Big Data for Chimps: A Guide to Massive-Scale Data Processing in Practice by Philip (flip) Kromer & Russell Jurney

Author:Philip (flip) Kromer & Russell Jurney [Kromer, Philip (flip)]
Language: eng
Format: epub
Publisher: O'Reilly Media
Published: 2015-09-27T21:00:00+00:00

Pattern in use

Where You’ll Use It

Anywhere you’re summarizing counts.

Standard Snippet

FOREACH (GROUP recs BY mykey) GENERATE group AS mykey, COUNT_STAR(recs) AS ct;.

Hello, SQL Users

SELECT key, COUNT(*) as CT from recs GROUP BY key;. Remember: COUNT_STAR(recs), not COUNT(*).

Important to Know

See “Pattern in use”.

Output Count

As many records as the cardinality of its key (i.e., the number of distinct values).

Records

Output is mykey, ct:long.

Dataflow

Map, combiner, and reduce; combiners are very effective unless cardinality is extremely high.

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