Expert Oracle Exadata - Parent Directory

CHAPTER 4 STORAGE INDEXES• For each column evaluated there is an id field that correlates to its position in thetable.• For each column evaluated there is a flg field. It appears that it is the decimalrepresentation of a bit mask. It also appears that the first bit indicates whethernulls are contained in the current column of the storage region. (That is, 1 and 3both indicate that nulls are present.)• For each column evaluated there is a lo and a hi value (stored as hex).• The lo and hi values are only 8 bytes, indicating that the Storage Indexes will beineffective on columns where the leading portion of the values are not distinct(empirical evidence bears this out, by the way).While generating and reading trace files is very informative, it is not very easy to do and requiresdirect access to the storage servers. On top of that, the approach is completely undocumented. It isprobably best used for investigations in nonproduction environments.Monitoring Wrap UpNeither the database statistic nor the tracing is a particularly satisfying way of monitoring Storage Indexusage. It would be nice to be able to track Storage Index usage at the statement level, via a column inV$SQL for example. In the meantime, the cell physical IO bytes saved by storage index statistic isthe best option we have.Controlling Storage IndexesThere is not much you can do to control Storage Index behavior. However, the developers have built in afew hidden parameters that provide some flexibility.There are three database parameters that deal with Storage Indexes (that we’re aware of):• _kcfis_storageidx_disabled (default is FALSE)• _kcfis_storageidx_diag_mode (default is 0)• _cell_storidx_mode (default is EVA)None of these parameters are documented, so you need to be careful with the methods we discussin this section. Nevertheless, we’ll tell you a little bit about some of these parameters and what they cando._kcfis_storageidx_disabledThe _kcfis_storageidx_disabled parameter allows Storage Indexes to be disabled. As with all hiddenparameters, it’s best to check with Oracle support before setting it, but as hidden parameters go, this oneis relatively innocuous. We have used it extensively in testing and have not experienced any negativeconsequences.You can set the parameter at the session level with the alter session statement:alter session set "_kcfis_storageidx_disabled"=true;Note that although setting _kcfis_storageidx_disabled to TRUE disables Storage Indexes for reads,the setting does not disable the maintenance of existing Storage Indexes. That is to say that existing111