I've heard that index compression is not so usefult, so index block is not compressed.
And in the manual, they don't mention about compression of index block.
So I've thoght that index block is not compressed by default.
There's no index compression options in Wiredtiger engine (I know we can compress index block by specifying special configString option).

But actually this is not the case,
After creating collection with a few secondary indexes, I have examined data file with hexdump.
Primary key index is not compressed, but all secondary indexes are compressed.

Is this MongoDB expected or not ?
Why secondary indexes are compressed ? and primary index is not ?
And (if this is expected case) could you mention this in mongodb manual ?