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From "pat chan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PIG-3341) Improving performance of loading datetime values
Date Tue, 04 Jun 2013 00:40:20 GMT

    [ https://issues.apache.org/jira/browse/PIG-3341?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13673876#comment-13673876
] 

pat chan commented on PIG-3341:
-------------------------------

I was looking in the docs for any documentation on this topic. I found the following in http://wiki.apache.org/pig/UDFManual

<quote>
The first thing to decide is what to do with invalid data. This depends on the format of the
data. If the data is of type bytearray it means that it has not yet been converted to its
proper type. In this case, if the format of the data does not match the expected type, a null
value should be returned. If, on the other hand, the input data is of another type, this means
that the conversion has already happened and the data should be in the correct format. This
is the case with our example and that's why it throws an error (line 16.) Note that WrappedIOException
is a helper class to convert the actual exception to an IOException.

Also, note that lines 10-11 check if the input data is null or empty and if so returns null.
</quote>

If I'm reading this correctly, it says that if the type of the input doesn't match the signature
of the UDF, a null should be returned. However, I get this:

  grunt> A = load 'o' as (a:bytearray);
  grunt> B = foreach A generate ToDate(a); dump B;
  2013-06-03 17:15:09,253 [main] ERROR org.apache.pig.tools.grunt.Grunt - ERROR 1046: 
  <line 2, column 23> Multiple matching functions for org.apache.pig.builtin.ToDate
with input schema: ({long}, {chararray}). Please use an explicit cast.

It also seems to be saying that if the types are right and the format is invalid, an error
should be thrown. I just checked and yes, I get an error. However, this doesn't match Rohini's
proposal to return a null instead. Also, as Dmitriy hinted, it's not philosophically consistent
with loading behavior where invalid things turn into nulls.

  2013-06-03 17:25:12,977 [main] INFO  org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MapReduceLauncher
- Failed!
  2013-06-03 17:25:12,981 [main] ERROR org.apache.pig.tools.grunt.Grunt - ERROR 1066: Unable
to open iterator for alias B


BTW, the note about lines 10-11 isn't quite right. The code in the example doesn't have a
check for null and so a null would cause an exception.

                
> Improving performance of loading datetime values
> ------------------------------------------------
>
>                 Key: PIG-3341
>                 URL: https://issues.apache.org/jira/browse/PIG-3341
>             Project: Pig
>          Issue Type: Improvement
>          Components: impl
>    Affects Versions: 0.11.1
>            Reporter: pat chan
>            Priority: Minor
>             Fix For: 0.12, 0.11.2
>
>
> The performance of loading datetime values can be improved by about 25% by moving a single
line in ToDate.java:
>     public static DateTimeZone extractDateTimeZone(String dtStr) {
>       Pattern pattern = Pattern.compile("(Z|(?<=(T[0-9\\.:]{0,12}))((\\+|-)\\d{2}(:?\\d{2})?))$");;
> should become:
>     static Pattern pattern = Pattern.compile("(Z|(?<=(T[0-9\\.:]{0,12}))((\\+|-)\\d{2}(:?\\d{2})?))$");
>     public static DateTimeZone extractDateTimeZone(String dtStr) {
> There is no need to recompile the regular expression for every value. I'm not sure if
this function is ever called concurrently, but Pattern objects are thread-safe anyways.
> As a test, I created a file of 10M timestamps:
>   for i in 0..10000000
>     puts '2000-01-01T00:00:00+23'
>   end
> I then ran this script:
>   grunt> A = load 'data' as (a:datetime); B = filter A by a is null; dump B;
> Before the change it took 160s.
> After the change, the script took 120s.
> ----------------
> Another performance improvement can be made for invalid datetime values. If a datetime
value is invalid, an exception is created and thrown, which is a costly way to fail a validity
check. To test the performance impact, I created 10M invalid datetime values:
>   for i in 0..10000000
>     puts '2000-99-01T00:00:00+23'
>   end
> In this test, the regex pattern was always recompiled. I then ran this script:
>   grunt> A = load 'data' as (a:datetime); B = filter A by a is not null; dump B;
> The script took 190s.
> I understand this could be considered an edge case and might not be worth changing. However,
if there are use cases where invalid dates are part of normal processing, then you might consider
fixing this.

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