[SPARK-7118] [Python] Add the coalesce Spark SQL function available in PySpark
[SPARK-7118] [Python] Add the coalesce Spark SQL function available in PySpark

This patch adds a proxy call from PySpark to the Spark SQL coalesce function and this patch comes out of a discussion on devspark with rxin

This contribution is my original work and i license the work to the project under the project's open source license.

Olivier.

Author: Olivier Girardot <o.girardot@lateral-thoughts.com>

Closes #5698 from ogirardot/master and squashes the following commits:

d9a4439 [Olivier Girardot] SPARK-7118 Add the coalesce Spark SQL function available in PySpark

(cherry picked from commit 068c3158ac0c66e20d90a45e6a2a0b93108e08d5)
Signed-off-by: Reynold Xin <c9d35a384bc579d1c6c8c055f2a2b5c04e11b521@databricks.com>

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