Introduction to NonStop SQL/MP

Other Architectural Features
NonStop SQL Architecture
3–22 113425 Tandem Computers Incorporated
system without first manipulating the data. The file system then performs any
projection and restriction required by the query.
If a query requires extensive projection and restriction—if only a few rows and
columns need to be retrieved—the data access manager can perform the single-table
query evaluation. However, instead of returning a row at a time, the data access
manager can build a virtual block containing only the qualified rows and columns. It
then returns the virtual block to the file system. This method, called virtual sequential
block buffering (VSBB), reduces both message traffic and the amount of data passed to
the file system.
The optimizer determines which I/O method to use when it builds a query plan. The
optimizer might choose VSBB for the following query:
SELECT FIRST_NAME,LAST_NAME,SALARY
FROM EMPLOYEE
WHERE EMPNUM <= 1000 AND SALARY > 75000 ;
Figure 3-9 shows a sample block returned by the data access manager when it uses
VSBB to execute the preceding query.
Figure 3-9. Virtual Sequential Block Buffering (VSBB) Access
1
23
599
Roger
Jerry
Joan
Green
Howard
Harris
175500.00
137000.00
85000.00
1
19
23
Roger
Susan
Jerry
Green
Amis
Howard
175500.00
55000.00
137000.00
568
570
599
Jessica
Peter
Joan
Criner
Smith
Harris
39500.00
42500.00
85000.00
File System Data Access Manager
Returns One Block of
Rows (With Projection
and Restriction)
Get Next Row Where
EMPNUM <= 1000 AND SALARY > 75000.00
015
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