can pg 9.6 vacuum freeze skip page on index?

Hello,

we upgraded our postgresql db to 9.6, as we know, pg9.6 doesn't need full table scan in vacuum freeze.

http://rhaas.blogspot.com/2016/03/no-more-full-table-vacuums.html

so we think if we have run vacuum freeze on the table, and there is no change on table after vacuum freeze, and we run vacuum freeze again, it should finish super faster.

However, it doesn't like we expect. the next run of vacuum freeze still take long time. Then we run vacuum freeze with verbose. we notice it spends long time on scanning index.

looks like even all rows are frozen on the data page, vacuum freeze still needs to scan all the index pages. if we drop the index, then vacuum freeze finishes immediately.

Does anyone know if it is true?

Btw, our table is large, and has about 40GB index files. is there anyway to make the vacuum freeze faster in this case?

Thanks,

James