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15
votes
2answers
14k views

What are database statistics, and how can I benefit from them?

I've heard mention of statistics that SQL Server keeps by default. What are they tracking, and how can I use this information to improve my database?
11
votes
3answers
3k views

Is there a reason to update statistics manually?

In SQL Server, statistics are updated automatically when Auto Update Statistics in True (which is the default). Is there a reason to update statistics manually and in what circumstances?
11
votes
2answers
11k views

When To Update Statistics?

I've inherited a Maintenance Plans that does the following: Cleanup old data Checks DB integrity Performs Database and Transaction Log Backups Reorganizes Our indexes Updates Statistics Delete old ...
11
votes
1answer
356 views

How does SQL Server know predicates are correlated?

While diagnosing SQL Server 2008 R2 queries with poor cardinality estimation (despite simple indexing, up-to-date statistics, etc.) and hence poor query plans, I found a perhaps-related KB article: ...
8
votes
1answer
1k views

SQL Server Index vs Statistic

What are the differences between CREATE INDEX and CREATE STATISTICS and when should I use each?
8
votes
2answers
795 views

Tracking stored procedure usage

Besides using SQL Server Profiler, is there any way to track which stored procedures are being used, or at least when they were last executed?
8
votes
2answers
2k views

Does a re-index update statistics?

I've been doing the MS10775A course this past week and one question that came up that the trainer couldn't answer reliably is: Does a re-index update the statistics? We found discussions online ...
7
votes
1answer
3k views

Fast general method to calculate percentiles

I want to find n>1 percentiles of an unsorted column in PostgreSQL. For example the 20th, 40th, 60th, 80th and 100th percentile. An obvious solution is to count and sort the column and then do a ...
7
votes
1answer
147 views

Auto Update Statistics Duration and Interruptions

Our production SQL Server 2008 R2 sometimes begins to perform slowly: all queries begin to work too slow (and sometimes all queries just hang and wait something then continue). This usually lasts ...
7
votes
1answer
490 views

How does sampling work when updating statistics?

I have several massive tables. I'd like to ensure their statistics are up to date via a weekly maintenance plan. However, doing so is taking too much time. If I specify WITH SAMPLE 50 PERCENT ...
6
votes
3answers
426 views

Are SQL Server statistics stored in database or buffer pool?

Just wondering are statistics kept in the database but not in the memory? If I backup/restore the database from a prod server to a development server, would it keep the same statistics so that ...
6
votes
5answers
4k views

SQL Select taking too much time to execute

It's a simple select from a temporary table, left joining an existing table on its primary key, with two sub selects using top 1 referring the joined table. In code: SELECT TempTable.Col1, ...
6
votes
1answer
277 views

AUTO_UPDATE_STATISTICS_ASYNC ON?

I have been advised that setting the AUTO_UPDATE_STATISTICS_ASYNC ON may be a good idea. I have been reading a couple of ideas and still not sure as this is a non default setting. Any further ...
6
votes
1answer
1k views

Medians, Modes, Percentiles and OLAP

I'm newbie trying to wrap my head around OLAP, and I have a few questions. Question 1: Can an OLAP cube store medians, modes, percentiles? Question 2: Can an user-written MDX query return a summary ...
6
votes
1answer
396 views

Extremely Slow StatMan query issue with #SP2010 farm

The issue I’m experiencing is explained in a blog-post I created for this, (easy to refer to and to manage the responses / solutions in 1 location) which can be found here: ...
6
votes
2answers
379 views

What can cause statistics to get out of line?

