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Add to this that the JOIN can be done using various strategies (nested loop, hash, merge) and you’ll see why optimization is such an important concept in SQL.
SQL Server uses a cost based optimizer, meaning that it will consider all (or at least many) of the possible alternatives, try to make an educated guess about the cost of each alternative, and then choose the one with the lowest cost.
Exploring all these alternatives can be time consuming and this is why once a query plan is created is also cached for future reuse.
Future similar requests can skip the optimization phase if they can find an already compiled and optimized query plan in the SQL Server internal cache.
Individual statements inside the SQL Batch will not create new tasks.As a side note: a common mistake done by developers is trying to come up with a single T-SQL query that cover many alternatives, usually by using clever expressions in the WHERE clause, often having many OR alternatives (eg. For developers trying to keep things DRY and avoiding repetition are good practices, for SQL queries they are plain bad.The compilation has to come up with an access path that work for any value of the input parameters and the result is most often sub-optimal.For a SQL batch request the worker that picks up that task will execute the entire SQL batch (every statement).This should settle the often asked question whether statements in a SQL batch (= 1) and create sub-tasks, each sub-task goes through exactly the same cycle: it is created as PENDING and a worker must pick it up and execute it (a different worker from the SQL batch worker, that is by definition occupied! The lists and state of workers inside SQL Server can be seen by querying .
This is the type of request Sql Client sends if you invoke any of the managed class.