Understanding Memory Grants

Erik Darling explains memory grants in SQL Server:

Our query memory grants range from around 8 MB to around 560 MB. This isn’t even ordering BY the larger columns, this is just doing the work to sort results by them. Even if you’re a smarty pants, and you don’t use unnecessary ORDER BY clauses in your queries, SQL may inject them into your query plans to support operations that require sorted data. Things like stream aggregates, merge joins, and occasionally key lookups may still be considered a ‘cheaper’ option by the optimizer, even with a sort in the plan.

Of course, in our query plans, we have warnings on the last two queries, which had to order the VARCHAR(8000) column.

This shows just how much difference a simple column size can make.

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Table Variables And Parallelism

Erik Darling shows your brain on table variables: Inserts and other modifications to table variables can’t be parallelized. This is a product limitation, and the XML warns us about it. The select could go parallel if the cardinality estimate were more accurate. This could potentially be addressed with a recompile hint, or with Trace Flag […]

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Non-Blocking Aggregations

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