Expert Oracle Exadata - Parent Directory

CHAPTER 3 HYBRID COLUMNAR COMPRESSIONData Type Base Table Name Characteristics Compression RatiosFinancial Data REV_ACCRUAL Wide Table, Many NULLs, Many Low NDVColumns4×-10×Retail Sales Data SALES Skinny Table, Mostly Low NDV Columns 15×-32×Hopefully this data gives you some feel for the range of compression ratios that you can expect fromHCC and the types of datasets that will benefit most. Of course the best way to predict how compressiblea particular table may be is to actually test it. This fact cannot be overemphasized.Restrictions/ChallengesThere are a few challenges with using HCC. Many of them have to do with the fact that HCC is notavailable on non-Exadata platforms. This fact poses challenges for recovery and high availabilitysolutions. The other major challenge is that HCC doesn’t play well with data that is being activelyupdated. In particular, systems characterized by lots of single-row updates, which we often describe asOLTP workloads, will probably not work well with HCC.Moving Data to a non-Exadata PlatformProbably the largest hurdle with using HCC has been moving the data to non-Exadata platforms. Forexample, while RMAN and Dataguard both support the HCC block format, and will happily restore datato a non-Exadata environment, a database running on such an environment will not be able to doanything with the data until it is decompressed. This can mean a lengthy delay before being able toaccess the data in a case where a failover to a standby on a non-Exadata platform occurs. The same issueholds true for doing an RMAN restore to a non-Exadata platform. The restore will work but the data inHCC formatted blocks will not be accessible until the data has been moved into a non-HCC format. Thiscan be done with the ALTER TABLE MOVE NOCOMPRESS command, by the way.■ Note: The ability to decompress HCC data on non-Exadata platforms only became available in Oracle databaseversion 11.2.0.2. Attempting this on version 11.2.0.1 would result in an error. (Check with Oracle Support for apatch that may enable this behavior on 11.2.0.1)In addition to the lengthy delay associated with decompressing data before being able to access it,there is also the issue of space. If HCC is providing a 10× compression factor, you will need to have 10times the space you are currently using available on the target environment to handle the increased sizeof the data. For these reasons, Dataguard is rarely set up with a standby on a non-Exadata platform.Disabling Serial Direct Path ReadsAs we discussed in Chapter 2, Serial Direct Path Reads allow nonparallelized scan operations to use thedirect path read mechanism, which is a prerequisite for enabling the Smart Scan features of Exadata.100