How can i optimize the below query?

Question

I have a table like this.

_id (integer)
event_name(varchar(20))
event_date(timestamp)

Here is some sample data given below.

---

ID         event_date                          event_name
101        2013-04-24 18:33:37.694818          event_A
102        2013-04-24 20:34:37.000000          event_B
103        2013-04-24 20:40:37.000000          event_A
104        2013-04-25 01:00:00.694818          event_B
105        2013-04-25 12:00:15.694818          event_A

I need the data from above table in below format.

Date          count_eventA                       count_eventB
2013-04-24    2                                   1
2013-04-25    1                                   1

hence basically in need the count of each event on each date.

I have tried below query for getting the desired result.

SELECT A.date1 AS Date ,
       A.count1 AS count_eventA,
       B.count2 AS count_eventB,
           FROM
           (SELECT count(event_name)AS count1,
            event_date::date AS date1
            FROM tblname
            WHERE event_name='event_A'
            GROUP BY (event_date::date))AS A 
           LEFT JOIN
           (SELECT count(event_name)AS count1,
            event_date::date AS date1
            FROM tblname
            WHERE event_name='event_B'
            GROUP BY (event_date::date))AS B ON A.date1=B.date2 

Can someone please suggest me to find out a better and optimized query? , or I am following a good approach .

Answer 1

Something on this lines should work:

select event_date::date AS Date ,
        count_eventA = sum(case when event_name = 'event_A' then 1 else 0 end),
     count_eventB = sum(case when event_name = 'event_B' then 1 else 0 end)
from tblname
GROUP BY (event_date::date))

If you have more events you only need to add more sum(case) lines :)

The DBEngine only runs through the table once to give you the totals, independiently of the number of the events you want to count: when you have a high rowcount you will observe significant delay with the original query. Should I add this to my answer, you think