Re: Add/Drop partition and CBO statistics

Reply in lines.

Zhu Chao
----- Original Message -----
To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Wednesday, December 03, 2003 7:29 PM

> Agreed that scanning one big index is faster than many partitions.
>
> Then raises the questions - I thought partitioning is for:
> 1) ease of archiving/dropping off old partitions - drop old and create new
> partitions in a sliding window. A single large global index negates a lot of
> this ease - even though it is true that deletes on non-partitioned tables
> would be even more inconvenient.

    Yes, that is why we use local index.not Global indexes.

> 2) efficiencies in partition pruning for queries. If you are "querying whole
> table" - why bother with partitioning?
> The point then becomes you don't need to partition in the first place, or
> your partitioning scheme is not appropriate?

    Sometimes it is constrained by complex real applications. A table has tens of columns and you can only partition by one key(or several columns), and to use partition elimination, the SQL must contains the partition key. So only these limited SQL can use partition pruning. While in complex real life application, there will always SQL with different where clause that do not use the partition key at all.     The other constraint is business logic.We should partition according to product online time, but we have ten tables to archive and only one table(products) has that key, all other tables do not have that column. Adding such a column to other tables need considrable application rewrite and is denied. So we use product_id(the primary key of most tables) as the partition key.      

> > When partitioning key is not a part of the index and you are querying
> whole table, then it is faster to scan one big index than many smaller ones.
> The difference is something like log rows to partcount*log (rows/partcount).
> >
> > > BTW, local indexes are the only way to go -- I've never
> > > understood the point
> > > of global indexes on partitioned tables -- maybe someone else can?

    Global indexes are faster than local index, so if you have schedule down time and need better performance, go to global index. OLTP is more suitable for GLobal index, tomas kyte said in his expert one on one.

Regards
Zhu Chao.

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Author: zhu chao
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