Wednesday, January 2, 2013

SQL Server: A Query Slow in SSMS, Fast in Application, WHY?



Today, a colleague asked me, why his simple select query is taking around 3000ms (3 Seconds) to execute while, same query is quite fast when executed from application.

Answer is simple: SQL Server Management Studio use RBAR-Row By Agonizing Row method to fetch rows and inform row by row to SQL Server that row is received while on other hand application which don’t use RBAR method, inform once after whole batch is received and reluctantly is fast as compared to SSMS or those applications which use RBAR method.

To confirm that query is running slow just because of RBAR factor, I have used extended events for single session waits analysis, a well defined method by Paul Randal. Output was as following: 

 NETWORK_IO is basically ASYNC_NETWORK_IO, when working with extended events. According to BOL “Occurs on network writes when the task is blocked behind the network. Verify that the client is processing data from the server.”

But a more proper definition for this type of wait you can find on Karthik PK’s Blog. He stats that “When a query is fired, SQL Server produces the results ,place it in output buffer and send it to client/Application. Client/Application then fetch the result from the Output buffer, process data  and sends an acknowledgement to SQL Server. If client/Application takes long time to send acknowledgement then SQL Server waits on ASYNC_NETWORK_IO (SQL 2005/2008) or  Network_IO (SQL 2000) before it produces additional results.

Hence proved that, our query delay was just because of NETWORK_IO wait (2870ms out of total 3000ms) and we were on the same machine where SQL Server was installed so no chances of any network problem and its only RBAR method of SQL Server Management Studio which was causing this delay. 

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