[SPARK-19712][SQL][FOLLOW-UP] reduce code duplication

[cloud-fan]

What changes were proposed in this pull request?

abstract some common code into a method.

How was this patch tested?

existing tests

[cloud-fan]

cc @dilipbiswal

[dilipbiswal]

LGTM
Thanks a lot for doing this @cloud-fan

[SparkQA]

Test build #104243 has finished for PR 24281 at commit cf68e6c.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #104244 has finished for PR 24281 at commit d5a17e9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

thanks, merging to master!