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Oracle10g Grid Computing with RAC
Chapter 12 - RAC Design Considerations

Proper Sequence Usage

If not used properly, sequences can be a major headache in RAC. Generally speaking, sequences should be either CACHED or ORDERED, but not both. The preferred sequence is a CACHED, non-ordered sequence. If the ordering of sequences is forced, performance in a RAC environment will suffer unless ordering the sequence to a single node in the RAC cluster isolates insert activity.

Another method to optimize the use of sequences is to use a staggered sequence insert trigger. A staggered sequence insert trigger is a specific constant added to the sequence value based on the instance number. This isolates each set of inserts and prevents inadvertent attempts to use the same sequence number. An example of a staggered sequence insert trigger is shown in Figure 13.5. A staggered sequence trigger will insert the values into indexes such that each instance’s values are staggered to prevent index node intra-node transfers. The formula to allow this is:

index key = (instance_number -1)* 100000+ Sequence number



For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.

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