Details
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Improvement
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Status: Closed
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Minor
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Resolution: Fixed
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None
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None
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None
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Reviewed
Description
The current DBInputFormat implementation uses SELECT ... LIMIT ... OFFSET statements to read from a database table. This actually results in several queries all accessing the same table at the same time. Most database implementations will actually use a full table scan for each such query, starting at row 1 and scanning down until the OFFSET is reached before emitting data to the client. The upshot of this is that we see O(n^2) performance in the size of the table when using a large number of mappers, when a single mapper would read through the table in O time in the number of rows.
This patch sets the number of map tasks to 1 in the MapReduce job sqoop launches.