Expert Oracle Exadata - Parent Directory

CHAPTER 12 MONITORING EXADATA PERFORMANCEan already running query is easy with Oracle 11.2.0.2 and later, because you no longer must resort to theORADEBUG ERRORSTACK command.The Time & Wait Statistics section in Figure 12-6 shows the familiar metrics of Duration andDatabase Time of a statement. Move your mouse over the different database time components to seehow much time was spent waiting inside the database compared to running on CPU. The Wait Activity %bar shows the breakdown of wait events. Note that the 100% in this bar means 100% of the wait timecomponent of database time, not of the entire database time (which also includes CPU time).Figure 12-6. Time and Wait Statistics in the SQL Monitoring detail pageThe IO Statistics section in Figure 12-7a shows some key I/O statistics for statement execution.Figure 12-7a. I/O Statistics in the SQL Monitoring detail pageThe Buffer Gets bar shows the total number of logical I/Os done in both the database layer and thecells (if Smart Scan kicked in). The IO Requests and IO Bytes statistics are self-explanatory, but note thatthose metrics show all I/O done for the given statement, which includes the Smart Scan I/O, any regularblock I/O, and also TEMP tablespace I/O (done for sorting, hash joins, and for any other work areaoperations that didn’t fit into allowed PGA memory). Now you should begin to understand why the CellOffload Efficiency is negative for the statement execution in the figure. This metric should ideally showthe percentage of disk-to-database-host interconnect traffic that was avoided thanks to the Smart Scansperforming filtering and projection early in the cells and returning only a subset of data. However, in acomplex execution plan there is much more going on than just a Smart Scan against a single table. Youhave joins, aggregate operations, sorting, and direct path data loads, which all use extra I/O bandwidth,driving the offload efficiency percentage down. This is a good example of why focusing on only a singleratio is not a good idea. The single percentage value for offload efficiency hides a lot of information, such386