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Multiple statistics sharing the same leading column

SQL Server will not prevent you from creating identical indexes or statistics. You may think that everything is under your control, but have you ever added a missing index that has been reported by the query optimizer, or the DTA? If so, you will certainly have created duplicate statistics. Have a look at the following example:

-- create test table with 3 columns
-- filler is only nedded to widen the row
create table T1
(
c1 int not null identity(1,1) primary key
,c2 int not null
,filler nchar(200) not null default '#'
)
go

-- insert 500000 rows with random
-- values between 0 and 49999 for c2
insert T1(c2)
select top(500000) abs(checksum(newid())) % 50000
from sys.fulltext_system_stopwords as a
,sys.fulltext_system_stopwords as b

Before invoking the following SELECT command, allow the actual execution plan being displayed (Strg-M or selecting Query/Include Actual Execution Plan from the menu).

-- get some data
select * from T1 where c2 = 100

If you look at the execution plan, you see that the optimizer complains about a missing index on column c2. The prognosticated improvement is about 99%, so adding the index it's certainly a good idea. Let's do it:

-- add missing index
create nonclustered index ix_1 on t1(c2)

Perfect! The query runs much faster now and needs a lot fewer resources. But have a look at the table's statistics:

image

You see three statistics, one for the primary key, a second one for our created index ix_1, and a third one that was automatically created during execution plan generation for the first SELECT statement. This is the statistics, named _WA_Sys.. If the AUTO CREATE STATISTICS option is set to ON, the optimizer will add missing statistics automatically. In our little experiment, the optimizer had to generate this column statistics on column c2 in order to make some assumptions about the number of rows that had to be processed.

And here's the problem: When creating the index on column c2, a statistics on this column is also created, since every index has a corresponding linked statistics. That's just the way it works. At the time the index was added, the column statistics on c2 (that _WA_Sys. statistics) already existed. If you don't remove it manually, this statistics will remain there forever, although it is needless now. All it's good for is to increase maintenance efforts during statistics updates. You can safely remove this statistics by executing:

drop statistics t1._WA_Sys_...

If you didn't think about this before, there's a chance that you'll find some of those superfluous statistics duplicates inside your database(s). Here's a query that finds index-related and column-statistics that match on the first column. Looking for matches on the first column is sufficient here, since the optimizer only automatically adds missing single-column statistics.

with all_stats as
(
select s.object_id, s.name as stats_name, s.stats_id, s.has_filter
,s.auto_created, sc.stats_column_id
,sc.column_id, c.name as column_name
from sys.stats as s
inner join sys.stats_columns as sc
on s.stats_id = sc.stats_id
and s.object_id = sc.object_id
and sc.stats_column_id = 1
inner join sys.columns as c
on c.object_id = s.object_id
and c.object_id = sc.object_id
and c.column_id = sc.column_id
where objectproperty(s.object_id, 'IsUserTable') = 1
)
select row_number() over(partition by s1.object_id, s1.column_name order by s1.column_name) as stats_no
,object_name(s1.object_id) as table_name
,s1.stats_name as stats_name
,s2.stats_name as identical_stats_name
,s2.column_name
from all_stats as s1
inner join all_stats as s2
on s1.object_id = s2.object_id
and s1.stats_id != s2.stats_id
and s1.stats_column_id = s2.stats_column_id
and s1.column_id = s2.column_id
and s1.has_filter = s2.has_filter
and s1.auto_created != s2.auto_created

With that query at hand, you may easily find redundant statistics. Here's a sample output:

image

If you'd like to find out more about SQL Server statistics, you may want to check out my series of two articles, published recently on the Simple-Talk platform.

Part 1: Queries, Damned Queries and Statistics
Part 2: SQL Server Statistics: Problems and Solutions

It's free, so you might want to give it a try. Feel free to vote, if you like it!

SQL Server Administration, SQL Server Performance
 Print article This entry was posted by Holger Schmeling on 04.11.10 at 19:12:15 . Follow any responses to this post through RSS 2.0.