[jira] [Created] (PIG-4007) GROUP ALL on an empty table produces no output

["Philip (flip) Kromer (JIRA)" <ji...@apache.org>]

Philip (flip) Kromer created PIG-4007:
-----------------------------------------

             Summary: GROUP ALL on an empty table produces no output
                 Key: PIG-4007
                 URL: https://issues.apache.org/jira/browse/PIG-4007
             Project: Pig
          Issue Type: Bug
            Reporter: Philip (flip) Kromer


Using GROUP ALL on an empty table produces no output. I would expect it to produce a single row with key 'all' and an empty bag. This seems inconsistent with PIG-514.

{code}
vals = LOAD 'test/data/pigunit/top_queries_input_data.txt' AS (name:chararray, val:int); 
empty = FILTER vals BY (1 == 2); 
empty_g_1 = GROUP empty ALL; 
empty_g_2 = GROUP empty BY 1;
empty_g_1_stats = FOREACH empty_g_1 GENERATE 
  COUNT_STAR(empty); 
DUMP empty_g_1; 
DUMP empty_g_2; 
DUMP empty_g_1_stats; 
{code}

None of the previous statements produce output. My workaround is to COGROUP with a one-line table:

{code} 
one_line = FOREACH (LIMIT vals 1) GENERATE 1 AS uno; 
empty_cog = COGROUP one_line BY uno, empty BY 1; 
DUMP empty_cog;
{code}

A practical example of where it complicates things is set equality. This is best done by testing whether the symmetric difference has zero size:

{code}
vals_a = LOAD 'test/data/pigunit/top_queries_input_data.txt' AS (name:chararray, val:int); 
vals_b = LOAD 'test/data/pigunit/top_queries_input_data.txt' AS (name:chararray, val:int); 
a_xor_b = FILTER (COGROUP vals_a BY name, vals_b BY name)
  BY ((COUNT_STAR(vals_a) == 0L) OR (COUNT_STAR(vals_b) == 0L));
a_equals_b = FOREACH (GROUP a_xor_b ALL) GENERATE
  ((COUNT_STAR(a_xor_b) == 0) ? 1 : 0) AS is_equal; 
DUMP a_equals_b;
{code}



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