Pass fields from base search to subsearch fails

Hi, I tried to do a base search, then pass fields to subsearch as both a filter and stat columns. I tested with following: index="_internal" | eval MyUser=user | table MyUser bytes | map search="search index="_audit" user=$MyUser$ | stats values(user), values($MyUser$) as MyUser, values($bytes$) as bytes, values(action) " user=$MyUser$ works but the columns for fields MyUser and bytes are empty. Please help. Thanks. Besides, will there be better and more efficient way to do so? Thanks a lot. /ST Wong