Estimated Number Of Rows To Be Read

Rob Farley discusses a successful Connect item’s implementation:

I opened it up, and sure enough, no sign of that 7,276 value. It looks just the same as the estimated plan I just showed.

Getting plans out of the cache is where the estimated values come into their own. It’s not just that I’d prefer to not actually run potentially-expensive queries on customer databases. Querying the plan cache is one thing, but running queries to get the actuals – that’s a lot harder.

With SQL 2016 SP1 installed, thanks to that Connect item, I can now see the Estimated Number of Rows to be Read property in estimated plans, and in the plan cache. The operator tooltip shown here is taken from the cache, and I can easily see that Estimated property showing 7,276. This is shown from Management Studio because Plan Explorer doesn’t yet call out this property explicitly:

If you’re looking to use SQL Server 2016 SP1, read the whole thing; this will make query tuning without running those horribly expensive queries a bit easier.

Wanted: Forced Parallelism

James Anderson would like a query hint to force parallelism:

I would like to see a hint that causes the optimiser to consider a parallel plan no matter the cost of the query. It’s possible to get this behaviour with trace flag 8649 but it’s unsupported by Microsoft and therefore unfit for production use.

I only tend to use query hints as a very last resort. It’s almost always better to allow the optimiser to make these decisions and continue to reevaluate these decisions as your data changes but sometimes they can be a get out of jail free card.

I’ve written about this before and so has Paul White. Paul created this connect item for something similar. Please vote it up if you think it could help with your query tuning.

Click through for the full argument, and then hit the Connect item if you agree.

Columnstore Query Patterns

Niko Neugebauer gives a couple hints on how to make columnstore queries as fast as possible:

On my VM with 4 cores it takes 33 seconds to execute this query on SQL Server 2016 with Service Pack 1, while it burns almost 48 seconds of the CPU Time.
The relevant part of the execution plan can be found below, showing so many performance problems that this query is suffering, such as INNER LOOP JOIN, INDEX SPOOL, besides even worse part that is actually hidden and is identifiable only once you open the properties of any of the lower tree (left side of the LOOP JOIN), seeing that it all runs with the Row Execution Mode actually.

To show you the problem, on the left side you will find the properties of the sort iterator that is to be found in the lower (left) part of the LOOP Join that was executed around 770.000 times in the Row Execution Mode, effectively taking any chances away from this query to be executed in a fast way. One might argue that it might that it might be more effective to do the loop part in Row Mode, but given that we are sorting around 3.1 Million Rows there – for me there is no doubt that it would be faster to do it within a Batch Execution Mode. Consulting the last sort iterator in the execution plan (TOP N SORT), you will find that it is running with the help of the Batch Execution Mode, even though it is processing around 770.000 rows.

There’s some valuable information here.

TempDB And Parallelism

Kendra Little looks at cases when a query uses multiple tempdb data files:

As you might guess, things may not always get evenly accessed, even if you have evenly sized tempdb files. One of my queries did a select into a temp table. Although it used all four tempdb files whether or not it went parallel, there were more file_read events against the first tempdb file than against the other four.

It’s an interesting look at this specific question, but also as pedagogical technique.

Computed Columns And Columnstore

Kendra Little exposes a gotcha with non-clustered columnstore indexes and computed columns:

Looking at the execution plan, SQL Server decided to scan the non-clustered columnstore index, even though it doesn’t contain the computed column BirthYear! This surprised me, because I have a plain old non-clustered index on BirthYear which covers the query as well. I guess the optimizer is really excited about that nonclustered columnstore.

Kendra links to a Connect item from Niko Neugebauer to add persisted computed columns to columnstore indexes.

OPTIMIZE FOR Without Parameter Sniffing

Grant Fritchey digs into an interesting combination of settings:

While presenting recently and talking about dealing with bad Parameter Sniffing, I got the question; what happens to OPTIMIZE FOR hints when parameter sniffing is disabled?

This is my favorite kind of question because the answer is simple: I don’t know.

Click through to join Grant in learning what happens.

READPAST Hints

Arun Sirpal explains the READPAST hint:

So, as the name of the hint suggests, it literally does “readpast” it.

I’ve never used READPAST in any production code.  I suppose there are good reasons to use it, but I haven’t run into any yet.

Scan Counts 0

Shane O’Neill learns more about SET STATISTICS IO’s scan count:

My previous intuitions about this guy was…

“Scan count is the number of hits the table/index received”
(THIS IS NOT CORRECT! …and why it isn’t down as a full quote).

Let’s check out the definition again and see what it has to say for itself.

This was a fun read.

SARGability And Date Math

Erik Darling explains performance ramifications of date math operations:

Both plans are helped by our thoughtfully named index on the OrderDate column, though the one with cheaper estimated cost is the bottom one. Yes, I know this can sometimes lie, but we’re not hiding any functions in here that would throw things off horribly. If you’re concerned about the Nested Loops join, don’t worry too much. There is a tipping point where the Constant Scan operator is removed in favor of just index access. I didn’t inflate this table to find exact row counts for that, but I’ve seen it at work elsewhere. And, yeah, the second query will still be cheaper even if it also scans.

Which side of the equation you put a function on can make all the difference.

Creating Plan Guides

Brent Ozar has a couple examples of creating plan guides to work around bad queries using OPTIMIZE FOR UNKNOWN:

You can see the actual execution plan here, and hover your mouse over various parts of it to see the estimated vs actual rows.

The OPTIMIZE FOR UNKNOWN hint tells SQL Server to use the density vector rather than column statistics, so it only estimates that 1,865 rows will come back – when in actuality, 3.3mm rows come back. In performance tuning, that’s what we call a “bad thing,” since SQL Server ends up doing around 10mm page reads due to that key lookup. It would have been much more efficient to just do a clustered index scan.

Creating plan guides can be ugly business, but sometimes they’re the best solution.

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