The Table Spool Operator In SQL Server

Hugo Kornelis digs into table spools:

The Table Spool operator is one of the four spool operators that SQL Server supports. It retains a copy of all data it reads in a worktable (in tempdb) and can then later return extra copies of these rows without having to call its child operators to produce them again. These copies can be made available in the same part of the execution plans, or in another part.

Table Spool is probably the most basic of the spool operators. The Index Spool operator is very similar to it, but indexes its data to allow it to return only a subset of the stored rows. The Row Count Spool operator is optimized for specific cases where the rows to be returned are empty. And the Window Spool operator is used to support the ROWS and RANGE specifications of windowing functions.

Typical use cases of a Table Spool are: to reproduce the same input multiple times without having to re-execute its child nodes (e.g. in the inner input of a Nested Loops); to make the same input available in multiple branches of an execution plan (e.g. in wide update plans); or to ensure that an original copy of the data is available after an insert, update, or delete operator changes the base data (“Halloween protection”).

Click through for a great deal more detail.

Related Posts

Tuning Apache Spark Applications

Vidisha Gupta has a few tips for tuning Apache Spark programs: Data Serialization – Serialization plays an important role in increasing the performance of any application. Spark provides two serialization libraries – Java Serialization: By default, spark uses Java’s ObjectOutputStream framework which can work with any class that implements java.io.serializable. This serialization is flexible but slow and […]

Read More

A Compendium Of Bad (Or Misleading) Performance Tips

Grant Fritchey responds to a long list of performance tips of greater or (mostly) lesser value: Index the predicates in JOIN, WHERE, ORDER BY and GROUP BY clauses What about the HAVING clause? Does the column order matter? Should we put a single column or multi-column index? INCLUDE statements? What kind of index, clustered, non-clustered, […]

Read More

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Categories

November 2018
MTWTFSS
« Oct  
 1234
567891011
12131415161718
19202122232425
2627282930