Does Sqoop use Reducer? - database

Question

Does Sqoop run reducer if there is a join/aggregation performed in the select query given with a --query parameter? Or is there any case in Sqoop where both mappers and reducers run?
Documentation specifies that each map task will need to execute a copy of the query, with results partitioned by bounding conditions inferred by Sqoop.
$ sqoop import \
--query 'SELECT a.*, b.* FROM a JOIN b on (a.id == b.id) WHERE $CONDITIONS' \
--split-by a.id --target-dir /user/foo/joinresults
In the example above, how does the JOIN take place where the table is first partitioned using $CONDITIONS?

Answer 1

Join/Computation will be executed on RDBMS and its result will be used by mapper to transfer to HDFS.
No reducer is involved
With --query parameter, you need to specify the --split-by parameter with the column that should be used for slicing
your data into multiple parallel map tasks. This parameter usually automatically defaults to
the primary key of the main table
$CONDITIONS will be automatically substituted this placeholder with the generated conditions specifying which slice of data to transfer

Answer 2

In your particular command, the sqoop does not use reducer.
However there are cases, when sqoop do use reducer. Check the below example taken from document here.
$ sqoop export \
-Dmapred.reduce.tasks=2
-Dpgbulkload.bin="/usr/local/bin/pg_bulkload" \
-Dpgbulkload.input.field.delim=$'\t' \
-Dpgbulkload.check.constraints="YES" \
-Dpgbulkload.parse.errors="INFINITE" \
-Dpgbulkload.duplicate.errors="INFINITE" \
--connect jdbc:postgresql://pgsql.example.net:5432/sqooptest \
--connection-manager org.apache.sqoop.manager.PGBulkloadManager \
--table test --username sqooptest --export-dir=/test -m 2