database - Data compression in RDBMS like Oracle, MySQL etc -

i reading in-memory database incorporates feature data compression. using that, instead of storing first name, last name, father's name etc. values in column (which leads lot of data duplication , waste of disk storage), creates dictionary , attribute vector table each column, unique values stored in dictionary , corresponding attribute vector stored in original table.
clear advantage of approach lot of space removing overhead of data duplication.

i want know:

1. does rdbms oracle, mysql etc. implicitly follow approach when store data on disk? or when use these rdbms have implement same if want take advantage of same?
2. as know there no free lunch, understand trade-offs if developer implements above explained data compression approach? 1 can think of in order fetch data database, have make join between dictionary table , main table. isn't it?

please share thoughts , inputs.

this answer based on understanding of query. appears mixing 2 concepts : data normalisation , data storage optimisation.

data normalisation : process needs undertaken application developer. here pieces of data need stored repeatedly stored once , referenced using identifiers typically integers. way database consumes spaces as needed store repeating data once. common practice while storing string , variable length data database tables. in order retrieve data, application have perform joins between related tables. , process contributes directly application performance depending on manner in related tables designed.

data storage optimisation : handled rdbms itself. involves various steps maintaining b-tree structures hold data, compressing data before storage, managing free space within data files etc. different rdbms systems handle them in different ways (some of them patented , proprietary while others more general); when speaking of rdbms oracle , mysql can assured follow best in class storage algorithms efficiently store data.

hope helps.