Expert Oracle Exadata - Parent Directory

CHAPTER 4 STORAGE INDEXESIn this example, a function was applied to a date column, which as you might expect, disables theStorage Index. The fact that applying a function to a column disables the Storage Index is not toosurprising, but application of the trunc function is a commonly seen coding technique. Many dates havea time component and many queries want data for a specific day. It is well known that truncating a datein this manner will disable normal B-Tree index usage. In the past, that generally didn’t matter. Queriesin many data warehouse environments were designed to do full scans anyway, so there was really noneed to worry about disabling an index. Storage Indexes change the game from this perspective and mayforce us to re-think some of our approaches. We’ll discuss this issue in more detail in Chapter 16.SummaryStorage Indexes are an optimization technique that is available when the database is able to utilizeSmart Table Scans. They can provide dramatic performance improvements. Storage indexes can bethought of as an alternate partitioning strategy, but without the normal restrictions associated withpartitioning. They are especially effective with queries that access data via an alternate key that tracksthe primary partition key.How the data is physically stored is an important consideration and has a dramatic impact on theeffectiveness of Storage Indexes. Care should be taken when migrating data to the Exadata platform toensure that the data is clustered on disk in a manner that will allow Storage Indexes to be usedeffectively.124