5. Query Optimization
The world of relational database theory is a world dominated by tables and sets, and operations on tables and sets. A database is a set of tables, and a table is a set of rows and columns. When you issue a SELECT statement to retrieve rows from a table, you get back another set of rows and columns—that is, another table. These are abstract notions that make no reference to the underlying representation a database system uses to operate on the data in your tables. Another abstraction in set theory is that operations on tables happen all at once; queries are conceptualized as set operations for which there is no concept of time.
The real world is quite different. Database management systems implement abstract concepts but ...
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