Philip (flip) Kromer created PIG-4007:
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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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