Interpreting Index Statistics On SQL 2005
Nov 28, 2006
I ran the DBCC SHOW_STATISTICS command for all of my indexes; I was told that high density numbers are bad, low numbers good. I have some questions about my results, though; I'm not sure how to interpret them.
Of 48 indexes, 14 have a density of 0. Does this mean that the indexes are not selective enough? Does it mean they're garbage and I should toss them?
6 have a density of NULL. They are all primary keys. I suppose this just means that they're never used because these tables are rarely queried. Would this assumption be correct?
13 have a density of 1. I have no idea what this means.
The others have densities ranging from 0.01210491 to 0.5841165. I was told that the lower this number is, the more selective and thus more useful an index is. I think 0.5841165 is too high a number. Would this be correct?
Thanks in advance.
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Jun 5, 2014
I'm designing a new database which will be the back-end to a heavily-used web-based application (all these terms are relative - I guess the use won't be that heavy in the grand scheme of things, I'm only talking 100 users or so at the very most). Data from the old application database will be migrated to this one, and the old database is around 7GB in size after 5 years of use.
I have two different ways of linking some tables in mind, one which is slightly more complex than the other but which potentially has benefits over the simpler method. However, I'm concerned that I might be 'over-cooking' the design, and that performance would suffer as a result, so I've tried creating the two different versions of the database (the part of it I'm concerned with, anyway), one for each of the solutions I've got in mind, migrated the data into the relevant tables and carried out some queries on the data to collect some statistics.
The problem is that, whilst I can see that the more complex method is more expensive, as expected, I don't really understand if the difference is significant. Since I don't know what the numbers in the Client Statistics window actually mean (there are no units! I'm guessing times are in milliseconds?), or how much of real-world impact the difference will have, I'm finding it hard to interpret my statistics and come to a decision.
Querying the entirety of my tables to return ~20,000 records listing one column from each of the main tables I'm playing with, the simpler method had a Total Execution Time of 199, and the more complex a Total Execution Time of 272. Is that the statistic I should be most concerned with? Is that a difference I should be concerned about? Is the difference likely to be magnified when the database is much larger and in use, such that a difference of 73 milliseconds in this test scenario could end up being as much as a whole second in production, for example?
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Dec 14, 2007
Hi,
I used SET STATISTICS TIME ON to get execution stats for a query. I found that the CPU Time was sometimes greater than the elapsed time. How is this possible? The query does not use any parallelism since I used the query option MAXDOP 1. Is the elapsed time wait time? Is the total execution time the sum of the CPU time and elapsed time?
SQL Server Execution Times:
CPU time = 797 ms, elapsed time = 162 ms.
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Jun 6, 2000
What are some ways to analyze index coverage and usage? I have a 18 GB database, half is data, other half is indexes and I want to cut down that number as much as I can without affecting performance. Thanks
Peter Karhatsu
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Feb 14, 2001
I have been monitoring some indexes on a table with a lot of inserts, no updates and no deletes. I was wanting to determine when to update the statistics on the index. Does anyone know what would be a good target range for the density when you run the dbcc show_statistics?
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Jun 9, 2000
I was wondering how often you should reindex. By looking at dbcc showcontig and statistics I see that I am heavily fragmented and scan density is between 10-30% on my important indexes. I'm thinking of scheduling this to be done nightly. nay help is much appreciated.
Pete Karhatsu
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Jan 13, 1999
At managing index SQL Server 6.5, there is distribution button. I have a database on production server, one of the table has 20 indexes. When I press the Distribution button, it reveals that most of the indexes have very poor selection of index, from 30% to 99%. There are 6 of them are very good. Based on this statistics, do you think I should remove these indexes? As the book says, if the statistics is higher than 5%, the optimizer will not use index but do a table scan. Removing those index should not affect the performance, is that right?
Your suggestion is very appreciated.
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Sep 5, 2007
Hi
I have a question regarding updating statistics for a primary key.
Background: An update statistics with fullscan is sometimes taking 30 minutes - the table is 80 million rows, with only 4 columns. The table is truncated, and then 80 million rows inserted all in one go.
Now why the update stats is taking that long is another question (I have no idea - any thoughts?), but my question is; Since you can't disable the "not automatically recompute statistics" option for a primary key, and you would think it would be imperitive for the stats to be kept up to date for a PK for inserts.... does this mean the stats would be kept up to date? and an update stat with fullscan isn't required?
