android - Null User Id's recorded by firebase -

we linking our firebase analytics data bigquery , upon analyzing found user id's null several users.

in scenarios can happen? , solution same.

the bad behavior when user_id property populated value. property user_id should null except if set explicitly call function:

• android function: setuserid(...)
• ios function: setuserid(...)

if don't explicitly call setuserid, common, id should use in bigquery app_instance_id.

my advice: call setuserid once have logged in user (in facebook, google sign-in or whatever method offer). give cross-device id. if user not logged in, don't call it, , use app_instance_id id instead.

note app_instance_id change if user uninstall & reinstall app.