Trying to figure out why there is such a significant difference between
select * from linkedserver..tablename
and
select * from openquery(linkedserver, select * from tablename).
4 minutes vs 13 seconds.
OPENQUERY connects to the destination server and runs the query on that server and returns the resultset. Whereas, I believe the Linked Server query is executed on the local server and runs across the connection.
Hope this helps.
Openquery connects directly to the destination server. Despite saying the RDBMS, you did not said what the other way to communicate with the data was.
Do you know if it is trying to enlist a distributed transaction over the linked server? If you watch the Trace output from Oracle when the calls are made, you should be able to see the difference in the requests happening. IIRC, the standard link query will request schema information from the oracle metadata in a different manner than the OPENQUERY call does. I don't know exactly why this happens, but I had a similar issue in the past that was resolved by using OPENQUERY.
Related
I have a server called ERP-SERVER, and a server called SQLDEV-SERVER.
They both have a blob instance, but we never copy over the complete blob to the SQLDEV-SERVER as that would be too much data.
So when trying to access a file on our test server, it should first check if that file exists on the SQLDEV-SERVER, and if not check if the file exists on the ERP-SERVER. This is where it goes wrong. This piece of code (SQL) used to work but somewhere along the way it broke. I have narrowed it down to the inter database query just returning completely different results.
so for instance i run this query on the ERP-SERVER instance in SQL management studio:
SELECT count(*)
FROM [erp-server].[Extranet_Blob].[dbo].[FileStorBlob]
this returns 223221 results.
When i run the same query on the SQLDEV-SERVER instance in SQL management studio, it returns 313 results.
It points to the same server and same database, yet a completely different count, which is why it is also not returning the files from the live environment when it is not found on the dev environment.
Any pointers as to where this problem could be situated?
Look very carefully at your linked server definition. When you are running the query on SQLDEV-SERVER it is using the linked server definition of that name rather than necessarily the ERP_Server. Is it possible that someone has fiddled with the definition?
A view that references a remote server
4part name ([ServerName], [DatabaseName], [Owner], [Object Name]
OpenQuery
Which is better performance?
Why is performance good?
AFAIK, it depends a lot on your remote server type.
With recent SQL version (2016) on both server (local and remote), I didn't noticed any difference.
If your remote server is anything else (postgres, mysql...) your really should use OpenQuery as it executes the query on the remote server, getting only the correct resultset. If you use the 4 part name, SQL server will order and filter on local.
For example, take a 4 million record table and execute a query like :
SELECT * FROM reoteserver.database.schema.table where id = 4
With openquery, sql server will get only the record with id 4. Without, it will get all the table, and then filter it to get the id 4.
Late to the party here, but the difference essentially is that 4 part queries are executed locally, thus cannot utilise indexes or keys since the local server doesn't know about them. Instead it essentially retrieves the entire object, then applies the filter. On a small table, you would be unlikely to notice a difference, but on a table with millions of rows, you'd notice a difference. Openquery essentially tells the remote server to execute the query on it's behalf then pass the result back.
General rule I would say is;
NEVER join on to a table using 4 part. Only join using Openquery and I would even avoid that where possible, but that's more of a personal preference.
However, 4 part SP execution i.e. EXEC ServerName.DBName.SchemaName.ObjectName is essentially the same since that also tells the remote server to execute the query on its behalf.
I'm looking for a query to get the current running queries in Azure SQL. All of the T-SQL I've found do not show the running queries when I test them (for instance, run a query in one window, then look in another window at the running queries). Also, I'm not looking for anything related to the time, CPU, etc, but only the actual running query text.
When I run ...
SELECT * FROM Table --(takes 2 minutes to load)
... and run a standard information query (like from Pinal Dave or this), I don't see the above query (I assume there's another way).
select * from sys.dm_exec_requests should give you what other sessions are doing.You can join this with sys.dm_exec_sql_text to get the text if needed. sys.dm_tran_locks gives the locks hold / waiting. If this is V12 server you can also use dbcc inutbuffer. Make sure that the connection you are running is dbo / server admin
I've written a SQL query that looks like this:
SELECT * FROM MY_TABLE WHERE ID=123456789;
When I run it in the Query Analyzer in SQL Server Management Studio, the query never returns; instead, after about ten minutes, I get the following error: System.OutOfMemoryException
My server is Microsoft SQL Server (not sure what version).
SELECT * FROM MY_TABLE; -- return 44258086
SELECT * FROM MY_TABLE WHERE ID=123456789; -- return 5
The table has over forty million rows! However, I need to fetch five specific rows!
How can I work around this frustrating error?
Edit: The server suddenly started working fine for no discernable reason, but I'll leave this question open for anyone who wants to suggest troubleshooting steps for anyone else with this problem.
According to http://support.microsoft.com/kb/2874903:
This issue occurs because SSMS has insufficient memory to allocate for
large results.
Note SSMS is a 32-bit process. Therefore, it is limited to 2 GB of
memory.
The article suggests trying one of the following:
Output the results as text
Output the results to a file
Use sqlcmd
You may also want to check the server to see if it's in need of a service restart--perhaps it has gobbled up all the available memory?
Another suggestion would be to select a smaller subset of columns (if the table has many columns or includes large blob columns).
If you need specific data use an appropriate WHERE clause. Add more details if you are stuck with this.
Alternatively write a small application which operates using a cursor and does not try to load it completely into memory.
We have 2 servers - one production, one test/development. I wanted to run some SQL checks and updates against production data but write the changes to test/development server, so people wouldn't see the changes.
Using SQL Server Management studio, I ran the cursor with the checks and updates in it. I was actively connected to test/development. However, I wrote my queries as follows.
SELECT * FROM [Production_Server].[Production_DB].[schema].[table]
I was under the impression this would look at the production server, however, it did not. It looks at the test/development server. I have access/rights in both environments.
Is there something I overlooked permission wise to get this to work? Or is just how it is intended to work?
"[Production_Server]" must be the wrong linked server name.
Run the below sproc to find the correct value to use
exec sp_linkedservers