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SQL Server internals information including my content from SQLBlog.com and SQLPerformance.com.
Showing posts with label Spool. Show all posts
Showing posts with label Spool. Show all posts

Sunday, 26 July 2020

A bug with Halloween Protection and the OUTPUT Clause

A bug with Halloween Protection and the OUTPUT Clause

Background

The OUTPUT clause can be used to return results from an INSERT, UPDATE, DELETE, or MERGE statement. The data can be returned to the client, inserted to a table, or both.

There are two ways to add OUTPUT data to a table:

  1. Using OUTPUT INTO
  2. With an outer INSERT statement.

For example:

-- Test table
DECLARE @Target table
(
    id integer IDENTITY (1, 1) NOT NULL, 
    c1 integer NULL
);

-- Holds rows from the OUTPUT clause
DECLARE @Output table 
(
    id integer NOT NULL, 
    c1 integer NULL
);

Wednesday, 28 July 2010

Ranking Function Optimizer Transformations

Ranking Function Optimizer Transformations

In my last post I showed how SQL Server 2005 and later can use a Segment Spool to implement aggregate window functions and the NTILE ranking function.

The query optimizer is also smart enough to recognise that some queries are logically equivalent to a window function, even if they are written using different syntax.

Partitioning and the Common Subexpression Spool

Partitioning and the Common Subexpression Spool

SQL Server 2005 introduced the OVER clause to enable partitioning of rowsets before applying a window function. This post looks at how this feature may require a query plan containing a ‘common subexpression spool’. This query plan construction is required whenever an aggregate window function or the NTILE ranking window function is used.