how is number of mappers determined in mapside join?

[Bruce Bian]

Hi there,
when I'm executing the following queries in hive

set hive.auto.convert.join = true;
CREATE TABLE IDAP_ROOT as
SELECT a.*,b.acnt_no
FROM idap_pi_root a LEFT OUTER JOIN idap_pi_root_acnt b ON
a.acnt_id=b.acnt_id

the number of mappers to run in the mapside join is 3, how is it
determined? When launching a job in hadoop mapreduce, i know it's
determined by the function
max(Min split size, min(Max split size, HDFS blockSize)) which in my
configuration is max(1B, min(256MB ,32MB)=32MB and the two tables are 460MB
and 1.5MB respectively.
Thus I thought the mappers to launch to be around 15, which is not the case.

Thanks
Bruce