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https://issues.apache.org/jira/browse/MAPREDUCE-885?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Aaron Kimball updated MAPREDUCE-885:
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Attachment: MAPREDUCE-885.patch
This patch introduces DataDrivenDBInputFormat. This class extends DBInputFormat
and reuses much of its common logic (e.g., setting up and tearing down
connections, configuration, DBWritable, etc). But it adds a
DataDrivenDBInputSplit class which splits queries based on data values, e.g.
{{"id >= 10 AND id < 20"}} for one split, and {{"id >= 20 AND id < 30"}} for
the next one. The resulting queries run significantly faster and parallelise
properly.
Instead of requiring a counting query like DBInputFormat, this InputFormat
requires a query that returns the min and max values of the split column on the
data to import. DataDrivenDBInputSplit is a subclass of DBInputSplit; the
original DBRecordReader family of classes has been modified to discriminate
between the new InputSplit class vs. the old one; if it detects a new one, it
submits the newer WHERE-based query rather than the LIMIT/OFFSET-based query to
the database.
The min and max values of the column are used to generate splits via linear
interpolation between the values. A DBSplitter interface has been added, which
takes the min and max values for the column, as well as the number of splits to
use. It then generates about this many splits, which subdivide the range of
values into roughly-even intervals. Several DBSplitter implementations are
provided which are applicable to different data types. For example, there is an
IntegerSplitter which can split INTEGER, BIGINT, TINYINT, LONG, etc. columns.
The FloatSplitter implementation works on DECIMAL, NUMBER, and REAL datatypes.
A TextSplitter implementation is provided, but its utility is
database-dependent. Databases may choose to sort strings via a number of
algorithms (e.g., case-sensitive vs. case-insensitive). The TextSplitter
assumes that strings are sorted in Unicode codepoint order. (e.g., "AAA" <
"BBB" < "aaa".) A warning will be logged if the TextSplitter is used.
Explicit tests have been added for some of the splitters. Sqoop has been
modified to use the new InputFormat with encouraging performance results.
Sqoop's existing regression test suite exercises the code paths for all the
splitters and isolated several bugs which were fixed prior to submitting this
patch. I will post the Sqoop patch separately after this JIRA issue is
committed.
> More efficient SQL queries for DBInputFormat
> --------------------------------------------
>
> Key: MAPREDUCE-885
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-885
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Reporter: Aaron Kimball
> Assignee: Aaron Kimball
> Attachments: MAPREDUCE-885.patch
>
>
> DBInputFormat generates InputSplits by counting the available rows in a
> table, and selecting subsections of the table via the "LIMIT" and "OFFSET"
> SQL keywords. These are only meaningful in an ordered context, so the query
> also includes an "ORDER BY" clause on an index column. The resulting queries
> are often inefficient and require full table scans. Actually using multiple
> mappers with these queries can lead to O(n^2) behavior in the database, where
> n is the number of splits. Attempting to use parallelism with these queries
> is counter-productive.
> A better mechanism is to organize splits based on data values themselves,
> which can be performed in the WHERE clause, allowing for index range scans of
> tables, and can better exploit parallelism in the database.
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