I have an installation of SQL Server 2008 R2 (one of a few) and it's reasonably busy system. I'm trying to optimise some of the indexes like I've done in the past by using information from missing indexes tables.
What seems to be strange is that sys.dm_db_missing_index_group_stats table is empty?!
Now I don't believe for a second that none of the databases on that server misses a beat and other tables like sys.dm_db_missing_index_group_stats, sys.dm_db_missing_index_group_stats etc contain plenty of records.
Just now I've seen 1 (one) record briefly appearing in that table and it's gone since then.
I wonder if something is seriously wrong with that server or if I'm missing a trivial
Seems to be a bug in SQL 2008 R2.
Search google for "sys.dm_db_missing_index_group_stats empty" (regards to Mike West).
I`ll try to explain what happens.
There are 4 DMOs (Dynamics Management Objects) which are used in the Missing Indexes Feature of SQL Server.
Missing index details are held in these DMOs, until purged.
The purge is triggered when you actually create the missing index (or restart the server).
But, there is a limit of 600 rows that can be held within these DMOs.
This, in conjuction with the fact that the sys.dm_db_missing_index_group_stats DMO gathers data only for executed queries, wheres the other three gather data for both compiled and for executed queries introduces this "bug".
The workaround suggests that the sys.dm_db_missing_index_details dmv be queried and all suggested indexes be created with "STATISTICS_ONLY = -1" and then dropped.
This would restart the index recommendations gathering.
More explanation here, and a workaround here.
Related
I have taken over an existing MVC website which uses entity framework and hangfire and is hosted on Azure and uses Azure database. Every so often the website times out.
I'm new to Azure portal, entity framework and hangfire.
If I increase the DTU's it clears the timeout issues?
I'm looking for ways of how to diagnose why the website times out. I have added error logging using elmah and checked hangfire but this doesn't give me any further information.
Is there anything in azure portal that can help?
If it "times out" and if "increasing DTU resolves timeouts" and these observations are true (I think it's on you to really convince yourself this is absolutely true, don't make this assumption lightly) then the usual and obvious candidate is "a slow sql query". Entity Framework is often used with linq to create sql queries without writing sql. These queries are often fine for very simple tasks, such as someData.Where(x=>x.Id == 1).First(), however, if linq is used to join tables, or create complex associations, the generated sql can become monstrously bad, from a performance perspective. You can add logging to write out the sql generated by linq, or you can try to trace the database to see what sql is running on it. If tracing is out of the question, there are still meta queries you can use to view things like cached query plans and SQL Server can give you estimated costs and cached execution counts.
You can still hang yourself without using linq. You can still use stored procedures with EF. Way too many developers are naive about SQL performance still; you need to comb over your back end and learn the schema, the stored procedures; inspect the sql contents of everything. Check for any database triggers (easy to miss). Red flags are subqueries, too many joining, too many results from a query, lots of string manipulation in a query, joining tables on strings, or XML/JSON-based SQL work.
Be aware that "slow sql queries" will become slower when load is high. And when slow sql queries build up, they only take more time to resolve. This can also cause debilitating table locking, depending on the nature of the query.
But queries can be performant and still cause locking. ie One table is being written to often and it's blocking other writes or reads from that table. This is a little harder to diagnose, but you can figure it out by carefully inspecting logs of database calls and how long they take to execute. There are also sql queries you can run on the database to diagnose long-running queries, or what tables are locked at a given point in time.
Finally, check for any back end webjobs for your application. If timeouts occur at reoccurring days or times, then somebody's batch SQL could be blocking your production database from being read.
But this is all speculation. I think you need to do more research to determine what is actually causing the site to become unresponsive. If you can log response times for common queries, you can rule out SQL-based latency as being the culprit or not and work from there. There's nothing inherently "amiss" about any of the technologies you specified.
If queries are perfomant but still causing issues, a long term solution is to add something like a message queue and batch your sql work intelligently, or just make the database work asynchronous and not block the UI.
You should correlate any logged timeouts with azure's monitoring. Azure can give you CPU/RAM/page visits and such on the dashboard.
SQL Azure is a bit of a different beast. It doesn't have the on-demand performance of a dedicated DB unless you're prepared to throw serious $$ at it. And even then ...
EF, when written for well can perform quite well. When written poorly it can be a dog, and those problems are compounded on a platform like SQL Azure.
