Lecture 3. Early approaches to the organization of the database. Systems based on inverted lists, hierarchical and network DBMS. Examples Strengths and weaknesses of early systems

Before moving on to a detailed and consistent study of relational database systems, we’ll briefly dwell on the early (pre-relational) DBMS. This makes sense for three reasons: first, these systems historically preceded relational ones, and in order to properly understand the reasons for the ubiquitous transition to relational systems, you need to know at least something about their predecessors; secondly, the internal organization of relational systems is largely based on the use of methods of early systems; thirdly, some knowledge in the field of early systems will be useful for understanding the development of post-relational DBMS.

Note that in this lecture we limit ourselves to the consideration of only general approaches to the organization of three types of early systems, namely, systems based on inverted lists, hierarchical and network database management systems. We will not touch on the features of any particular systems; this would result in many technical details that, while interesting, are somewhat apart from the main goal of our course. Details can be found in the recommended literature.

Let's start with some of the most common characteristics of early systems:

1. These systems have been actively used for many years, longer than any of the relational database management systems. In fact, some of the early systems are used even in our time, huge databases have been accumulated, and one of the actual problems of information systems is the use of these systems in conjunction with modern systems.
2. All early systems were not based on any abstract models. As we mentioned, the concept of a data model actually came into use in the field of database specialists only with the relational approach. Abstract representations of early systems appeared later on the basis of the analysis and identification of common features in various specific systems.
3. In early systems, access to the database was made at the record level. The users of these systems carried out explicit navigation in the database using programming languages ​​extended by the functions of the DBMS. Interactive access to the database was supported only by creating appropriate application programs with their own interface.
4. We can assume that the level of early DBMS tools corresponds with the level of file systems in much the same way as the level of the Cobol language corresponds with the level of the Assembly language. Note that in this view, the level of relational systems corresponds to the level of Hell or APL.
5. The navigation nature of early systems and data access at the record level forced the user to perform the entire database access optimization without any system support.
6. After the advent of relational systems, most of the early systems were equipped with "relational" interfaces. However, in most cases this did not make them truly relational systems, since it remained possible to manipulate the data in the natural way for them.