I've just worked through a problem at a clients site which it turned out was caused by the statistics being wrong which caused the Optimizer to time out. Running exec sp_updatestats fixed the problem ...
5
votes
2answers
932 views

DTA Recommends to CREATE STATISTICS

I just ran a T-SQL query through DTA and one of the recommendations is to CREATE STATISTICS on one of the columns that is part of many of the queries in the SQL code file. My question is, how exactly ...
5
votes
2answers
374 views

Get NULL statistics for a column in MS SQL Server

In MS SQL Server 2005, how can I extract statistical information about NULL fraction (or NULL count) for a given column? Same information is available in Oracle and PostgreSQL, and I heard that MS ...
5
votes
2answers
455 views

Estimating distribution of row access in mysql

I am trying to get an idea of the size of the "hot data" part of a rather large table, and I was wondering if this could be done directly in mysql. I know that with the percona version of mysql, I can ...
5
votes
2answers
990 views

Frequency distributions segmented by a field

Some background: I have a sample population data file. Each record in the data file has a frequency weight (FIELD NAME: wgt) indicating how many times the records needs to be replicated to get the ...
5
votes
2answers
227 views

If a query triggers a statistics update and times out are the statistics still updated?

I have a fulltext query which is usually very fast but may time out when it causes a statistics update since statistics updating is very slow on this database. Usually the query "recovers" to normal ...
5
votes
1answer
182 views

New query slower even if it has less logical reads than the old one

I have a query in a stored procedure that must list all dates in a range and JOIN on it date from a table, if it exists for that day. The procedure was created before I was employed as the DBA, so I ...
5
votes
1answer
804 views

How to calculate cache misses for PostgreSQL

I'm administering a server where a tool using a PostgreSQL runs. The tool takes care of most PostgreSQL configurations by itself, but I'm observing some performance problems. I could confirm at OS ...
4
votes
2answers
690 views

Performance degradation after rebuilding indexes

I am optimizing some server application task that uses database (querying, complex calculations, data insertions...). Execution of this task spends about 16 minutes (I have tested it 3 more times) and ...
4
votes
2answers
1k views

Estimate average and median efficiently in Postgres?

I have a Postgres database with tables in the billion scale. So any aggregate functions such as count() and avg(), as well as "order by random()" are very time consuming. Postgres has pg_catalog which ...
4
votes
1answer
382 views

Getting ratio of reads to writes on SQL Server

Sorry I am not a DBA so bear with me. I wanted to get a better understanding of the ratio of reads to writes in our archiecture. The best to get this is at the database. We are using SQL Server ...
4
votes
2answers
162 views

SQL Server 2012 autoupdate statistics default behaviour

I found, that since SQL Server 2008 R2 there is a trace flag 2317 available. It turns on dynamical autoupdate statistics threshold, instead of default threshold of 20%. So, it greatly improves ...
4
votes
1answer
66 views

Why is an additional filtered statistic being ignored (EAV schema)?

I'm trying to improve a row estimate for this sub-query (of a larger query). The estimate is showing 1266 rows. The actual is 117k rows. This particular property (EAV schema) only has two values ...
3
votes
2answers
527 views

How do I replicate SQL Server index INCLUDE and STATISTICS functionality on PostgreSQL?

I'm working on a project that must support two database engines; SQL Server and PostgreSQL. We are using NHibernate as the ORM. We are running into performance issues with certain queries. Using ...
3
votes
2answers
753 views

Parallel Statistics Update

In SQL Server 2008 or later, is UPDATE STATISTICS WITH FULLSCAN a single threaded operation or it can use parallelism? How about update statistics with default sampling - can it use parallelism? I ...
3
votes
2answers
138 views

Multicolumn statistics on large table yielding poor estimations

I have a rather large table (~200 million rows), and although the statistics are up to date using WITH FULLSCAN, is it possible that my histogram (limited to 200 steps) is simply too broad for the ...
3
votes
1answer
356 views

Advisability of using STATISTICS_NORECOMPUTE

I've recently become involved in maintaining a set of databases with some interesting index problems. One of the ones that aggravates me the most is the differences in the indexes between ...
3
votes
1answer
393 views

Does creating / updating statistics have an impact on the transaction log?

Does creating or updating statistics, assuming on a large table, create significant (or at all) transaction log churn? I would think not as it does not change the underlying data or structure. If it ...
3
votes
1answer
277 views

Query requires 'Update statistics' very often in SQL Server

We have a few tables in our database. Since most of our queries are date based, we have constructed clustered index on 'date + primary key (surrogate)' combination and we enforce joins on date as well ...
3
votes
1answer
474 views

SQL Azure dm_exec_query_stats clearing?

