Memory has always been my favorite topic when comes to troubleshooting performance in SQL Server :)
I have been contributing and posting few articles on Buffer and Data pages in cache and how it works and all that! But this time I would like to share what exactly query memory is
and how and where to look at, when query performance is suffering!
Let me try to explain in easy language, what happens when query enters the door of SQL Server RDBMS:
1. SQL Server receives query
2. SQL Server creates compiled plan before it creates execution plan
3. SQL server when create compiled plan evaluate the need of two memory parameters
(Required memory * degree of parallelism) + Additional memory
5. If above needed memory exceeds the query memory limit then server reduce the memory to fit the need
6. Now this memory which is able to fit a.k.a requested memory need is fulfilled by RESOURCE SEMAPHORE
7. If RESOURCE_SEMAPHORE can not satisfy this memory need then query is put under RESOURCE_SEMAPHORE wait type
8. And cycle goes on..
If you see high RESOURCE_SEMAPHORE wait types, it is an indicator that query need more requested memory.
This opens up the case to optimize the query!
I have been contributing and posting few articles on Buffer and Data pages in cache and how it works and all that! But this time I would like to share what exactly query memory is
and how and where to look at, when query performance is suffering!
Let me try to explain in easy language, what happens when query enters the door of SQL Server RDBMS:
1. SQL Server receives query
2. SQL Server creates compiled plan before it creates execution plan
3. SQL server when create compiled plan evaluate the need of two memory parameters
· Required memory( Min memory need to run sort and hash operations)
· Additional memory(need to store rows in memory)
4. Server calculates how much memory query need. Formula is (Required memory * degree of parallelism) + Additional memory
5. If above needed memory exceeds the query memory limit then server reduce the memory to fit the need
6. Now this memory which is able to fit a.k.a requested memory need is fulfilled by RESOURCE SEMAPHORE
7. If RESOURCE_SEMAPHORE can not satisfy this memory need then query is put under RESOURCE_SEMAPHORE wait type
8. And cycle goes on..
If you see high RESOURCE_SEMAPHORE wait types, it is an indicator that query need more requested memory.
This opens up the case to optimize the query!
