Github user mengxr commented on the pull request:
https://github.com/apache/spark/pull/268#issuecomment-39116485
@yinxusen I mean a column. You don't need to check the type as you already
use `activeIterator` to get the non-zero elements. The approach I suggested is
to only update mean/variance/min/max for non-zeros. After we scan all the
items, we know how many zeros we need to append to each column, and then we do
a constant-time update for each column.
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