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What are Memory Wave Protocol grants? Memory grants, also referred to as Question Execution (QE) Reservations, Query Execution Memory, Workspace Memory, and Memory Reservations, describe the usage of memory at question execution time. To provide some context, throughout its lifetime, a query could request memory from completely different memory allocators or clerks relying on what it must do. For example, when a query is parsed and compiled initially, it consumes compilation memory. As soon as the question is compiled, that memory is launched, and the ensuing question plan is stored in the plan cache memory. As soon as a plan is cached, the query is ready for execution. If the question does any kind operations, hash match operations (Be a part of or aggregates), or insertions right into a COLUMNSTORE indexes, it uses Memory Wave from query execution allocator. Initially, the query 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 type outcomes or hash buckets. This memory allocated throughout query execution is what is known as memory grants.
As you possibly can think about, once the question execution operation completes, the memory grant is released back to SQL Server to use for other work. Subsequently, memory grant allocations are temporary in nature but can still final a very long time. For instance, if a question execution performs a kind operation on a really large rowset in memory, the sort may take many seconds or minutes, and the granted memory is used for the lifetime of the query. This question selects a rowset of over 300,000 rows and types it. The type operation induces a memory grant request. For those who run this question in SSMS, you can view its query plan. XML aspect that shows the identical memory grant info. Several terms want explanation here. A question might need a certain amount of execution memory (DesiredMemory) and would generally request that amount (RequestedMemory). At runtime, SQL Server grants all or part of the requested memory depending on availability (GrantedMemory).
Ultimately, the question could use kind of of the initially requested memory (MaxUsedMemory). If the question optimizer has overestimated the quantity of memory wanted, it uses lower than the requested size. However that memory is wasted as it could have been used by one other request. Then again, if the optimizer has underestimated the size of memory wanted, the surplus rows may be spilled to disk to get the work done at execution time. Instead of allocating more memory than the initially requested size, SQL Server pushes the additional rows over to disk and uses it as a short lived workspace. For extra data, see Workfiles and Worktables in Memory Grant Issues. Let's assessment the completely different phrases chances are you'll encounter relating to this memory shopper. Once more, all these describe concepts that relate to the same memory allocations. Query Execution Memory (QE Memory): This term is used to focus on the fact that type or hash memory is used in the course of the execution of a query.
Commonly QE memory is the largest shopper of memory throughout the life of a query. Question Execution (QE) Reservations or Memory Reservations: When a query needs memory for sort or hash operations, it makes a reservation request for memory. That reservation request is calculated at compile time primarily based on estimated cardinality. Later, when the question executes, SQL Server grants that request partially or totally depending on memory availability. In the long run, the query may use a proportion of the granted memory. Memory Grants: When SQL Server grants the requested memory to an executing query, it is stated that a memory grant has occurred. There are a couple of performance counters that use the term "grant." These counters, Memory Grants Outstanding and Memory Grants Pending, show the count of Memory Wave grants satisfied or waiting. They do not account for the memory grant dimension. One query alone could have consumed, for instance, 4 GB of memory to carry out a sort, but that isn't mirrored in either of these counters.
Workspace Memory is one other term that describes the same memory. Usually, you may even see this term within the Perfmon counter Granted Workspace Memory (KB), which reflects the overall amount of memory at present used for sort, hash, bulk copy, and index creation operations, expressed in KB. The maximum Workspace Memory (KB), one other counter, accounts for the maximum quantity of workspace memory accessible for any requests that may must do such hash, kind, bulk copy, and index creation operations. The time period Workspace Memory is encountered infrequently exterior of these two counters. Normally, when a thread requests memory inside SQL Server to get something done and the memory isn't out there, the request fails with an out of memory error. Nevertheless, there are a few exception situations where the thread does not fail but waits till memory does develop into obtainable. One of those eventualities is memory grants, and the opposite is query compilation memory. SQL Server uses a thread synchronization object called a semaphore to maintain track of how much memory has been granted for query execution.