Expert Oracle Exadata - Parent Directory

CHAPTER 3 HYBRID COLUMNAR COMPRESSIONsum 1,460.31 row selected.Elapsed: 00:00:00.02SYS@SANDBOX1> -- It says ARCHIVE HIGH, but obviously the data has not been compressed.SYS@SANDBOX1>SYS@SANDBOX1> alter table kso.skew move compress for query low parallel 32;Table altered.Elapsed: 00:00:09.60SYS@SANDBOX1> @table_size2Enter value for owner: KSOEnter value for table_name: SKEWOWNER SEGMENT_NAME TOTALSIZE_MEGS COMPRESS_FOR-------------------- ------------------------------ -------------- ------------KSO SKEW 301.3 QUERY LOW--------------sum 301.3Elapsed: 00:00:00.02You could probably guess that the first ALTER command (without the MOVE keyword) didn’t really doanything to the existing data. It only took a few hundredths of a second, after all. And as you can seewhen we looked at the size of the table, it did not compress the existing data, even though the datadictionary now says the table is in ARCHIVE HIGH mode. When we added the MOVE keyword, the table wasactually rebuilt with the new compression setting using direct path inserts and as you can see, the datahas been compressed from the original 1,406MB to 301MB.In this section we briefly described each of the three types of compression available in Oracle. Sincethis chapter is focused on HCC, we won’t discuss the other methods further except as they relate to howHCC works.HCC MechanicsHCC works by storing data in a nontraditional format—nontraditional for Oracle, anyway. Data storedusing HCC still resides in Oracle blocks, and each block still has a block header. But the data storage hasbeen reorganized. In the first place, the blocks are combined into logical structures called compressionunits, or CUs. A CU consists of multiple Oracle blocks (usually adding up to 32K or 64K). Figure 3-5shows a logical representation of how CUs are laid out.72