Expert Oracle Exadata - Parent Directory

CHAPTER 3 HYBRID COLUMNAR COMPRESSIONEstimated Compression Ratio using "Compress For Archive Low": 43.5Elapsed: 00:01:01.70SYS@SANDBOX1> @get_compression_ratio.sqlEnter Value for owner: KSOEnter Value for table_name: SKEW3Enter Value for compression_type (OLTP): ARCHIVE HIGHCompression Advisor self-check validation successful. select count(*) on both Uncompressed andEHCC Compressed format = 1000001 rowsEstimated Compression Ratio using "Compress For Archive High": 54.7Elapsed: 00:01:18.09Notice that the procedure prints out a validation message telling you how many records were usedfor the comparison. This number can be modified as part of the call to the procedure if so desired. Theget_compression_ratio.sql script prompts for a table and a Compression Type and then executes theDBMS_COMPRESSION.GET_COMPRESSION_RATIO procedure. Once again the pertinent data is a little hard topick out of the listing, so Table 3-5 compares the actual compression ratios to the estimates provided bythe Compression Advisor.Table 3-5. Compression Advisor AccuracyTable NameCompress ForActualCompressionRatioEstimatedCompressionRatioError PercentageSKEW3_OLTP OLTP 1.4 1.4 0%SKEW3_HCC1 QUERY LOW 4.0 4.0 0%SKEW3_HCC2 QUERY HIGH 45.6 42.4 7%SKEW3_HCC3 ARCHIVE LOW 45.6 43.5 5%SKEW3_HCC4 ARCHIVE HIGH 55.9 54.7 2%The estimates are fairly close to the actual values and while they are not 100 percent accurate, thetradeoff is probably worth it in cases where the objects are very large and a full test would be too timeconsumingor take up too much disk space. The ability to run the Compression Advisor on non-Exadataplatforms is also a real plus. It should be able to provide you with enough information to makereasonable decisions prior to actually migrating data to Exadata.Real World ExamplesAs Yogi Berra once said, you can learn a lot just by watching. Marketing slides and book author claimsare one thing, but real data is often more useful. Just to give you an idea of what kind of compression isreasonable to expect, here are a few comparisons of data from different industries. The data should94