How to count events in groups before a specific event

Suppose I have a table like the following:

\d events 
    Table "public.events
   Column   |           Type           | Modifiers 
------------+--------------------------+-----------
 my_id      | bigint                   | 
 tstamp     | timestamp with time zone | 
 event_type | text                     |

With example data:

   my_id     |           tstamp           | event_type 
------------+----------------------------+------------
 1111111111 | 2015-11-14 09:02:46.185+02 | A
 1111111111 | 2015-11-14 17:32:58+02     | B
 1111111111 | 2015-11-28 15:06:30.895+02 | A
 1111111111 | 2015-12-05 15:22:31.582+02 | A
 2222222222 | 2015-11-17 15:06:07.481+02 | A
 2222222222 | 2015-11-17 20:30:03+02     | B
 2222222222 | 2015-12-04 15:36:31.532+02 | A
 3333333333 | 2015-11-20 15:06:01.621+02 | A
 3333333333 | 2015-11-20 19:15:09.908+02 | A
 3333333333 | 2015-11-21 15:06:01.621+02 | A
 3333333333 | 2015-11-26 09:07:45.134+02 | B
 3333333333 | 2015-11-27 14:39:31.657+02 | A
 4444444444 | 2015-12-05 10:21:21.441+02 | A
 4444444444 | 2015-12-05 20:00:40.772+02 | B

I would like to count all events A per my_id before event B.

The expected output would be:

   my_id   | events_before_B 
-----------+-----------------
1111111111 | 1
2222222222 | 2
3333333333 | 3
4444444444 | 1

Postgres version 9.4

Solution

SELECT my_id
      ,position('B' IN string_agg(event_type, '')) - 1 events_before_B
FROM events
GROUP BY my_id
ORDER BY my_id

SQLFIDDLE-DEMO

Explanation:

string_agg()

select my_id,string_agg(event_type,'') from events group by my_id yields

my_id      string_agg 
---------- ---------- 
1111111111 ABAA       
2222222222 ABA        
3333333333 AAABA      
4444444444 AB

position()

select position('B' in 'AAABAA') can be used to find out the position of B in string AAABAA

position 
-------- 
4

So select position('B' in 'AAABAA')-1 yields the position of A just before B

position 
-------- 
3