Il est communément admis que le fait de conserver les index et les tables dans des espaces de table séparés améliore les performances. Ceci est maintenant considéré comme un mythe par de nombreux experts respectables (voir ce fil Ask Tom - recherchez "mythe" ), mais reste une pratique courante car les vieilles habitudes ont la vie dure !
Modification tierce
Extrait de asktom :"Tablespace d'indexation" à partir de 2001 pour Oracle version 8.1.6 la question
- Est-ce toujours une bonne idée de conserver les index dans leur propre tablespace ?
- Cela améliore-t-il les performances ou s'agit-il plutôt d'un problème de récupération ?
- La réponse diffère-t-elle d'une plate-forme à l'autre ?
Première partie de la réponse
Yes, no, maybe.
The idea, born in the 1980s when systems were tiny and user counts were in the single
digits, was that you separated indexes from data into separate tablespaces on different
disks.
In that fashion, you positioned the head of the disk in the index tablespace and the head
of the disk in the data tablespace and that would be better then seeking 2 times on the
same disk.
Drives back then were really slow at seeking and typically measured in the 10's to 100's
of megabytes (if you were lucky)
Today, with logical volumes, raid, NN gigabyte (nn is rapidly becoming NNN gigabytes)
drives, hundreds/thousands of concurrent users, thousands of tables, 10's of thousands of
indexes - this sort of "optimization" is sort of impossible.
What you strive for today is to be able to manage things, to spread IO out evenly
avoiding hot spots.
Since I believe all things should be in locally managed tablespaces with UNIFORM extent
sizes, I would say that yes, indexes would be in a different tablespace from the data but
only because they are a different SIZE then the data. My table with 50 columns and an
average row size of 4k might belong in a tablespace that has 5meg extents whereas the
index on a single number column might belong in a tablespace with 512k or 1m extents.
I tend to keep my indexes separate from the data but for the above sizing reason. The
tablespaces frequently end up on the same exact mount points. You strive for even io
across your disks and you may end up with indexes and data on the same devices.
