What are memory grants? Memory grants, additionally known as Query Execution (QE) Reservations, Query Execution Memory, Workspace Memory, and Memory Reservations, describe the usage of memory at query execution time. To supply some context, during its lifetime, a question might request memory from completely different memory allocators or clerks depending on what it needs to do. For example, when a query is parsed and compiled initially, it consumes compilation memory. As soon as the query is compiled, that memory is launched, and the ensuing question plan is stored in the plan cache memory. Once a plan is cached, the query is ready for execution. If the query does any type operations, hash match operations (Join or aggregates), or insertions right into a COLUMNSTORE indexes, it makes use of memory from question execution allocator. Initially, the question asks for that execution memory, and later if this memory is granted, the query makes use of all or a part of the memory for kind results or hash buckets. This memory allocated throughout query execution is what's referred to as memory grants.
As you may imagine, once the question execution operation completes, the memory grant is launched again to SQL Server to make use of for other work. Due to this fact, memory grant allocations are momentary in nature but can nonetheless final a very long time. For example, if a question execution performs a kind operation on a very massive rowset in memory, the type may take many seconds or minutes, and the granted memory is used for the lifetime of the query. This query selects a rowset of over 300,000 rows and sorts it. The type operation induces a Memory Wave grant request. If you run this query in SSMS, you possibly can view its question plan. XML component that exhibits the identical memory grant information. A number of terms need explanation here. A question could desire a certain amount of execution memory (DesiredMemory) and would generally request that quantity (RequestedMemory). At runtime, SQL Server grants all or a part of the requested memory relying on availability (GrantedMemory).
In the long run, the question might use kind of of the initially requested memory (MaxUsedMemory). If the query optimizer has overestimated the quantity of memory wanted, it makes use of less than the requested measurement. However that memory is wasted as it may have been used by another request. However, if the optimizer has underestimated the dimensions of memory wanted, the surplus rows could also be spilled to disk to get the work executed at execution time. As a substitute of allocating extra memory than the initially requested dimension, SQL Server pushes the additional rows over to disk and makes use of it as a brief workspace. For more info, see Workfiles and Worktables in Memory Grant Issues. Let's assessment the completely different terms you could encounter relating to this memory client. Once more, all these describe concepts that relate to the identical memory allocations. Query Execution Memory (QE Memory): This term is used to focus and concentration booster on the truth that sort or hash memory is used throughout the execution of a query.
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Generally QE memory is the biggest shopper of memory during the life of a question. Question Execution (QE) Reservations or Memory Reservations: When a query wants memory for type or hash operations, it makes a reservation request for memory. That reservation request is calculated at compile time based mostly on estimated cardinality. Later, when the query executes, SQL Server grants that request partially or totally relying on memory availability. In the long run, the question could use a proportion of the granted memory. Memory Grants: Memory Wave When SQL Server grants the requested memory to an executing query, it is mentioned that a memory grant has occurred. There are a number of performance counters that use the term "grant." These counters, Memory Grants Excellent and Memory Grants Pending, display the depend of memory grants glad or ready. They don't account for the memory grant size. One query alone could have consumed, for example, 4 GB of memory to carry out a sort, but that is not reflected in either of those counters.
Workspace Memory is another term that describes the identical memory. Often, you may see this time period within the Perfmon counter Granted Workspace Memory (KB), which displays the overall quantity of memory at present used for type, hash, bulk copy, and index creation operations, expressed in KB. The maximum Workspace Memory (KB), another counter, accounts for the utmost amount of workspace memory obtainable for any requests that will have to do such hash, type, bulk copy, and index creation operations. The time period Workspace Memory is encountered infrequently exterior of those two counters. Most often, when a thread requests memory inside SQL Server to get something executed and the memory is not obtainable, the request fails with an out of memory error. However, there are a few exception scenarios the place the thread would not fail but waits until memory does turn out to be out there. One of those scenarios is memory grants, and the opposite is query compilation memory. SQL Server uses a thread synchronization object referred to as a semaphore to maintain observe of how a lot memory has been granted for query execution.