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Hello Dear Reader, yesterday I posed the question to you
what are Statistics? We could get down
and dirty of how the internals of SQL Server use Statistics (and we will), but
first let’s talk about the concept because it is at heart a very simple one.
In America we are in my favorite season, Fall. The weather is cool but not to cold. The leaves turn beautiful colors, the smell
of wood burning in a fire place, fire pit, or the general smokey scent that
goes with the great out doors this time of year always springs to mind.
Unfortunately we are also in my least favorite time of the every
4 years, Election Season. But Election
Season does tie nicely into our subject of the day. Statistics are nothing but POLLS! In election season they say things like, “Do
you like this Candidate? Do you like
this particular issue? Do you believe they kill puppies when people aren’t
looking?”, and we get a break down of Yes, No, and Undecided.
In SQL Server Statistics are SQL’s way of Polling our
Data.
GATHERING STATISTICS:
SQL Server: “I See you are a column Called First Name”
Registered Column: “Yes I am”
SQL Server: “Mind if I ask you a few questions (QUERIES) and
create a Poll based off of how you answer?”
Registered Column: “Go Right Ahead”
Breaking New Folks Registered Column has 28% of his data
between Alex and David, only 5% between David and Nick, and a WHOPPING 77%
between Opie and Zachary!
USING STATISTICS:
Now the next time we ask a question (QUERY) we (THE QUERY
OPIMIZER) have an expected number of people in a particular demographic (VALUE
RANGE). We know if our candidate wants
to know how all the Opie’s through Zachary’s will answer a question (QUERY), we
can plan on how to best collect that information (AKA HOW THE QUERY OPTIMIZER
CREATES A QUERY PLAN). We can then
figure out how many people we need to send out (WORK THAT NEEDS TO BE DONE
SORT, SPOOLS, HASH’s) in order to collect that data. For example we need less people to collect
data from David to Nick (NESTED LOOP JOIN) than we do to collect data from Opie
to Zachary (HASH JOIN).
OUT OF DATE
STATISTICS:
Now that we have our Poll, the next time we have a question
(QUERY), the folks on the new screen will say, our expected result was 77% when
we selected the range of Opie’s and Zachary’s however we found that only 58%
actually resided there.
Our population was moving (DATA WAS BEING UPDATED/INSERTED/DELETED)
and our Statistics were not up to date.
If we had a plan to collect our data (QUERY PLAN) using a lot of people
to go out in the community and collect polls we may have sent out to many and
over allocated our resources (PICKED A BAD PLAN IE HASH JOIN INSTEAD OF NESTED
LOOP). If we still had 77% of Opie’s
to Zachary’s our polling plan (ESTIMATED
ROWS RETURNED)
would be good, but it wasn’t (ACTUAL ROWS RETURNED).
UPDATING STATITICS:
So our Statistics were out of wack on our poll. Something was off. If we had a big plant closure in our town or
a big company laid off a lot of people (PURGE PROCESS ON A TABLE), then we
would expect some population shift. If
we knew 20% of people (20% OF ROWS IN A TABLE) were going to be laid off we
could expect some would move in with other family members or move to find new
work. We would probably send people out
in the community to get new polls (AUTO UPDATE STATITICS) and find out what the
new data was for. We found 58% Alex and
David, 25% between David and Nick, and 17% between Opie and Zachary
Regeneration of Statistics causes us to re-think our plan to
Poll Opie’s to Zachary’s (QUERY RECOMPLIATION TO GET A NEW QUERY PLAN) in order
to send the right amount of people out to ask questions (QUERY) to get our
candidate some information (GET OUR DATA).
Now we see that we need much less people (NESTED LOOP JOIN) to poll Opie
thru Zachary than we previously did (MERGE JOIN) and our polling plan (QUERY
PLAN) reflects that.
GETTING INTERNAL
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Now that we have a general idea of how things work let’s
spell it out a purely in SQL Server Language.
Clustered Indexes and Non Clustered Indexes automatically have
statistics generated for their key columns.
However there are more columns in a table than just indexed columns.
SQL places those Statistics in an object named a Histogram. A Histogram contains entries (will only ever
have a max of 200) that show data values spread over a range. This allows the Query Optimizer when
constructing a plan to say, “Statistics, I’m going to run this query on this
table how many rows can I expect to get back?” and then plan accordingly.
We have the following table named Students with columns
StudentID, SSN, FirstName, LastName, MiddileIntial, BirthDate, and Gender. Clustered Index on StudentID (no debate in
indexes right now this is just a demo J
).
*All of the code to create a Students table along with other
and generate random data was uploaded to my resources page yesterday as a part
of my Trimming Indexes Getting Your Database In Shape presentation. Download that code and play around with it
however you like!
create table students(
studentID int identity(100000,1)
,ssn char(9)
,FirstName varchar(50)
,MiddileInitial char(1)
,LastName varchar(100)
,BirthDate datetime
,Gender char(1)
,constraint pk_students_studentID primary
key clustered (studentID)
)
If we insert a couple rows into this table (*go get the
code!) and then go look at SSMS.
We see that we have statistics created for my primary
key. If you right click on the
Statistics and open them up and then click on Details you will see a whole host
of information. You can see when the
statistics were generated, when they were last updated and what the range is.
You can see that my Average Length is 4. That is because my Primary Key on column
StudentID is an INT or a 4 byte fixed length value. You can see in my range what my RANGE_HI_KEY
is to my RANGE_ROWS.
For my 200
different samples you can see how many rows fall in that data DISTINCT RANGE
ROWS.
If I said to the Query Optimizer
SELECT
studentid
FROM
dbo.students
WHERE
studentid between
104030 and 108969
I would expect to get back 4940 rows, BUT my statistics are
OUT OF DATE and do not refelect that. So
when I execute my query, and include actual execution plan, this is what I get
back.
My options at this point are to update my statistics.
UPDATE STATISTICS dbo.students pk_students_studentID
WITH FULLSCAN
And now my query plan looks like this.
As you can see the Optimizer expected the number of results
it got back. In my Query Plan (a simple
trivial one), the statistics did not shift my outcome. But had I joined on the Courses table or the
Grades table it could have completely changed my plan.
TWITTER IT UP
So the question on twitter yesterday that spawned all of
this was should I delete old statistics?
My answer to that is no. You
should update them. The Histogram is not
normally a big space consuming object.
They are not like unused Indexes.
Unused indexes occur IO, they must be maintained as the base structure
is updated. This costs your system. Statistics just off the query optimizer a
path, if the statistics are old and the range is still valid leave them
be.
Whenever a query comes along you will save the optimizer the
trouble of regenerating them. Because if
they are not there we have to create them, but that is an example for another
day.
As always Thanks for stopping by!
Thanks,
Brad
