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Hive, mail # user - Re: Custom UserDefinedFunction in Hive


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Re: Custom UserDefinedFunction in Hive
Jan Dolinár 2012-08-07, 17:37
I'm afraid that  he query

SELECT * FROM REALTIME where dt= yesterdaydate('yyyyMMdd') LIMIT 10;

will scan entire table, because the functions is evaluated at runtime, so
Hive doesn't know what the value is when it decides which files to scan. I
am not 100% sure though, you should try it.

Also, you might want to try to add annotation to your UDF saying that the
function is deterministic:
@*UDFType(deterministic=false*)

I think Hive might be clever enough to evaluate it early enough to use the
partition pruning correctly, since it operates on constant expression. But
again, I'm not really sure, maybe someone with deeper knowledge of Hive
optimizations will tell us more. It is actually quite interesting question.

Another way to help Hive with the optimizations might be to skip passing
the format string argument, if you have all dates in same format, you can
call the function just like 'yesterdaydate()' and hardcode the format in
the function.

Jan
On Tue, Aug 7, 2012 at 7:20 PM, Raihan Jamal <[EMAIL PROTECTED]> wrote:

> Hi Jan,
>
>
> I figured that out, it is working fine for me now. The only question I
> have is, if I am doing like this-
>
>
>
> SELECT * FROM REALTIME where dt= yesterdaydate('yyyyMMdd') LIMIT 10;
>
>
>
> Then the above query will be evaluated as below right?
>
>
>
> SELECT * FROM REALTIME where dt= ‘20120806’ LIMIT 10;
>
>
>
> So that means it will look for data in the corresponding dt partition *(20120806)
> *only right as above table is partitioned on dt column ? And it will not
> scan the whole table right?**
>
>
>
> *Raihan Jamal*
>
>
>
> On Mon, Aug 6, 2012 at 10:56 PM, Jan Dolinár <[EMAIL PROTECTED]> wrote:
>
>> Hi Jamal,
>>
>> Check if the function really returns what it should and that your data
>> are really in yyyyMMdd format. You can do this by simple query like this:
>>
>> SELECT dt, yesterdaydate('yyyyMMdd') FROM REALTIME LIMIT 1;
>>
>> I don't see anything wrong with the function itself, it works well for me
>> (although I tested it in hive 0.7.1). The only thing I would change about
>> it would be to optimize it by calling 'new' only at the time of
>> construction and reusing the object when the function is called, but that
>> should not affect the functionality at all.
>>
>> Best regards,
>> Jan
>>
>>
>>
>>
>> On Tue, Aug 7, 2012 at 3:39 AM, Raihan Jamal <[EMAIL PROTECTED]>wrote:
>>
>>> *Problem*
>>>
>>> I created the below UserDefinedFunction to get the yesterday's day in
>>> the format I wanted as I will be passing the format into this below method
>>> from the query.
>>>
>>>
>>>
>>> *public final class YesterdayDate extends UDF {*
>>>
>>> * *
>>>
>>> *                public String evaluate(final String format) { *
>>>
>>> *                                DateFormat dateFormat = new
>>> SimpleDateFormat(format); *
>>>
>>> *                                Calendar cal = Calendar.getInstance();*
>>>
>>> *                                cal.add(Calendar.DATE, -1);     *
>>>
>>> *                                return
>>> dateFormat.format(cal.getTime()).toString(); *
>>>
>>> *                } *
>>>
>>> *}*
>>>
>>>
>>>
>>>
>>>
>>> So whenever I try to run the query like below by adding the jar to
>>> classpath and creating the temporary function yesterdaydate, I always get
>>> zero result back-
>>>
>>>
>>>
>>> hive> create temporary function *yesterdaydate* as
>>> 'com.example.hive.udf.YesterdayDate';
>>>
>>> OK
>>>
>>> Time taken: 0.512 seconds
>>>
>>>
>>>
>>> Below is the query I am running-
>>>
>>>
>>>
>>> *hive> SELECT * FROM REALTIME where dt= yesterdaydate('yyyyMMdd') LIMIT
>>> 10;*
>>>
>>> *OK*
>>>
>>> * *
>>>
>>> And I always get zero result back but the data is there in that table
>>> for Aug 5th.**
>>>
>>>
>>>
>>> What wrong I am doing? Any suggestions will be appreciated.
>>>
>>>
>>>
>>>
>>>
>>> NOTE:- As I am working with Hive 0.6 so it doesn’t support variable
>>> substitution thing, so I cannot use hiveconf here and the above table has
>>> been partitioned on dt(date) column.**