Expert Oracle Exadata - Parent Directory

CHAPTER 4 STORAGE INDEXES• _cell_storage_index_partial_rd_sectors=512• _cell_storage_index_partial_reads_threshold_percent=85• _cell_storage_index_sizing_factor=2You’ve already seen the tracing parameters (_CELL_STORAGE_INDEX_DIAG_MODE and_CELL_SI_MAX_NUM_DIAG_MODE_DUMPS) in the section “Monitoring Storage Indexes.” These two parametersare the most useful in our opinion, although you should get the idea from the list that there is also somebuilt in ability to modify behaviors such as the amount of memory to allocate for storage indexes and thenumber of columns that can be indexed per table.BehaviorThere is not a lot you can do to control when Storage Indexes are used and when they are not. Otherthan the parameter for disabling them, there is little you can do. There is no specific hint to enable ordisable their use. And unfortunately, the OPT_PARAM hint does not work with the_KCFIS_STORAGEIDX_DISABLED parameter, either. The fact that there is no way to force the use of a StorageIndex makes it even more important to understand when this powerful optimization will and will not beused.In order for a storage index to be used, a query must include or make use of all the following:Smart Scan: Storage Indexes can only be used with statements that do SmartScans. This comes with a whole set of requirements, as detailed in Chapter 2.The main requirements are that the optimizer must choose a full scan and thatthe I/O must be done via the direct path read mechanism.At Least One Predicate: In order for a statement to use a Storage Index, theremust be a WHERE clause with at least one predicate.Simple Comparison Operators: Storage Indexes can be used with the followingset of operators:=, , BETWEEN, >=,