Hope someone can help
Thanks
James
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Feb 26, 2008
Is it neccessary to schedule a update statistics on index in sql server 2005 on daily basis
Is it neccessary to schedule a rebuild index on index in sql server 2005 on daily basis
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Sep 7, 2004
Hello!
Is there any way to determine index usage statistics for a given table?
For examle, I have a table, with three indices. I need to know how many times each index was used. Is it possible?
And second part of question: I need to know, which user overloads my base with their giantic queries. Is there any way to determine, how many system resources each of user's sessions uses?
MS SQL Server 2000 Enterprise Edition.
Thank you!
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Jul 30, 2007
We have a 20 GB database and reorganize indexes and update statistics maintainance takes about 4 hours and the log files grows out of control what is a serious problem since it can not be truncated (database mirroring).
Ivan
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Dec 12, 2006
Hi ..
Please advice whether i have to do update statistics along with Index rebuild (online /off line)
Thanks in advance
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Jan 15, 1999
I have an index that shows distribution statistics of 98.20%, which is very poor. I set show query plan and show statis I/O on. This table has 1113675 rows of data.
*************
select orderID, custId, intertcsi from tblorders
where intertcsi = '2815'
STEP 1
The type of query is SELECT
FROM TABLE
tblorders
Nested iteration
Index : indxInterTCSI
orderID custId intertcsi
----------- ----------- ---------
1015245 1011313 2815
2556392 2556392 2815
....
Table: tblOrders scan count 1, logical reads: 104, physical reads: 58, read ahead reads: 0
***************
Then I use the same select statement to force a table scan:
select orderID, custId, intertcsi from tblorders (index=0)
where intertcsi = '2815'
STEP 1
The type of query is SELECT
FROM TABLE
tblorders
Nested iteration
Table Scan
orderID custId intertcsi
----------- ----------- ---------
60472 61084 2815
102184 102333 2815
...
Table: tblOrders scan count 1, logical reads: 110795, physical reads: 6891, read ahead reads: 103980
When the index is not provided, the logical reads and physical reads increased dramatically. Does this tell me that I should keep that index though it is a poor selection? Is that because a huge table like this make the optimizer use the index. The query without using index takes longer time to run.
Any idea or comment would be very appreciated.
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Nov 21, 2014
We have implemented a very small reporting database which has a main table that started off small and has now grown to around half a million rows. Initially, there were no indexes on the table apart from a clustered index, but as the data has grown, performance has dropped and so we have added a number of indexes. This has resolved the performance issues.
Before creating the indexes SQL Server had auto created a number of statistic objects (_WA_Sys_000... etc). After creating the indexes, new statistic objects where created for the new indexes. In some cases, there are duplicate statistics (auto and index) for the same columns.Should I go through and drop the duplicate auto statistics? Will having duplicates cause issues at all?
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Jul 16, 2007
I want to be able to reproduce my production execution plans on development with copying data.
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Feb 3, 2015
At one of your client sides we have configured Always on with synchronous mode.Also we have schedule rebuild index and update statistics job which runs in night every alternate day. the issue is there are more then 100 sleeping queries which is blocking update statistics job.
I have to stop update statistics job manually once i come to office manually.
Once I have killed blocking sleeping query but then other sleeping query blocked it and so on.
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Sep 5, 2015
I am really puzzled by an apparent difference between table index key column order and its statistics order. I was under understanding that index statistics mirror index definition. However, in my db 2470 index ordinal definitions match statistics definition but 66 do not. I also can reproduce such discrepancy in 2008 R2, 2012 and 2014.
As per definition,
stats_column_id
int
1-based ordinal within set of stats columns
This script duplicates this for me.
BEGIN TRAN
GO
use tempdb
GO
CREATE TABLE [dbo].[ItemProperties](
[itmID] [int] NOT NULL,
[cpID] [smallint] NOT NULL,
[ipuID] [tinyint] NOT NULL,
[Code] ....
The result I get is this:
object_id      stats_name                                    Â
stats_column_list
1525580473 PK_ItemProperties_itmID_ipuID_cpID itmID, cpID, ipuID,
and
object_id      index_name                                    Â
index_column_list
1525580473 PK_ItemProperties_itmID_ipuID_cpID itmID, ipuID, cpID,
Also a query I used to discover this in my db is:
WITH stat AS
(
SELECT
s.object_id
,s.name as stats_name
,(
SELECT
c.name + ', ' as [data()]
FROM sys.stats_columns as sc
[Code] .....
