Minimum Memory Grant Hint

James Anderson looks at memory grant query hints:

This is one of a pair of query hints that can manage the memory grants of individual queries. The maximum memory grant hint allows you to limit the amount of memory an individual query can use. If the hint specifies a value lower than the amount of memory required to execute the query, the query will request the required amount. This is like a granular resource governor. For the rest of this post I will concentrate on the minimum memory grant query hint.

When the query optimiser builds an execution plan for a query, it calculates how much memory is required for the query to execute. This calculation is based on the estimated number of rows (used in sort or hash joins) and the average size of these rows. The optimiser calculates the least amount of memory required to perform the operation (required memory) and the amount of memory required to perform the whole operation in memory (desired memory). Each time the query is executed it requests a memory grant from the server. The query will perform much better if it’s granted its desired memory or more. If at run time the query has to perform sorts or hash operations on more than the estimated number of rows, it will spill to tempdb. Spilling to tempdb can be orders of magnitude worse for performance as your data has to be written to disk causing more IO.

This feels like the type of thing you might want to do once for a particularly recalcitrant query, but not something you want to do regularly.

Related Posts

Performance Concern: Anti-Join And Top

Paul White explains a scenario in which an innocent-looking execution plan can hide something sinister: Not every execution plan containing an apply anti join with a Top (1) operator on its inner side will be problematic. Nevertheless, it is a pattern to recognise and one which almost always requires further investigation. The four main elements […]

Read More

Character Columns And MAX Vs TOP+ORDER Differences

Kendra Little digs into a tricky performance problem: Most of the time in SQL Server, the MAX() function and a TOP(1) ORDER BY DESC will behave very similarly. If you give them a rowstore index leading on the column in question, they’re generally smart enough to go to the correct end of the index, and […]

Read More


May 2016
« Apr Jun »