[Pig Wiki] Update of "FAQ" by DavidPhillips

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The following page has been changed by DavidPhillips:
http://wiki.apache.org/pig/FAQ

The comment on the change is:
this page is old, the new one is PigFaq

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- Pig FAQ
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- 1. I'm using PigStorage to parse my input files. Can I make it use control characters as delimiters?
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- A. Yes. Examples: PigStorage('\u0001') for Ctrl+A or '\u007C' for this character: |
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- 2. Can I do a numerical comparison while filtering?
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- A. Yes, you can choose between numerical and string comparison. For numerical comparison use the operators =, <>, <  etc. and for string comparisons use eq, neq etc. 
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- 3. How do I make my jobs run on multiple machines?
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- A. Use the PARALLEL clause. For example =C = JOIN A by url, B by url PARALLEL 50=
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- 4. Does Pig support NULLs?
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- A. Pig currently has no support for NULL values but it is on the roadmap.
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- 5. Does pig support regular expressions?
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- A. Pig does support regular expression matching via =matches= keyward. Tt uses java.util.regexp matches which means your pattern has to match the entire string (ie if your string is "hi fred" and you want to find "fred" you have to give a pattern of ".*fred" not "fred").
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- 6. How to prevent failure if some records don't have the needed number of columns.
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- You can filter away those records by including the following in your Pig program:
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- A = load 'foo' using PigStorage('\t');
- B = FILTER A BY ARITY(*) < 5;
- .....
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- This code would drop all the records that has less than 5 columns.
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- 7. Is there any difference between == and eq for numeric comparisons?
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- For equality, there is no difference while you stay in integers. However 11.0 and 11 will be equal with == but not with eq. 
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- 8. Is there an easy way for me to figure out how many rows exists in a dataset from its alias?
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- You can run the following set of commands:
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- a = load 'bla' ... ;
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- b = group a all;
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- c = foreach b generate COUNT(a.$0);
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- This is equivalent to select count(*) in SQL.
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- 9. Does Pig allow grouping on expressions
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- Currently, Pig only allows to group on data fields rather than expressions. Allowing grouping on expressions is on our road map. Stay tuned!
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