Introduction to Database Systems1

2 Basic Definitions Mini-world Some part of the real world about which data is stored in a database. Data Known facts that can be recorded and have an implicit meaning. Database A collection of related data. Database Management System (DBMS) A software package/system to facilitate the creation and maintenance of a computerized database. Database System The DBMS software together with the data itself. Sometimes, the applications are also included.

Introduction to Database Systems3 Data Models A data model is a collection of concepts for describing data. A schema is a description of a particular collection of data, using a given data model. The relational data model is the most widely used model today. Main concept: relation, basically a table with rows and columns. Every relation has a schema, which describes the columns, or fields.

Introduction to Database Systems4 Levels of Abstraction Many views, single conceptual (logical) schema and physical schema. Views describe how users see the data. Conceptual schema defines logical structure Physical schema describes the files and indexes used. View 1View 3View 2 Conceptual Schema Physical Schema Database

Introduction to Database Systems5 Example: University Database Conceptual schema Students(sid: string, name: string, login: string, age: integer, gpa: real) Courses(cid: string, cname: string, credits: integer) Enrolled(sid: string, cid: string, grade: string) Physical schema Relations stored as unordered files. Index on first column of Students. External Schema (View) Course_info(cid: string, enrollment: integer)

Introduction to Database Systems6 File Systems vs. DBMS Large main memory Application must stage large datasets between main memory and secondary storage (e.g., buffering, page- oriented access, 32-bit addressing of 4 GB, etc.) Query processing Special code for different queries Concurrency control Must protect data from inconsistency due to multiple concurrent users Crash recovery Security and access control

Introduction to Database Systems7 Advantages of the Database Approach Data independence Applications insulated from how data is structured and stored. Logical data independence: Protection from changes in logical structure of data. Physical data independence: Protection from changes in physical structure of data. Efficient data access Data integrity and security Uniform data administration Concurrent access and crash recovery Reduced application development time

Introduction to Database Systems8 When not to use a DBMS Main costs of using a DBMS High initial investment and possible need for additional hardware. Overhead for providing generality, security, concurrency control, recovery, and integrity functions. When a DBMS may be unnecessary If the database and applications are simple, well defined, and not expected to change. If there are stringent real-time requirements that may not be met. If access to data by multiple users is not required. When no DBMS may suffice Limitation of its modeling capability. Special operations not supported by the DBMS.

Introduction to Database Systems9 Query Languages Query languages: Allow manipulation and retrieval of data from a database. Relational model supports simple, powerful query languages To specify "what" instead of "how" A query is applied to relation instances, and the result of a query is also a relation instance. Several ways of expressing a given query; a query optimizer should choose the most efficient version.

Introduction to Database Systems10 Concurrency Control Concurrent execution of user programs is essential for good DBMS performance. Disk accesses are frequent and relatively slow. Interleaving actions of different user programs can lead to inconsistency Check is cleared while account balance is being computed. DBMS ensures such problems don’t arise Users can pretend they are using a single-user system.

Introduction to Database Systems11 Transaction A transaction is a sequence of database actions (reads/writes). Desirable Properties: ACID Atomicity: A transaction is an atomic unit of processing; it is either performed in its entirety or not performed at all. Consistency preservation: A correct execution of the transaction must take the database from one consistent state to another. Isolation: A transaction should not make its updates visible to other transactions until it is committed. Durability or permanency: Changes made by a committed transaction must never be lost because of subsequent failure.

Introduction to Database Systems12 Scheduling Concurrent Transactions DBMS ensures that execution of {T1,..., Tn} is equivalent to some serial execution T1’... Tn’. Before reading/writing an object, a transaction requests a lock on the object, and waits till the DBMS gives it the lock. Idea: If an action of Ti (say, writing X) affects Tj (which perhaps reads X), one of them, say Ti, will obtain the lock on X first and Tj is forced to wait until Ti completes; this effectively orders the transactions. What if Tj already has a lock on Y and Ti later requests a lock on Y? (Deadlock!)

Introduction to Database Systems13 Ensuring Atomicity DBMS ensures atomicity (all-or-nothing property) even if system crashes in the middle of a transaction. Idea: Keep a log (history) of all actions carried out by the DBMS while executing a set of transactions: Before a change is made to the database, the corresponding log entry is forced to a safe location (Write-Ahead Log or WAL protocol). After a crash, the effects of partially executed transactions are undone using the log.

Introduction to Database Systems14 The Log Actions to be recorded in the log Ti writes an object: The old value and the new value. Ti commits/aborts: A log record indicating this action. Log records chained together by transaction id, so it’s easy to undo a specific transaction. Periodic checkpointing can reduce the time needed to recover from a crash. All log related activities (and in fact, all concurrency control related activities such as lock/unlock, dealing with deadlocks etc.) are handled transparently by the DBMS.

Introduction to Database Systems15 Structure of a DBMS Database Files and Access Methods Buffer Management Disk Space Management Concurrency Control Recovery Manager Query Optimizer Operator Evaluator Queries