The first thing is to check that your EF contexts are set up to use an execution strategy suited to Azure: https://learn.microsoft.com/en-us/ef/ef6/fundamentals/connection-resiliency/retry-logic
The next thing would be to see what kinds of SQL tracing you can run on Azure. Tracing is essential to see what EF is doing behind the scenes. I'm not familiar with tools available for Azure, in my case my Azure experience was running SQL Server on VMs because SQL Azure was too immature, not HIPAA compliant at the time, and expensive for the DTU estimates we were able to get. Worst case, can you restore an database backup into an SQL Server instance and point a copy of your application environment temporarily at that to run through common usage scenarios? Using an SQL Trace you can pick up on exactly when and how often EF is executing queries, and what queries it is executing.
Things to look at:
How many queries are running? If you are loading a set of records and expect one query, are there a whole heap of queries getting sent? This would indicate lazy-load calls being triggered.
What queries are being run? Is it selecting a lot more fields than are being displayed? This would be potentially a case where entire entities are being loaded where a .Select() could be used to reduce the amount of data. Perhaps even the case where entire sets of entities are being loaded that aren't relevant to what is displayed/done, such as cases where someone is using .ToList() prior to just doing a .Count() or .Any() or doing a .FirstOrDefault() just to do a != null check.
Is the database properly indexed? Copy some of the heavier queries into SQL Manager and execute them with an execution plan. Are there indexing suggestions?
The common sins of developing with EF and other ORMs boil down to "pulling too much, too often." It's surprising how many clients I've worked with have development teams that have not used a profiler to inspect their ORM use efficiency. (and I'm talking 0% so far.)
I have been working on a reporting database in DB2 for a month or so, and I have it setup to a pretty decent degree of what I want. I am however noticing small inconsistencies that I have not been able to work out.
Less important, but still annoying:
1) Users claim it takes two login attempts to connect, first always fails, second is a success. (Is there a recommendation for what to check for this?)
More importantly:
2) Whenever I want to refresh the data (which will be nightly), I have a script that drops and then recreates all of the tables. There are 66 tables, each ranging from 10's of records to just under 100,000 records. The data is not massive and takes about 2 minutes to run all 66 tables.
The issue is that once it says it completed, there is usually at least 3-4 tables that did not load any data in them. So the table is deleted and then created, but is empty. The log shows that the command completed successfully and if I run them independently they populate just fine.
If it helps, 95% of the commands are just CAST functions.
While I am sure I am not doing it the recommended way, is there a reason why a number of my tables are not populating? Are the commands executing too fast? Should I lag the Create after the DROP?
(This is DB2 Express-C 11.1 on Windows 2012 R2, The source DB is remote)
Example of my SQL:
DROP TABLE TEST.TIMESHEET;
CREATE TABLE TEST.TIMESHEET AS (
SELECT NAME00, CAST(TIMESHEET_ID AS INTEGER(34))TIMESHEET_ID ....
.. (for 5-50 more columns)
FROM REMOTE_DB.TIMESHEET
)WITH DATA;
It is possible to configure DB2 to tolerate certain SQL errors in nested table expressions.
https://www.ibm.com/support/knowledgecenter/en/SSEPGG_11.5.0/com.ibm.data.fluidquery.doc/topics/iiyfqetnint.html
When the federated server encounters an allowable error, the server allows the error and continues processing the remainder of the query rather than returning an error for the entire query. The result set that the federated server returns can be a partial or an empty result.
However, I assume that your REMOTE_DB.TIMESHEET is simply a nickname, and not a view with nested table expressions, and so any errors when pulling data from the source should be surfaced by DB2. Taking a look at the db2diag.log is likely the way to go - you might even be hitting a Db2 issue.
It might be useful to change your script to TRUNCATE and INSERT into your local tables and see if that helps avoid the issue.
As you say you are maybe not doing things the most efficient way. You could consider using cache tables to take a periodic copy of your remote data https://www.ibm.com/support/knowledgecenter/en/SSEPGG_11.5.0/com.ibm.data.fluidquery.doc/topics/iiyvfed_tuning_cachetbls.html
I've upgraded a server from SQL Server 2005 to SQL Server 2008 but the database runs slower when running certain stored procedures especially against records which contain more data than others.
