Don Burleson recently sent out to one of his Oracle DBA mailing lists some information about rebuilding indices (or is it indexes?) to his Oracle tips mailing list. He also provided a link to more information on his web site: http://www.dba-oracle.com/t_index_rebuilding_issues.htm. His information is valuable, and I have a few more observations from my experience at my previous job.
Burleson notes that Oracle recommends rebuilding indexes when the b-level is more than four or the percent deleted is greater than 20.
At my previous employer, the hardware budget was always very tight. We couldn’t always lay out the tablespaces in the ideal manner to optimize performance, but I think we did a pretty good job within the constraints we had. We rarely hit a b-level higher than three, but we discovered that in Oracle Versions 8i, 9i, and 10g, the larger the table the more noticeable the performance degradation on index reads as the percentage of deletes goes up. That is, on larger tables the performance degradation was much quicker and more noticeable than on smaller tables. A large table would see noticeable performance degradation on as few as 5% delete, whereas on smaller tables it would need to hit 10-15% before the degradation became noticeable. These were not minor differences, measured in milliseconds. The performance changes were dramatic, highly noticeable to end users. For the purposes of this article, a smaller table is from one hundred thousand to one million rows, a medium size table could be up to five million rows, and a large table is more than five million rows.
Obviously this sort of statistic is highly dependent on hardware, but I suspect there are plenty of Oracle shops that have seen similar results.
I eventually wrote a simple query that generates a list of candidates for index rebuilds, and the commands necessary to rebuild the indexes once the tables reached a point where it was necessary. The query reads a table we built called TABLE_MODIFICATIONS that we loaded each night from DBA_TAB_MODIFICATIONS before we ran statistics. Monitoring must be turned on to use the DBA_TAB_MODIFICATIONS table.
round(((sum(t.updates)+sum(t.inserts)+sum(t.deletes))/(decode(a.num_rows,0,1,a.num_rows)) *100.0),2) per_chg
from analyzedb.table_modifications t,
where t.timestamp >= to_date(‘&from_date’,’dd-mon-yyyy’) and
t.table_owner = a.owner and t.table_owner not in (‘SYS’,’SYSTEM’) and
having (sum(t.deletes)/(decode(a.num_rows,0,1,a.num_rows)))*100.0 >=5
group by t.table_owner, t.table_name, a.num_rows
order by num_rows desc, t.table_owner, t.table_name;
Of course, your mileage will vary. Reorg_a_table4 was the stored procedure that actually generated the DDL. It just reads the data dictionary to get the index name and tablespace name and generates the alter index rebuild command.
This was a site with a traditional one night per week operations window, and we usually didn’t have enough time to rebuild everything, so the list was sorted so that those tables most in need of work were listed first.