How much time it take to rebuild an index for a table with 20 millions rows

Question

Hi all,
i need to rebuild the index of a table containing 20 000 000 row (i don't know why the other people working on this didn't think of rebuilding the index regularly, because i asked and apparently it has never been done :cry: :cry:) i am not a sql developper nor a DBA so i can't mesure how long it take to rebuild the index, does any one have an idea (aproximativly of course :aie:), the other question is there any formula to use in order to calculate how often to rebuild the indexes (i can for example retieve how much rows are delated or inserted daily ...)
Thanks again
Taha

Answer 1

taha wrote:
:aie: that's why i am asking because i don't know (and to be sure which solution is best)
so the table is like this (the columns) :
45 varchar2, 5 timestamp, 30 Number no LOB columns, (15 indexes : 5 unique indexes and that those indexes uses at a maximum 4 columns)15 indexes - 100,000 deletes: this could mean 1,500,000 block visits to maintain index leaf blocks as the table rows are deleted. If you're unlucky this could turn into 1,500,000 physical block read requests; if you're lucky, or the system is well engineered this could be virtually no physical I/O. The difference in time could be huge. At any rate it is likely to be 1,500,000 redo entries at 250 - 300 bytes per entry for a total of about 400MB of redo (so how large are your redo logs and how many log switches are you going to cause).
yes the tables is used by an application so (update, insert ) can take place at any time
for the deletion , there is the batch which does a mass delete on the table ( 4 or 5 time each day)
You haven't answered the question - how long does it take to do a sample batch delete.
If you can enable SQL tracing, or take a before/after snapshot of v$sesstat or v$session_event for the session as it does the delete then you can get some idea of where the time is going - for all you know it might be spending most of its time waiting for a lock to do away.
>
"How many leaf blocks are currently allocated to the index(es) ?" how can i answer to this question ? may be if i check the all_objects table ?
If you keep your statistics up to date then dba_indexes is a good place, cross-checked with dba_segments, and you can use the dbms_space package for more detail. I have a code sample on my blog which allows you to compare the current size of your indexes with the size they would be if rebuilt at some specific percentage: http://jonathanlewis.wordpress.com/index-sizing/ (It's such good code that Oracle Corp. has copied it into MOS note 989186.1)
Regards
Jonathan Lewis