Sequential,index squential,random access......

2. SYLLABUS
• SEQUENTIAL
• INDEX SEQUENTIAL
• RANDOM ACCESS
• HASHING TECHNIQUES

3. SEQUENTIAL
The easiest method for file Organization is Sequential method. In this method the file are
stored one after another in a sequential manner. There are two ways to implement this
method:
1.Pile File Method – This method is quite simple, in which we store the records in a
sequence i.e one after other in the order in which they are inserted into the tables.

4. CONT….
• Sorted File Method –In this method, As the name itself suggest whenever a new record has to be inserted, it is always
inserted in a sorted (ascending or descending) manner. Sorting of records may be based on any primary key or any other key.

5. PROS AND CONS OF SEQUENTIAL FILE
ORGANIZATION –
• PROS –
• FAST AND EFFICIENT METHOD FOR HUGE AMOUNT OF DATA.
• SIMPLE DESIGN.
• FILES CAN BE EASILY STORED IN MAGNETIC TAPES I.E CHEAPER STORAGE MECHANISM.
• CONS –
• TIME WASTAGE AS WE CANNOT JUMP ON A PARTICULAR RECORD THAT IS REQUIRED, BUT WE HAVE TO MOVE
IN A SEQUENTIAL MANNER WHICH TAKES OUR TIME.
• SORTED FILE METHOD IS INEFFICIENT AS IT TAKES TIME AND SPACE FOR SORTING RECORDS.

6. INDEX SEQUENTIAL
• IN THIS SEARCHING METHOD, FIRST OF ALL, AN INDEX FILE IS CREATED, THAT
CONTAINS SOME SPECIFIC GROUP OR DIVISION OF REQUIRED RECORD WHEN
THE INDEX IS OBTAINED, THEN THE PARTIAL INDEXING TAKES LESS TIME
CAUSE IT IS LOCATED IN A SPECIFIED GROUP.
• NOTE: WHEN THE USER MAKES A REQUEST FOR SPECIFIC RECORDS IT WILL
FIND THAT INDEX GROUP FIRST WHERE THAT SPECIFIC RECORD IS
RECORDED.

7. CHARACTERISTICS OF INDEXED
SEQUENTIAL SEARCH:
• IN INDEXED SEQUENTIAL SEARCH A SORTED INDEX IS SET ASIDE IN ADDITION TO THE ARRAY.
• EACH ELEMENT IN THE INDEX POINTS TO A BLOCK OF ELEMENTS IN THE ARRAY OR ANOTHER EXPANDED
INDEX.
• THE INDEX IS SEARCHED 1ST THEN THE ARRAY AND GUIDES THE SEARCH IN THE ARRAY.
• NOTE: INDEXED SEQUENTIAL SEARCH ACTUALLY DOES THE INDEXING MULTIPLE TIME, LIKE CREATING THE
INDEX OF AN INDEX.

8. Explanation by diagram “Indexed
Sequential Search”:

9. RANDOM ACCESS
• DIRECT ACCESS FILE IS ALSO KNOWN AS RANDOM ACCESS OR RELATIVE FILE
ORGANIZATION.
• IN DIRECT ACCESS FILE, ALL RECORDS ARE STORED IN DIRECT ACCESS STORAGE
DEVICE (DASD), SUCH AS HARD DISK. THE RECORDS ARE RANDOMLY PLACED
THROUGHOUT THE FILE.
• THE RECORDS DOES NOT NEED TO BE IN SEQUENCE BECAUSE THEY ARE UPDATED
DIRECTLY AND REWRITTEN BACK IN THE SAME LOCATION.
• THIS FILE ORGANIZATION IS USEFUL FOR IMMEDIATE ACCESS TO LARGE AMOUNT OF
INFORMATION. IT IS USED IN ACCESSING LARGE DATABASES.
• IT IS ALSO CALLED AS HASHING

10. ADVANTAGES OF DIRECT ACCESS FILE
ORGANIZATION
• DIRECT ACCESS FILE HELPS IN ONLINE TRANSACTION PROCESSING SYSTEM
(OLTP) LIKE ONLINE RAILWAY RESERVATION SYSTEM.
• IN DIRECT ACCESS FILE, SORTING OF THE RECORDS ARE NOT REQUIRED.
• IT ACCESSES THE DESIRED RECORDS IMMEDIATELY.
• IT UPDATES SEVERAL FILES QUICKLY.
• IT HAS BETTER CONTROL OVER RECORD ALLOCATION.
• DISADVANTAGES OF DIRECT ACCESS FILE ORGANIZATION
DIRECT ACCESS FILE DOES NOT PROVIDE BACK UP FACILITY.
• IT IS EXPENSIVE.
• IT HAS LESS STORAGE SPACE AS COMPARED TO SEQUENTIAL FILE.

11. HASHING TECHNIQUES
• IN DATABASE MANAGEMENT SYSTEM, WHEN WE WANT TO RETRIEVE A
PARTICULAR DATA, IT BECOMES VERY INEFFICIENT TO SEARCH ALL THE INDEX
VALUES AND REACH THE DESIRED DATA. IN THIS SITUATION, HASHING
TECHNIQUE COMES INTO PICTURE.
HASHING IS AN EFFICIENT TECHNIQUE TO DIRECTLY SEARCH THE LOCATION
OF DESIRED DATA ON THE DISK WITHOUT USING INDEX STRUCTURE. DATA IS
STORED AT THE DATA BLOCKS WHOSE ADDRESS IS GENERATED BY USING
HASH FUNCTION. THE MEMORY LOCATION WHERE THESE RECORDS ARE
STORED IS CALLED AS DATA BLOCK OR DATA BUCKET.

12. HASH FILE ORGANIZATION :
• DATA BUCKET – DATA BUCKETS ARE THE MEMORY LOCATIONS WHERE THE RECORDS ARE
STORED. THESE BUCKETS ARE ALSO CONSIDERED AS UNIT OF STORAGE.
• HASH FUNCTION – HASH FUNCTION IS A MAPPING FUNCTION THAT MAPS ALL THE SET OF
SEARCH KEYS TO ACTUAL RECORD ADDRESS. GENERALLY, HASH FUNCTION USES PRIMARY
KEY TO GENERATE THE HASH INDEX – ADDRESS OF THE DATA BLOCK. HASH FUNCTION CAN
BE SIMPLE MATHEMATICAL FUNCTION TO ANY COMPLEX MATHEMATICAL FUNCTION.
• HASH INDEX-THE PREFIX OF AN ENTIRE HASH VALUE IS TAKEN AS A HASH INDEX. EVERY
HASH INDEX HAS A DEPTH VALUE TO SIGNIFY HOW MANY BITS ARE USED FOR COMPUTING A
HASH FUNCTION. THESE BITS CAN ADDRESS 2N BUCKETS. WHEN ALL THESE BITS ARE
CONSUMED ? THEN THE DEPTH VALUE IS INCREASED LINEARLY AND TWICE THE BUCKETS
ARE ALLOCATED.

13. BELOW GIVEN DIAGRAM CLEARLY DEPICTS
HOW HASH FUNCTION WORK:

14. HASHING IS FURTHER DIVIDED INTO TWO
SUB CATEGORIES :

15. THANK YOU
• THANK YOU