Question

I’m working on some upgrades to an internal web analytics system we provide for our clients (in the absence of a preferred vendor or Google Analytics), and I’m working on the following query:

select      path as EntryPage,      count(Path) as [Count]  from      (         /* Sub-query 1 */         select              pv2.path         from              pageviews pv2                  inner join                     (                         /* Sub-query 2 */                         select                             pv1.sessionid,                             min(pv1.created) as created                         from                             pageviews pv1                                  inner join Sessions s1 on pv1.SessionID = s1.SessionID                                 inner join Visitors v1 on s1.VisitorID = v1.VisitorID                         where                             pv1.Domain = isnull(@Domain, pv1.Domain) and                             v1.Campaign = @Campaign                         group by                             pv1.sessionid                     ) t1 on pv2.sessionid = t1.sessionid and pv2.created = t1.created     ) t2 group by      Path; 

I’ve tested this query with 2 million rows in the PageViews table and it takes about 20 seconds to run. I’m noticing a clustered index scan twice in the execution plan, both times it hits the PageViews table. There is a clustered index on the Created column in that table.

The problem is that in both cases it appears to iterate over all 2 million rows, which I believe is the performance bottleneck. Is there anything I can do to prevent this, or am I pretty much maxed out as far as optimization goes?

For reference, the purpose of the query is to find the first page view for each session.

EDIT: After much frustration, despite the help received here, I could not make this query work. Therefore, I decided to simply store a reference to the entry page (and now exit page) in the sessions table, which allows me to do the following:

select     pv.Path,     count(*) from     PageViews pv         inner join Sessions s on pv.SessionID = s.SessionID             and pv.PageViewID = s.ExitPage         inner join Visitors v on s.VisitorID = v.VisitorID where     (         @Domain is null or          pv.Domain = @Domain     ) and     v.Campaign = @Campaign group by pv.Path; 

This query runs in 3 seconds or less. Now I either have to update the entry/exit page in real time as the page views are recorded (the optimal solution) or run a batch update at some interval. Either way, it solves the problem, but not like I’d intended.

Edit Edit: Adding a missing index (after cleaning up from last night) reduced the query to mere milliseconds). Woo hoo!

Answer 1

For starters,

    where pv1.Domain = isnull(@Domain, pv1.Domain)  

won’t SARG. You can’t optimize a match on a function, as I remember.