SQL/MP Query Guide

Improving Query Performance With Environmental
Options
HP NonStop SQL/MP Query Guide—524488-003
4-23
Real Sequential Block Buffering (RSBB)
Real Sequential Block Buffering (RSBB)
If a query accesses most of the rows in a table and most of the columns in each row, it
can be beneficial for the disk process to return a complete block of rows to the file
system rather than returning one row at a time. This process is called real sequential
block buffering (RSBB).
RSBB reduces the number of messages it would otherwise take to return all qualifying
rows, one at a time, to the file system. Figure 4-3 shows the disk process returning a
copy of a physical block of rows to the file system. When the executor requests the
next row, the file system does the projection and restriction of data and returns the next
row to the executor. Therefore, the number of requests (messages) between the file
system and the disk process is reduced by the file’s physical blocking factor (that is,
the number of rows per block).
The disk process returns a physical block of rows to the file system. The file system
examines each row in the returned block and tests the rows against the predicate.
After all the rows in the block have been processed, the file system requests another
block.
The efficiency of RSBB access can be estimated by dividing the number of useful
bytes per physical block by the block size.
Slack Space
RSBB efficiency is diminished if there is slack (unused) space within the data blocks.
In the preceding example, 100 rows are transferred from the disk process to the file
system; however, the file system sends only 12 messages to the disk process
(assuming four-kilobyte data blocks with no slack).
If there is fifteen percent slack space within data blocks, then each physical block
contains eight actual data rows instead of nine and the number of messages increases
from 12 to 13.
For more information about slack space and how you can specify slack space, see the
SQL/MP Reference Manual. The FUP INFO filename, STAT command displays the
Figure 4-3. Real Sequential Block Buffering (RSBB)
File System
Get Next Real Block Where
RETA IL_PRICE > 10
Disk Process
Returns One Physical
Block of Rows
5
...
11
...
20
...
PEACH
PEAR
APPLE
10
11
12
5
...
11
...
20
...
PEACH
PEAR
APPLE
10
11
12
0
16
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