Expert Oracle Exadata - Parent Directory

CHAPTER 3 HYBRID COLUMNAR COMPRESSIONCU total length: 21406CU flags: NC-U-CRD-OPncols: 5nrows: 32759algo: 0CU decomp length: 17269 len/value length: 1040997row pieces per row: 1num deleted rows: 1deleted rows: 0,START_CU:00 00 1f 44 1f 02 00 00 00 02 00 00 1f 26 02 17 cb d4 00 00 00 00 3e 76 0217 cb d5 00 00 00 4b 44 5a 30 82 95 52 f0 00 00 53 9e eb 06 00 05 7f f7 00Notice that this block shows that it is compressed at level 3 (ARCHIVE LOW). Also notice that onerecord has been deleted from this block (migrated would be a more accurate term, as this is the recordthat we updated earlier). The line that says deleted rows: actually shows a list of the rows that have beenmigrated. Remember that we updated the record with rowid 7.3171084.0, meaning file 7, block 3171084,slot 0. So this line tells us that the one deleted row was in slot 0.■ Kevin Says: I like to remind people to be mindful of the potential performance irregularities that may occurshould a table comprised of EHCC data metamorphose into a table that is of mixed compression types withindirection. There should be no expected loss in functionality; however, there is an expected, yet unpredictable,impact to the repeatability of scan performance. The unpredictable nature is due to the fact that it is completelydata-dependent.Expected Compression RatiosHCC can provide very impressive compression ratios. The marketing material has claimed 10×compression ratios and believe it or not, this is actually a very achievable number for many datasets. Ofcourse the amount of compression depends heavily on the data and which of the four algorithms isapplied. The best way to determine what kind of compression can be achieved on your dataset is to testit. Oracle also provides a utility (often referred to as the Compression Advisor) to compress a sample ofdata from a table in order to calculate an estimated compression ratio. This utility can even be used onnon-Exadata platforms as long as they are running 11.2.0.2 or later. This section will provide someinsight into the Compression Advisor and provide compression ratios on some sample real worlddatasets.Compression AdvisorIf you don’t have access to an Exadata but still want to test the effectiveness of HCC, you can use theCompression Advisor functionality that is provided in the DBMS_COMPRESSION package. TheGET_COMPRESSION_RATIO procedure actually enables you to compress a sample of rows from a specifiedtable. This is not an estimate of how much compression might happen; the sample rows are insertedinto a temporary table. Then a compressed version of that temporary table is created. The ratio returnedis a comparison between the sizes of the compressed version and the uncompressed version. As of91