Today, Jonathan Kehayias wrote about how fragmentation impacts execution plans. I’m really into performance tuning, so I read the post carefully, really interested to see how it impacted query runtime. You should read it too – it’s a great post with a solid demo.
But oddly, he didn’t mention the query speeds.
I thought that was kinda odd since that’s what users usually complain about – their queries running slow – so I fired open SSMS to check query runtimes, and…
The fragmented table ran faster. 3-4x faster!
- Fragmented table: ~500ms
- Rebuilt table: 1.5-2 seconds
Jonathan’s post explains why – SQL Server lowballs the cost estimates on the rebuilt table, causing the query plan to go single-threaded. With the fragmented table, it correctly estimates that a lot of work is going to be involved to scan those millions of rows, so it breaks the work across multiple cores – finishing it way faster.
What’s the Moral of the Story?
Rebuilding your indexes will slow queries down.
Just kidding – although the evidence does point to that. The real moral of the story is that it’s really hard to build demo code that effectively conveys the point you want to make. Jonathan’s right in that fragmentation changes the query plan. In this one case, it happens to make the query go faster.
“But what happens if you run tons of this query at exactly the same time?” you ask? As a consultant, my answer wouldn’t be to rebuild the indexes – because now, not only is your individual query slower, but the server’s still in bad shape due to the work involved with scanning 7M rows. This query maxes out an individual CPU core every time it runs – so concurrency is a hot mess.
In that case, the answer’s simple: create a nonclustered columnstore index. The query finishes in ~100ms with near-no CPU time and >10x less logical reads. That kind of magic doesn’t work in every situation – but as long as we’re using specific demos to prove a point, that one does the job quite nicely.
And that’s why in my defragmenting your indexes isn’t helping session, I explain that defragging won’t get your users to carry you through the hallways on their shoulders. The right indexes will.
Are you being paid fairly? Let's find out: the 2018 Data Professional Salary Survey is open.