It's been suggested that I run a basic reindex to see if this resolves.
Can someone take a look at the screenshot and advise if this will remove any data from my database - if so then this isn't the right thing to do.
Thanks James
p.s I will now attach a screen-shot if I can as not done that before using this Forum
Those actions won't remove any data from the database, but generally I wouldn't advise trying to shrink the database unless you really need the space as this can cause more fragmentation of indexes. The only options that you have ticked there that have the ability to improve performance are the rebuild/reorganise indexes and the update statistics options.
Rather than maintenance plans though I would generally recommend using Ola Hallengren's DB maintenance scripts though as they offer more flexibility and are generally a lot better than these plans:
Ola Hallengren - SQL Server Maintenance Solution
We are in a migration process from a Progress DB to use the Dataserver to a SQL Server database, and we have had a lot of issues, specifically with performance where the dataserver is not able to produce server side joins for a lot of queries.
In the datasheet for Openedge 11 it says this has been improved, but anyone has an idea of how much improvement they've made.
As an example, every query involving two buffers where the second one is FIRST/LAST wouldn't be joined at server side, has this been changed?
Many thanks,
Check page 175 of the OpenEdge Data Management for MS SQL Server PDF for an answer to this question - there are a whole pile of conditions that apply, as well as a number of control settings that'll affect performance.
We're considering using SSIS to maintain a PostgreSql data warehouse. I've used it before between SQL Servers with no problems, but am having a lot of difficulty getting it to play nicely with Postgres. I’m using the evaluation version of the OLEDB PGNP data provider (http://www.postgresql.org/about/news.1004).
I wanted to start with something simple like UPSERT on the fact table (10k-15k rows are updated/inserted daily), but this is proving very difficult (not to mention I’ll want to use surrogate keys in the future).
I’ve attempted (Link) and (http://consultingblogs.emc.com/jamiethomson/archive/2006/09/12/SSIS_3A00_-Checking-if-a-row-exists-and-if-it-does_2C00_-has-it-changed.aspx) which are effectively the same (except I don’t really understand the union all at the end when I’m trying to upsert) But I run into the same problem with parameters when doing the update using a OLEDb command – which I tried to overcome using (http://technet.microsoft.com/en-us/library/ms141773.aspx) but that just doesn’t seem to work, I get a validation error –
The external columns for complent.... are out of sync with the datasource columns... external column “Param_2” needs to be removed from the external columns.
(this error is repeated for the first two parameters as well – never came across this using the sql connection as it supports named parameters)
Has anyone come across this?
AND:
The fact that this simple task is apparently so difficult to do in SSIS suggests I’m using the wrong tool for the job - is there a better (and still flexible) way of doing this? Or would another ETL package be better for use between two Postgres database? -Other options include any listed on (http://en.wikipedia.org/wiki/Extract,_transform,_load#Open-source_ETL_frameworks). I could just go and write a load of SQL to do this for me, but I wanted a neat and easily maintainable solution.
I have used the Slowly Changing Dimension wizard for this with good success. It may give you what you are looking for especially with the Wizard
http://msdn.microsoft.com/en-us/library/ms141715.aspx
The External Columns Out Of Sync: SSIS is Case Sensitive - I encountered this issue multiple times and it makes me want to pull my hair out.
This simple task is going to take some work either way. SSIS is by no means an enterprise class ETL product yet, but it does give you some quick and easy functionality, and is sufficient for most ETL work. I guess it is also about your level of comfort with it as well.
SCD is way too slow for what I want. I need to use set based sql.
It turned out that a lot of my problems were with bugs in the provider.
I opened a forum topic (http://www.pgoledb.com/forum/viewtopic.php?f=4&t=49) and had a useful discussion with the moderator/support/developer person.
Also Postgres doesn't let you do cross db querys, so I solved the problem this way:
Data Source from Production DB to a temp Archive DB table
Run set based query between temp table and archive table
Truncate temp table
Note that the temp table is not atchally a temp table, but a copy of the archive table schema to temporarily stored data in.
Took a while, but I got there in the end.
This simple task is going to take some work either way. SSIS is by no means an enterprise class ETL product yet, but it does give you some quick and easy functionality, and is sufficient for most ETL work. I guess it is also about your level of comfort with it as well.
What enterprise ETL solution would you suggest?