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Jul 20, 2005
Hello group.I have an issue, which has bothered me for a while now:I'm wondering why the column statistics, which SQL Server wants me tocreate, if I turn off auto-created statistics, are so important to theoptimizer?Example: from Northwind (with auto create stats off), I do the following:SELECT * FROM Customers WHERE Country = 'Sweden'My query plan show a clustered index scan, which is expected - no indexexists for Country. BUT, the query plan also shows, that the optimizer ismissing a statistic on Country, which tells me, that the optimizer wouldbenefit from knowing this.I cannot see why? (and I've been trying for a while now).If I create the missing statistics, nothing happens in the query plan (andwhy should it?). I could understand it, if the optimizer suggested an indexon Country - this would make sense, but if creating the missing index, queryanalyzer creates the statistics with an empty index, which seems to me to beless than usable.I've been thinking long and hard about this, but haven't been able to reacha conclusion :) It has some relevance to my work, because allowing theoptimizer to create missing statistics limits my options for designingindexes (e.g. covering) for some rather wide tables, so I'm thinking why notturn it off altogether. But I would like to know the consequences - hopesomebody has already delved into this, and knows a good explanation.RgdsJesper
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Aug 1, 2006
What is the unit of the numbers you get in the Time Statistics-part when running a query in Microsoft SQL Server Management Studio with Client Statistics turned on?
Currently I get mostly 0´s, but if I try and *** up a query on purpose I can get
it up to around 30... Is it milliseconds or som made up number based on clockcycles or... ?
I would also like to know if it´s possible to change the precision.
- Nikolaj
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Jan 1, 2008
hi all
I am working on query performance and tuning.
I want to save current statistics for the latter use. do SQL Server provide any utility or command (like exec dbms_stats in ORACLE and OPTDIAG in sybase) to do this?
thanks
Gourav
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Apr 28, 2008
can someone tell me what the folowing SQL does?
SELECT MIN(QUOTENAME(TABLE_SCHEMA) + '.' + QUOTENAME(TABLE_NAME))
FROM INFORMATION_SCHEMA.TABLES
Thanks in advance, Ralph
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Aug 14, 2007
Dear Jamie,
Thanks for the reply.
We have another problem to solve.
on the node we are getting product A -2 >=1.978
What does it mean (-2) ?
It is mentioned as two time slices ago. Please help me to undertand this.
From
menik
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Feb 29, 2000
I have a job whose first step is to run a DTS package via a DTSRUN Operating System Command. I get the following message.
DTSRun: Loading... DTSRun: Executing... Error: -2147220499 (800403ED); Provider Error: 0 (0) Error string: No Steps have been defined for the transformation Package. Error source: Microsoft Data Transformation Services (DTS) Package Help file: sqldts.hlp Help context: 700. Process Exit Code 1. The step failed.
Prior to 2/29/2000, it had run dozens of times successfully, the last time on 2/23/2000.
I would be most appreciative of any help.
Thanks.
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Aug 10, 2005
Folks,
I'm going to write an advanced whitepaper on interpreting the results of CHECKDB in SQL Server 2005 (mostly applicable to SQL Server 2000 as well), should be available before end of the year. Couple of questions for you:
1) would this be interesting/useful to you?
2) anything in particular you'd like to see covered?
Thanks
Paul Randal
Dev Lead, Microsoft SQL Server Storage Engine
(Legalese: This posting is provided "AS IS" with no warranties, and confers no rights.)
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Sep 15, 2006
Hi,
I used a decision-tree mining-model to describe and predict fraud. The table contains 1039 records with 775 distinct value of A-number (the calling party). I used 9 columns in the model. SQL Server reports that only 3 columns are significant in predicting the fraud
- BPN_is_too_short (called party-number is too short)
- Duration_is_zero
- Invalid_area_code
The key-column in A-number, and the predicted column is Is_Fraud with the range of values are only 0 and 1. There's no record with NULL (missing-value) in the column Is_Fraud.
Mining Legend shows in the first split
[-] 625 cases of fraud
[-] 150 cases of non-fraud
[-] 0 cases of missing
In addition to that, Mining Legend shows
[-] 79.69% of fraud
[-] 19.64% of non-fraud
[-] 0.67% Missing
Now when I compare those values, they don't match.
(A) 625/775 is 80.645%, not 79.69%
(B) 150/775 is 19.355%, not 19.64%
(C) 0 cases of NULL (missing value) should imply 0% of missing, not 0.67% of missing
Furthermore in one node (with the split on duration_is_zero), there are 541 cases of fraud and 0 cases of non-fraud. This implies the node is leaf-node. However, Mining Legend shows
514 cases of fraud, 99.35%
0 cases of non-fraud, 0.33%
[F] 0 cases of missing, 0.33%
My questions
(1) Why the values don't match like in cases A through C ?
