SQL SERVER - Execution plan

Question

I have a VIEW in both Databases. At one database, takes less then 1 second to run and but in the other database 1 minute or more to go. I check indexes and everything is the same. The diference between the number of rows is below than 10 millions of rows from each other database.

I check de exectuion plan, and what i found is that, the database that takes more time, i have 3 Hash Match(1 aggregate and 2 right outer join) that is responssible for 100% on the query batch. On the other database i don't have this in the execution plan.

Can anyone tell me where can i begin to search the problem?

Thank you, sorry for the bad english.

Answer 1

You can check this link here for a quick explanation on different types of joins.

Basically, with the information you've given us, here are some of the alternatives for what might be wrong:

1. One DB has indexes the other doesn't.
2. The size difference between some of the joined tables in one DB over the other, is dramatic enough to change the type of join used.
3. While your indexes might be the same on both DB table groups, as you said.. it's possible the other DB has outdated / bad statistics or too much index fragmentation, resulting in sub-optimal plans.

EDIT:

Regarding your comment below, it's true that rebuilding indexes is similar to dropping & recreating indexes. And since creating indexes also creates the statistics for those indexes, rebuilding will take care of them as well. Sometimes that's not enough however.

While officially default statistics should be built with about 20% sampling rate of the actual data, in reality the sampling rate can be as low as just a few percents depending on how massive the table is. It's rarely anywhere near 20%. Because of that, many DBA's build statistics manually with FULLSCAN to obtain a 100% sampling rate.

The statistics take equally much storage space either way, so there are really no downsides to this aside from the extra time required in maintenance plans. In my current project, we have several situations where the default sampling rate for the statistics is not enough, and would still produce bad plans. So we routinely update all statistics with FULLSCAN every few weeks to make sure the performance stays top notch.