Databases Reference
In-Depth Information
the table spool separates the Clustered Index Scan from the Clustered Index Update, as
shown in Figure 6-8.
The spool operator scans the original data and saves a copy of it in a hidden spool
table in tempdb before it is updated. A Table Spool operator is usually used to avoid
the Halloween problem as it is a cheap operator. However, if the plan already includes
another operator that can be used, such as a Sort, then the Table Spool operator is not
needed, and the Sort can perform the same blocking job instead.
Figure 6-8:
An update with Halloween protection.
Finally, drop the table you have just created.
DROP
TABLE
dbo
.
Product
Listing 6-9.
Data Warehouses
A data warehouse is a decision support system for business decision making, designed to
execute queries from users as well as running reporting and analytical applications. It is
also structurally different from an online transaction processing (OLTP) system, which
focuses on operational transaction processing (we'll look at some of these differences in
a just a moment). Because of these different purposes, both systems also have different
workloads: a data warehouse will usually have to support complex and large queries,
compared to the typically small transactions of an OLTP system.
