July 2010
Intermediate to advanced
448 pages
12h 22m
English
Content preview from Physical Database Design
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3.4 Query Execution Plan Development 41
= read TEMP5
= 13.
In summary, the total cost of answering the query using this approach is as follows:
Number of block accesses for the query (option 2A)
= 2 + 261 + 2 + 97 + 23 + 13
= 398.
We now compare the two approaches:
It is clearly seen that the use of table reduction techniques (early selections and pro-
jections) has the potential of greatly reducing the I/O time cost of executing a query.
Although in this example indexes were not required, in general they can be very useful
to reduce I/O time further.
3.4 Query Execution Plan Development
A query execution plan is a data structure that represents each database operation (selec-
tions, projections, and joins) as a distinct node. The sequence of operations ...
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