How can we query the Eventstore by aggregate root Id?
If I want to retrieve all the events related to an user how can I do that?
By putting all those events in the same stream (eg stream per user)
or when you have cross cutting things you want to query from many streams …
you can write a projection to reindex them based on say the user. Let’s imagine you had metadata with userId in it …
fromAll().when($any :function(s,e) {linkTo(“byuser-” + e.metadata.userId, e)
This will create a stream per user of all events having metadata with a userId associated to that user.
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Stream per user does it really scale if I have a million users?
Streams are essentially free in eventstore its intended to have many many millions of streams…
btw for a real fun one there is a system I know of that has approaching a Billion yes that’s with a B streams 
That is impressive! BTW I am big fan of you. Watched all your talks on event sourcing.
If I read all events from the partitions, it is just looping through n-items before i could find a event associated for an user. Is it really
efficient[?].
Yes, it is very efficient.
The projection will loop through the “n-items” only ONCE (or AGAIN when you reset), and re-index the matching events into a new stream. You can then subscribe to (or use another projection on**)** the new stream to get the shape of data (i.e. state) you desire.
Projecting from (or Subscribing to) a filtered stream is very efficient.
Hope this helps.
Foluso.