I'm doing a lot of work with SQL Azure at the moment, trying to improve performance of a web app. I am using the dm_exec_query_stats view to look for poor performing queries. Does anyone know a way ...
3
votes
1answer
493 views

Teradata : Disadvantage of Collect stats

I want to know any disadvantage can occur by doing collect stats on a column that is being widely used in join condition across so many procedures ? (Eg. the column is accessed 300+ times access in 60 ...
3
votes
1answer
389 views

Can't understand results of pg_stat_statements view

I configured my postgresql.conf (my version is 9.1) with the normal persistent configuration: shared_preload_libraries = 'pg_stat_statements,plperl' custom_variable_classes = ...
3
votes
1answer
1k views

Executing the query “UPDATE STATISTICS [dbo].[Contact] WITH FULLSCAN …” failed

My maintenance plan fails with the following error on one of my databases does anyone know how I can fix this problem? is is a corrupt table? Executing the query UPDATE STATISTICS [dbo].[Contact] ...
3
votes
1answer
536 views

Oracle: How to gather stats in a logical standby database?

I have a Primary and a (logical) Standby Database. The Schema 'APP' gets synced. Now a User has performance issues running a query against some tables in this 'APP' schema of the Standby Database. On ...
2
votes
2answers
357 views

Problem with sp_Updatestats

I am having a problem when running sp_updatestats on my database it generates an error when my emp table is updating and disconnects I will paste the error here Updating [dbo].[emp] Msg 0, Level 11, ...
2
votes
1answer
963 views

Why is DBA_SEGMENTS presenting twice the amount of data than the DBA_TABLES?

I tried to compute the audit log table usage using the following steps: Truncate all the audit log table data (i.e. AUD$) Perform test case that inserts and update data, also delete statement but it ...
2
votes
1answer
348 views

Will Ola's default IndexOptimizer job update my stats?

I just installed Ola Hallengren's MaintenanceSolution.sql maintenance scripts. After looking at the output of the log generated by the IndexOptimize - USER_DATABASES job, I'm concerned that it's not ...
2
votes
2answers
929 views

SQL Server: Do Statistics Updates cause query plans to be flushed? Should they?

I've found that a big, nasty data-extraction query that runs daily needs updated stats to avoid making horrible query plans based on incorrect rowcount estimates (let's not worry about whether or not ...
2
votes
2answers
138 views

How to improve row estimate of 1 row in join for newly inserted data

A custom statistic exists for the CacheId column of a table. After an overnight statistics update: Statistics for INDEX 'ST_TableName_CacheId'. ...
2
votes
1answer
1k views

What's the impact of running sp_updatestats in production server?

Is it safe to run sp_updatestats on an SQL Server in the production environment? Or rather, what is the impact of updating all statistics on a sql server? Can it "choke" the sql server while it runs ...
2
votes
3answers
135 views

how to trigger a statistics update

I have a certain table in my OLTP database, which is bulk-updated by several users. There is no way to know when they will update the table (somewhere between 5 times a day and once in a week). The ...
2
votes
1answer
219 views

Strange things with Avg_row_length

The data from one tables is dumped to file and inserted into calls_old table with load data infile it shows Avg_row_length 7366. Later I dumped table with mysqldump, droped table, then created again ...
2
votes
2answers
77 views

Get count of processed queries

I have been searching a while to find a way to retrieve the amount of queries my postgres database is processing and responding to. I found various statistics about disk I/O, insert/update statistics ...
2
votes
0answers
85 views

Do OR conditions in Oracle SQL queries spoil index use / cause full table scan?

I have a query on Oracle 11g of the following type, which results in a series of highly inefficient full table scans: select t.Id, t.ObjectId, t...[other Tasks columns] , od1...[ObjectData1 ...
2
votes
0answers
243 views

Oracle 11g bug ? not returning the record until I triggered index to invisible and then to visible

We are using Oracle 11g, 11.2.0.3. We know a record exists in a table but a select is not returning it for some odd reason. The execution plans the general query, not forcing any index, shows that ...