Expert Oracle Exadata - Parent Directory

CHAPTER 11 UNDERSTANDING EXADATA PERFORMANCE METRICScell index scansThis statistic is incremented every time a Smart Scan is started on a B*Tree or bitmap index segment.Note that in order to use Smart Scans on index segments, the index fast full scan execution plan rowsource operator must be used together with direct path reads. This statistic is updated at the start of aSmart Scan session, so if you monitor its value for a session that has been executing a long-runningquery for a while, then you might not see this statistic incrementing for your session.When running just a serial session with Smart Scan on a nonpartitioned index segment, this statisticwould be incremented by one. However, when running a Smart Scan on a partitioned index segment,then the cell index scans statistic would be incremented for each partition scanned using Smart Scan.This is because the Smart Scan is a segment-level decision made at runtime, for each segment (table,index or partition). Because the Smart Scan kicks in only when doing direct path reads to PGA, and thedirect path reads decision is made based on the scanned segment size (among other things), differentpartitions of the same table accessed may be scanned using different methods. You might find that somepartitions in your multipartition table or index are not scanned with Smart Scan/direct path reads, asOracle has decided to use buffered reads for them thanks to their smaller size. In this case, the cell indexscans statistic would not increment as much and you would see the cell multiblock physical read waitevent pop up at the table/index scan row source path in an ASH or SQL Monitoring report.cell IO uncompressed bytesThis statistic shows the uncompressed size of the data scanned in the cell. So if you scan through a 10GBcompressed segment, physical read total bytes statistic would increase by 10GB, but the cell I/Ouncompressed bytes may well increase by 10 GB if that’s the total uncompressed size of the data. Thisstatistic is incremented only when performing a Smart Scan compression offloading, not when you readthe compressed blocks directly to the database layer with block I/O.cell num fast response sessionsThis statistic shows how many times Oracle started the Smart Scan code but then chose not to set up theSmart Scan session immediately, but instead to do a few block I/O operations first in hope to findenough rows to satisfy the database session. This optimization is used for FIRST ROWS execution planoptions, either when using a FIRST_ROWS_x hint (or equivalent init.ora parameter) or a WHERE rownum