(2) Why the values don't match even in cases D through F when we have no subtree at all ?
I've searched explanation by reading the mathematical reasoning, entropy, Gini index; but it does not answer the discrepancies of those values and percentages in the Mining Legend.
Regards,
Bernaridho
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May 16, 2014
We have a gaming application which generates transactional data in MongoDB which eventually sends the data to SQL Server and it is in JSON format. This data needs to be used for reporting tool but visualizing this data in forms of a table is proving to be difficult. One example of a column we receive is:
{responseCode:0 transactionId:null amount:200.00 message:account balance }
We need to build a sort of ETL or batch job but need to interpret this in a form which SQL Server can understand.
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Jul 7, 2015
We run std 2008.  In my ssrs log I see this for one of our most critical reports...
library!ReportServer_0-64!2244!07/07/2015-08:24:53:: Call to GetPermissionsAction(/somedirectory/somedirectory1).... which I assume is an indication of a report starting to render by first checking permissions.
Around the time my user says he still saw the revolving arrow and he stopped the report because he felt it was running too long, I see...
webserver!ReportServer_0-64!1dbc!07/07/2015-08:54:44:: i INFO: Processed report. Report='/somedirectory/somedirectory1/importantreport', Stream=''
How can it be true that he stopped it and ssrs reports that it processed the report?
About 4 minutes later I see this entry in the log...
webserver!ReportServer_0-64!15e4!07/07/2015-08:58:34:: i INFO: Processed report. Report='/somedirectory/somedirectory1/importantreport', Stream=''
Which processed report message is right? Could there be multiples cuz of subreports? I see a number of errors and exceptions around these same times but do not know how to tie either to a specific report. Is there a way?
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Oct 20, 2006
please explain the differences btween this logical & phisicall operations that we can see therir graphical icons in execution plan tab in Management Studio
thank you in advance
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Jan 21, 2008
We have been using Sql Server 2005 Compact Edition 3.1 RDA synchronization method successfully on Sql Server 2000 database. Recently we moved the database to Sql Server 2005, sync doesn't work anymore, it just hangs on one table. On further investigation, we found out that it's the index on that table that causes this. We removed the index, it works fine. We are wondering the root cause, removing the index is not a solution for us. Any thoughts?. Thanks.
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Mar 21, 2007
how do you create an index in SQL over the Student table to lookup for example by last name and what is the syntax for the SQL lookup statement.
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Jan 26, 2007
I have a very very strange situation with a particular application and sqlserver 2005 enterprise.This application combines numerical data from multiple tables. User can makequery over this kind of tables, can build queries with "group by" "order by"and "join", "sum, count(*) ecc. on many many columns.I cannot know the query that application is going to build, so I do not knowhow to create indexes.I cant make indexes on all the columns of course, so I'm creating some indexover columns that should be statistically used in the join, but when thereis a group by on a column chosen from the user, I realize that the planbecome non efficient with the famigerate "TABLE SCAN".Can somebody, give me an idea, to optimize this situation.ThanxMassimo
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Jan 30, 2008
Is there a simple way to find properties of an index for a table? Specifically I am looking for the unique property. I currently have code to to do this, however I feel like there must be some easier way.
I am using .net 2005 and SQL 2005 coding in c#. Thanks for any help!
Code Snippet
_conn.Open();
SqlCommand cmd = new SqlCommand("sp_helpindex", _conn);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.Add(new SqlParameter("@objname", "TABLENAME"));
SqlDataAdapter da = new SqlDataAdapter(cmd);
da.Fill(results);
foreach (DataRow row in results.Tables[0].Rows)
{
ArrayList names = new ArrayList();
DataTable dtUnique = new DataTable();
SqlCommand cmd2 = this._conn.CreateCommand();
cmd2.CommandText = String.Format("SELECT INDEXPROPERTY(OBJECT_ID('TABLENAME'), 'INDEXNAME', 'IsUnique') AS isUnique");
dtUnique.Load(cmd2.ExecuteReader());
bool unique = Convert.ToBoolean(dtUnique.Rows[0]["isUnique"]);
if (unique)
names.Add(tableName);
}
_conn.Close();
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Jul 24, 2006
Hi,
Does anyone here know of any on-line references on how to optimize index maintenance in sql 2005? Also do you know of any good DBA books that will explain database maintenance and or best practices?
Thanks.
:D
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