If you do not specify fill_factor or reservepagegap, rebuilding an index requires additional space of approximately 256 pages, or less in the data segment.
The amount of log space required is larger than that required to drop the index and re-create it using create index, but it should be only a small fraction of the actual index size. The more additional free space is available, the better the index clustering will be.
Running reorg rebuild table_name updates the statistics for all leading index columns. However, running reorg rebuild table_name index_name does not automatically update the statistics. Instead, SAP ASE automatically updates index statistics when you run reorg rebuild index_name if the update includes a suffiecient change in data to affect its plan choice and performance.