Batch Execution Mode And Window Functions

Kevin Feasel

2017-07-24

T-SQL

Chris Adkin shows how taking advantage of batch execution mode on rowstore tables can lead to faster performance as degree of parallelism increases:

The SQL Server execution engine fundamentally acts like a cursor, control flow is exerted from the root node of the plan down to right most child node or iterators. The (logical) flow of data through the plan is in the opposite direction.

I say ‘Logical’ because in practise the run time uses buffers in order to minimise the data movement whilst executing the plan. However, up until SQL Server 2012, which first introduced batch mode, execution plans were executed by iterators processing data in a row by agonising row manner. Batch mode changes all of this, if we can ferry rows around in batches, this reduces the number of CPU cycles it takes to process an iterator in the plan (providing it supports batch mode). Also by sizing batches such that they fit inside the level 2 cache of the CPU, we gain even more performance by minimizing CPU last level cache misses or worse still main memory.

Adkin credits Niko Neugebauer for the insight and shows how you can use this on normal rowstore tables.

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