英语翻译This model is more flexible than the hierarchical or net

英语翻译
This model is more flexible than the hierarchical or network model.In a hierarchical or network database,record definitions are fixed in order to embed linked list data pointers in the records.Since the relational model uses keyed field to relate records,record definitions may be easily changed and new records created.However,this flexibility comes with an integrity and performance price.To ensure integrity,user-defined integrity constraints must be established and checked before processing transactions.In the hierarchical or network model,the constraints are automatically established when the sets are implemented,while in the relational model,the constraints are user-defined in the application program.Since it is so easy to get the system up and running,the design of the user-integrity constraints may not be well thought out in advance and as the database grows in size and complexity,more tables are often added to the system,and therefore more joins are needed to access information in the database.(A join is one of the set operators of relational algebra used to concatenate data found in different tables.) The more joins needed to process a query,the slower the system responds.Another disadvantage for some information management systems based on the relational model is that complex data cannot be adequately described in simple rows and columns.In the LIMS environment,using the relational model to describe complex data,it is not uncommon to do 10 or more record joins to process a single information request.Despite its disadvantages,this model requires less initial planning and design to implement and is the quickest and easiest of the models to set up.It soon became the most recognized and widely accepted database model for microcomputers and minicomputers.The relational model became the panacea for every information storage problem.Whenever changes or additions to a database were needed,new tables were added or existing tables extended,often without determining the performance impact on the as information systems system.Consequently,changed over time,the performance often degraded dramatically.