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Instead of talking about storage efficiency, other database vendors talk about compression and compression ratios. Compression ratio is a good measure to provide a relative comparison of database size before and after compression. However, it can be often misleading to compare storage efficiency of different database products solely based on their compression ratios. This is due to the fact that databases have different internal structures for storing data and the original size for the same data can vary significantly across different products. It's the absolute size of the compressed database, and not the ratio alone, that ultimately determines the cost savings and other associated benefits.
In current discussions about compression techniques, "compression" is always understood as something that needs administrator intervention: administrators need to decide, whether or not – and if yes, for which objects – to use it, and then they need to implement it. If we accept this definition, the main difference between storage efficiency and compression consists in the fact that a database system may or may not offer technologies which could be called "implicit compression": technologies which make sure that the least possible amount of disk space is used to store data, but which are switched on by default and therefore simply do their job.
Oracle Database comes with the most comprehensive set of "implicit compression" features, and every new version adds new features. As a consequence, SAP customers find that Oracle needs up to 40% less disk space for the same data than other databases even without (explicit) compression.
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