Tables with LONG, BLOB,
CLOB, and NCLOB datatypes are likely to have spanned rows. The ???table fetch continued row???
statistic is also available in the AWR reports (or STATSPACK reports in Oracle Database 10g and
earlier).
In addition to chaining rows, Oracle will occasionally move rows. If a row exceeds the space
available to its block, the row may be inserted into a different block. The process of moving a row
from one block to another is called row migration, and the moved row is called a migrated row.
During row migration, Oracle has to dynamically manage space in multiple blocks and access the
freelist (the list of blocks available for insert operations). A migrated row does not appear as
a chained row, but it does impact the performance of your transactions. See Chapter 6 for an
example of using the DBMS_ADVISOR to find and reorganize tables with chained rows.
TIP
Accessing a migrated row increments the count in the ???table fetch
continued row??? statistic.
Increasing the Oracle Block Size
The effect of increasing the database block size is significant. Doubling the database block size
may improve the performance of query-intensive operations by up to 50 percent.
260 Oracle Database 11g DBA Handbook
The performance benefit has few costs. Because there will be more rows per database block,
there is a greater likelihood of block-level contention during data manipulation commands. To
address the contention problems, increase the settings for freelists and initrans at the table and
index level.
Pages:
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439