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Fun with Query Tuning in SQL Server 2019

Erik Darling has just wrapped up a nice series on tackling a problem which looks like parameter sniffing but isn’t. Part 2 covers the issue:

This isn’t always the exact case, but generally speaking you’ll observe something along these lines.

It’s definitely not the case for what we’re going to be looking at this week.

This week is far more interesting.

That’s why it’s a monstrosity.

Part three digs in:

It’s not parameter sniffing, but it sure could feel like it.

– When the procedure compiles and runs with VoteTypeId 5, it runs for 12 minutes
– Other VoteTypeIds run well with the same plan that VoteTypeId 5 gets
– When VoteTypeId 5 runs with a “small” plan, it does okay at 10 seconds

Part four gives us a solution without using OPTIMIZE FOR MEDIOCRE:

This is what happens when we optimize for unknown. The density vector guess is 13,049,400.

That guess for Vote Types with very few rows ends up with a plan that has very high startup costs.

This version of the query will run for 13-17 seconds for any given parameter. That sucks in zero gravity.

Part 5 looks into something which occasionally pops up with this query:

You see, there’s a mystery plan.

It only shows up once in a while, like Planet X. And when it does, we get bombarded by asteroids.

Just like when Planet X shows up.

I wouldn’t call it a good all-around plan, but it does something that we would want to happen when we run this proc for VoteTypeId 5.

Read on for an educational romp through the SQL Server 2019 optimizer.

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