a full table scan on my large data file. If this is billions of records then it's
still going to be slow, even if it only returns a handful of records.

Also I don't know of any way to get Hive to do a join without performing a
Map/Reduce. And, as I mentioned before, just the overheads of setting
up a Map/Reduce, even if it's map only and does practically nothing, makes the
elapsed time too high. I want it to be interactive. (I guess that something Tez
when it becomes available might solve this problem...)

My ugly "hack" approach works in seconds, the overhead for setting up Map/Reduce takes this into minutes.

Indexing looks promising but, as far as I can see, it can't be done without a Map/Reduce.

If I could find a way to perform a join or use indexing without a Map/Reduce I would be happy to use that approach.

Partitioning and ORC would be helpful but I can't assume anything about the original data format.

Z

From: Nitin Pawar [mailto:[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>]
Sent: 27 June 2013 09:52

To: [EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>
Subject: Re: Table Wrapper

few thoughts:
If you have a smaller file (in size of MB's) have you tried considering map only join?
also if you are interested in particular records from a table and do not want to go through entire table to find them, then partitioning + indexing will be handy.

ORCFile Format (still very new) can help you in this regard as well.

On Thu, Jun 27, 2013 at 2:16 PM, Peter Marron <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:
Well, I'm not very good at keeping things brief, unfortunately.
But I'll have a go, trying to keep things simple.

Suppose that I have a data table in Hive and it has many rows - say billions.
I have another file stored in HDFS (it can be a Hive table too if it helps)
and this file is small and contains file offsets into the data, Stored as binary,
8 bytes per offset. Now suppose that I want to read the records from the data
defined by the offsets in the small file, in the order defined in the small file.

How can I do that?

The obvious way is to turn the small file into a Hive table and provide a custom
InputFormat which can read the binary. I've done that, that's the easy part and
then I could form a query like this:

                SELECT * FROM data JOIN small ON data. ON data.BLOCK__OFFSET__INSIDE__FILE = small.offset;

But, when it works, this performs awfully.

The approach that I have taken is to create a "copy" of the data table which is "hacked" to use a custom input
format which knows about the small file and which overrides the record reader to use the offsets
as seeks before it reads the records. This is awkward, for various reasons, but it works well. I can
avoid a full table scan, in fact I can suppress any Map/Reduce and so the query runs very quickly.

So I was just trying to "wrap" the data table so that I didn't have to create the copy.

I hope that you don't regret asking too much.

Regards,

Z

From: Stephen Sprague [mailto:[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>]
Sent: 25 June 2013 18:37
To: [EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>
Subject: Re: Table Wrapper

Good luck, bro. :)   May i ask why are you doing this to yourself? I think your instincts are correct going down the path you describe sounds a tad more painful than just hitting yourself in the head with a hammer. Different strokes for different folks though.
so can we back up? what - briefly if possible - do you want to achieve with a "wrapper"? (i'm going to regret asking that i know.)
On Tue, Jun 25, 2013 at 7:29 AM, Peter Marron <Peter.Marron@trillium