If you have the parallel query option installed, you can parallelize index creation. Because multiple processes work together to create the index, Oracle can create the index more quickly than if a single server process created the index sequentially. When creating an index in parallel, storage parameters are used separately by each query server process. Oracle8 Server Tuning. The time it takes to create the index is decreased.
Performance improves for parallel creation of large indexes.
In general, the relative performance improvement is greater for larger unrecoverable indexes than for smaller ones. Creating small unrecoverable indexes has little affect on the time it takes to create an index. However, for larger indexes the performance improvement can be significant, especially when you are also parallelizing the index creation. From these estimates, you can make correct hardware purchases and other decisions. You can use the estimated size of an individual index to better manage the disk space that the index will use.