You are viewing an old version of this page. View the current version.

Compare with Current View Page History

« Previous Version 6 Next »

Q: What shall I do if I saw "Failed to create DataStorage"?

This usually happens when you are connecting hadoop cluster other than standard Apache hadoop 20.2 release. Pig bundles standard hadoop 20.2 jars in release. If you want to connect to other version of hadoop cluster, you need to replace bundled hadoop 20.2 jars with compatible jars. You can try:

  1. do "ant"
  2. copy hadoop jars from your hadoop installation to overwrite ivy/lib/Pig/hadoop-core-0.20.2.jar and ivy/lib/Pig/hadoop-test-0.20.2.jar
  3. do "ant" again
  4. cp pig.jar to overwrite pig-*-core.jar

Some other tricks is also possible. You can use "bin/pig -secretDebugCmd" to inspect the command line of Pig. Make sure you are using the right version of hadoop.
And this issue will be solved in Pig 0.9.1 and beyond.

Q: How can I load data using Unicode control characters as delimiters?

The first parameter to PigStorage is the dataset name, the second is a regular expression to describe the delimiter. We used `String.split(regex, -1)` to extract fields from lines. See java.util.regex.Pattern for more information on the way to use special characters in regex.

If you are loading a file which contains Ctrl+A as separators, you can specify this to PigStorage using the Unicode notation.

LOAD 'input.dat' USING PigStorage('\u0001')as (x,y,z);

Q: How do I make my jobs run on multiple machines?

Use the PARALLEL clause:

C = JOIN A by url, B by url PARALLEL 50;

Q: How do I make my Pig jobs run on a specified number of reducers?

You can achieve this with the PARALLEL clause. For example: 

C = JOIN A by url, B by url PARALLEL 50.

Besides PARALLEL clause, you can also use "set default_parallel" statement in Pig script, or set "mapred.reduce.tasks" system property to specify default parallel to use. If none of these values are set, Pig will only use 1 reducers. (In Pig 0.8, we change the default reducer from 1 to a number calculated by a simple heuristic for foolproof purpose)

Q: Can I do a numerical comparison while filtering?

Yes, you can choose between numerical and string comparison. For numerical comparison use the operators =, <>, < etc. and for string comparisons use eq, neq etc.

Q: Does Pig support regular expressions?

Pig does support regular expression matching via the `matches` keyword. It uses java.util.regex matches which means your pattern has to match the entire string (e.g. if your string is `"hi fred"` and you want to find `"fred"` you have to give a pattern of `".*fred"` not `"fred"`).

Q: How do I prevent failure if some records don't have the needed number of columns?

You can filter away those records by including the following in your Pig program:

A = LOAD 'foo' USING PigStorage('\t');
B = FILTER A BY ARITY(*) < 5;
.....

This code would drop all records that have fewer than five (5) columns.

Q: Is there any difference between `==` and `eq` for numeric comparisons?

There is no difference when using integers. However, `11.0` and `11` will be equal with `==` but not with `eq`.

Q: Is there an easy way for me to figure out how many rows exist in a dataset from it's alias?

You can run the following set of commands, which are equivalent to `SELECT COUNT(star)` in SQL:

a = LOAD 'mytestfile.txt';
b = GROUP a ALL;
c = FOREACH b GENERATE COUNT(a.$0);

Q: Does Pig allow grouping on expressions?

Pig allows grouping of expressions. For example:

grunt> a = LOAD 'mytestfile.txt' AS (x,y,z);
grunt> DUMP a;
(1,2,3)
(4,2,1)
(4,3,4)
(4,3,4)
(7,2,5)
(8,4,3)

b = GROUP a BY (x+y);
(3.0,{(1,2,3)})
(6.0,{(4,2,1)})
(7.0,{(4,3,4),(4,3,4)})
(9.0,{(7,2,5)})
(12.0,{(8,4,3)})

If the grouping is based on constants, the result is the same as GROUP ALL except the group-id is replaced by the constant.

grunt> b = GROUP a BY 4;
(4,{(1,2,3),(4,2,1),(4,3,4),(4,3,4),(7,2,5),(8,4,3)})

Q: Is there a way to check if a map is empty?

In Pig 2.0 you can test the existence of values in a map using the null construct:
m#'key' is not null

Q: How can I specify the number of nodes Pig allocates?

> pig -Dhod.param='-m 3' my_script.pig

Three (3) nodes is the minimum.

Q: I load data from a directory which contains different file. How do I find out where the data comes from?
You can write a LoadFunc which append filename into the tuple you load.

Eg, 

A = load '*.txt' using PigStorageWithInputPath();

Here is the LoadFunc:

public class PigStorageWithInputPath extends PigStorage {
    Path path = null;

    @Override
    public void prepareToRead(RecordReader reader, PigSplit split) {
        super.prepareToRead(reader, split);
        path = ((FileSplit)split.getWrappedSplit()).getPath();
    }

    @Override
    public Tuple getNext() throws IOException {
        Tuple myTuple = super.getNext();
        if (myTuple != null)
            myTuple.append(path.toString());
        return myTuple;
    }
}

In Pig 0.8.0 and beyond, you need to set "pig.splitCombination" to false for PigStorageWithInputPath work correctly.

  • No labels