What can I do to help improve the estimates with this query?
I would be more concerned with the 3 RID lookups on the same object8 farther to the left. can you change the index and INCLUDE those columns so the seek on Object8.Index4 returns those columns too?
If the optimizer thought that a NL join was the correct way and it turns out the rows are less (way less in this instance) then it is alright. Had it been the other way and the actual row count was way higher then you would have really been in a fix. A hash or merge join would possibly produced quicker results.
(Reasonable chance that is a terrible explanation)
So my remaining question would be: Say it can only use a nested loop and not a hash/merge join…. what impact would the wrong estimate have on the actual query if it is going to do a nested loop join anyway? Maybe the join order would be impacted, but I'm still unclear on how better estimation on this can greatly impact the performance. Maybe it's a case of belated monday's… I know cardinality is important, but I'm having trouble remembering exactly where it will impact the query if it's still do the same join operation.
But I doubt this is worth it. A more accurate estimate is unlikely to change the plan shape much, from what I can see.
The query already executes pretty swiftly, given the number of joins. The 867ms of NETWORK_IO suggests the bulk of the time is spent waiting for the large rows (LOB data, lots of columns) to pass across the network. You might get a better measure of the raw query performance by writing the results to variables (or a temp table).
If doing that really does drop the elapsed time for the query down to 1389 – 867 ~= 500ms, I'd ne asking myself how much quicker I really need it to go.
Is a lower actual than the estimated less of a concern to you? Any insight on this would be appreciated.