What happens if the number of reducers is 0 in Hadoop?
If we set the number of reducer to 0, then no reducer will execute and no aggregation will take place. In such case, we will prefer “Map-only job” in Hadoop. In a map-Only job, the map does all task with its InputSplit and the reducer does no job. Map output is the final output.
Between map and reduce phases there is key, sort, and shuffle phase. Sort and shuffle phase are responsible for sorting the keys in ascending order. Then grouping values based on same keys. This phase is very expensive. If reduce phase is not required we should avoid it. Avoiding reduce phase would eliminate sort and shuffle phase as well. This also saves network congestion. As in shuffling an output of mapper travels to the reducer, when data size is huge, large data travel to the reducer.
