SQL Server have different types of components (Plan Cache, Buffer pool and other memory components) in the memory area and requires memory for many things. We also require memory to process our queries. Each operator in the execution plan requires memory to process the rows from one operator to the other operator. Some of the iterators need to be buffered as the rows come into them before sending them to the next iterator in the execution plan. These iterators needs more memory. These are called memory consuming iterators. The grants that we given to these memory consuming iterators are called Query memory grants.
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