Re: HowTo find out used (useful) and unused (usesless) indexes?
Date: Sun, 20 Apr 2008 13:33:34 -0700
> "DA Morgan" <damorgan_at_psoug.org> schreef in bericht > news:1208624947.696580_at_bubbleator.drizzle.com...
>> Shakespeare wrote:
>>> But an index may become useful over time, true?
>> On that basis alone one could justify putting an index on every
>> column of every table so I will respectfully disagree unless you
>> write a very broad definition of "may."
>> You need to understand your data and how it is being accessed.
>> The extra overhead of an unused index is not value added.
>> My recommendation would be to use the DBMS_STATS.SET.... procedures
>> to see how queries will react to the expected future growth of both
>> tables and indexes.
>> Daniel A. Morgan
>> Oracle Ace Director & Instructor
>> University of Washington
>> damorgan_at_x.washington.edu (replace x with u to respond)
>> Puget Sound Oracle Users Group
> > I was aiming at data with for example a 'year' column. This column could be > indexed by design, but in the first year (all records same value) this index > is not useful and won't be used. But on the first entry in the second year > it is useful to find entries of that year and so on. A script to remove or > disable unused indexes would remove/disable this index in the first year. > > Shakespeare
First year the index is worthless. Second year it is valuable for some short amount of time and then, again, may become useless as the percentage of records in each of the two years approaches 50%. Go for the third year ... again possibly usable for some period of time and then all three years probably tend toward 33.3%. By the end of year four you are going for 25%.
I would not be all that ready to build that index unless I know, again using DBMS_STATS.SET_TABLE_STATS and SET_INDEX_STATS how Oracle was going to use it. That is not to say the index might not make a lot of sense ... I just would insist on testing any assumption before deciding to either build it or not.
-- Daniel A. Morgan Oracle Ace Director & Instructor University of Washington damorgan_at_x.washington.edu (replace x with u to respond) Puget Sound Oracle Users Group www.psoug.orgReceived on Sun Apr 20 2008 - 15:33:34 CDT