I live for questions. And my favorite questions are the ones where I’m not completely sure of the answer. Those are the questions that make me stop presenting in order to take a note so I can try to answer the question later, usually in a blog post. Guess where we are today?

I was asked at SQL Bits in London about the direct impact of the PERSISTED operator on calculated columns, both inserts and selects. I didn’t have a specific answer, so I wrote it down for later (and asked the, self-described, persisting Dane, to email me to remind me. He did, so I put together a few tests to try to answer his question.

First, I created three tables:

CREATE TABLE dbo.PersistTest (
PersistTestID INT IDENTITY(1,1) NOT NULL PRIMARY KEY CLUSTERED,
RandomValue1 DECIMAL(5,3),
RandomValue2 DECIMAL(5,3),
CalcValue AS (RandomValue1 + RandomValue2),
PersistValue AS (RandomValue1 + RandomValue2) PERSISTED
);
GO
CREATE TABLE dbo.PersistTest2 (
PersistTestID INT IDENTITY(1,1) NOT NULL PRIMARY KEY CLUSTERED,
RandomValue1 DECIMAL(5,3),
RandomValue2 DECIMAL(5,3),
CalcValue AS (RandomValue1 + RandomValue2)
);
GO
CREATE TABLE dbo.PersistTest3 (
PersistTestID INT IDENTITY(1,1) NOT NULL PRIMARY KEY CLUSTERED,
RandomValue1 DECIMAL(5,3),
RandomValue2 DECIMAL(5,3),
PersistValue AS (RandomValue1 + RandomValue2) PERSISTED
);
GO

The first table has two calculated columns, one PERSISTED and one not. The second table has a non-persisted column only. The third table has a persisted column only. This way, I can see the behavior across all these tables and compare them to see where PERSISTED adds to, or removes from, performance.

I took a very simple set of queries and ran these a few times on each table to get an average execution time:

INSERT dbo.PersistTest (
RandomValue1,
RandomValue2) VALUES (
42.2 , -- RandomValue1 - decimal
44.4  -- RandomValue2 - decimal
);

The results were:

On average, across about five executions, you can see a distinct difference of about 600 microseconds between Persist 2 and Persist 3, and an even larger average on the single Persist table. Let’s try the same thing with a few SELECT statements:

SELECT  pt.CalcValue,
        pt.PersistValue
FROM    dbo.PersistTest AS pt
WHERE   pt.PersistTestID = 3;
GO
SELECT  pt.CalcValue
FROM    dbo.PersistTest2 AS pt
WHERE   pt.PersistTestID = 3;
GO
SELECT  pt.PersistValue
FROM    dbo.PersistTest3 AS pt
WHERE   pt.PersistTestID = 3;
GO

While the results are spectular:

You can see that making the calculation on the fly is more costly with both Persist and Persist2 being higher than Persist3, which had no calculations to retrieve data.

Now, let’s run the same thing, but, how about, 10,000 times so we can really see what the differences are over time. First, the inserts:

Still measured in microseconds, you can see that the cumulative totals are higher for the persisted columns than they are for the non-persisted column. So, there is an overhead for storing the extra information. But, what about queries?

I’d say that’s pretty conclusive. When I hit the SELECT statement 10,000 times instead just five or six, we clearly have differences in behavior. Interestingly enough, the logical reads were identical at 20,000 each. So it really did come down to the time spent calculating the values versus the time spent just retrieving them.

Caveats. This is a very simple test with simple calculations and small persisted values. Even doing something 10,000 times isn’t the same as doing it one million or one hundred million. In short, your mileage may vary, some assembly required, for queries lasting longer than four hours call your physician, it depends.

You can see that a PERSISTED column is going to negatively impact INSERT, but, it’s going to positively impact SELECT. The question is, which one is giving you pain at the moment.

UPDATE: Transposed the data in the final table between Persist 2 & Persist 3. Fixed it.

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