It's hot here. I wish I was at the beach and I feel like a rant.
"oracle actually accesses indexes and tables serially"
Is it just me or is this blindingly obvious?
You cannot access the table data until you have completed accessing the
index data
because the index data contains the location of the table data.
During an indexed query on a single table the index will be accessed, then
the table,
then the index,then the table, then the index,then the table then the
index,then the table.
If the index and the table are on the same disk then a lot of time will be
taken up by
head seek movement.
If they are on the different disks then the "index" heads can locate their
data and stay
there - and the "data" heads can locate their data and stay there.
Less head movement, less wasted time.
That is the argument for what it is worth. Real life is of course vastly
more complex than
this and we are swimming in very muddy waters, which is why there is so
much
argument on the subject (raid salesmen - spit).
Thanks for the vent
Garry Gillies
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15/07/03 15:49
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Subject: should you seperate indexes from tables in seperate datafiles?
There has been alot of literature stating that you will recieve
performance improvements by seperating indexes and tables across multiple
I/O points.
Ie... you have a tables tablespace and an index tablespace. If you put
them on seperate hard drives, you will have less I/O contention.
Now Im seeing some articles stating that this is not true. That oracle
actually accesses indexes and tables serially. Now it might be useful
seperate indexes from tables for maintenance purposes but this wont lower
I/O contention.
Can anyone chime in on this? Curious to see where the evidence is leading?
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