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  • DBMS Part-5

    •  Query Optimization in DBMS 

     Query Optimization kya hota hai?

    Query Optimization ek process hai jisme DBMS (Database Management System) decide karta hai ki ek SQL query ko fastest aur sabse efficient way se kaise run kiya jaaye.

    Matlab: Jab aap koi SQL query likhte ho, toh use chalane ke multiple tareeke ho sakte hain. Query Optimizer unme se best aur sabse sasta (kam time aur kam resource lene wala) tareeka choose karta hai.

     Query Optimization ka Goal:

    Query ko is tareeke se execute karna ki:

    • Minimum time lage

    • Kam memory ya CPU use ho

    • Fast result mile

     Query Optimization zaruri kyu hai?

    Sochiye aapne query likhi:

    SELECT * FROM students WHERE age > 20;

    Ye query do tareeke se chal sakti hai:

    1. Poora table scan kare (har row check kare)

    2. Agar age pe index hai, toh sirf relevant rows direct pick kare

    Obviously, second option fast hoga – aur yahi kaam karta hai Query Optimizer.

     Real-Life Example (Analogy)

    Jaise aap Google Maps pe ek jagah search karte ho – wo aapko multiple routes dikhata hai, aur aap fastest route choose karte ho.

    DBMS bhi wahi karta hai – multiple query plans me se fastest choose karta hai.

     Query Optimization ke Steps

    Query optimize karne ke kuch main steps hote hain:

     Step 1: Parsing aur Translation

    • Pehle DBMS query ko check karta hai ki sahi hai ya nahi (syntax + meaning).

    • Fir usse convert karta hai internal format me – jaise query tree ya relational algebra expression.

    Example:

    SELECT name FROM students WHERE age > 20;

    Iska Relational Algebra expression:

    π_name (σ_age > 20 (students))

     Query Tree Diagram:

     π_name
       |
     σ_age > 20
       |
     students

     Step 2: Query Tree Optimization

    Ab system query tree ko optimize karta hai taaki fast ho jaye. Isse kehte hain logical optimization.

     Kuch Common Optimizations:

    • -> Select conditions ko neeche le jaate hain (predicate pushdown)

    • -> Duplicate operations hata dete hain

    • -> Index use karne ki koshish karte hain

     Step 3: Execution Plans Banana

    DBMS multiple execution plans banata hai – alag alag tareeko se:

    • Table scan

    • Index scan

    • Nested loop join

    • Merge join

    • Hash join

     Example:

    SELECT name
    FROM students
    WHERE age > 20 AND department = 'CSE';

    Do possible plans:

    1. Plan A: Full table scan → har row check karo

    2. Plan B: Index use karo age pe → fir filter karo department = 'CSE'

    Agar age indexed hai, toh Plan B fast hoga

     Step 4: Cost Estimation

    Har execution plan ka cost calculate kiya jaata hai:

    • CPU time

    • Disk I/O

    • Memory use

    • Kitne rows scan karni padengi

    DBMS apne statistics (jaise table size, rows count, index info) ka use karta hai cost nikalne ke liye.

     Step 5: Best Plan Select Karna aur Execute Karna

    Last step me optimizer sab plans me se lowest cost wale plan ko select karta hai aur execution engine us plan ko run karta hai.

     Query Optimization Flow – Diagram

         SQL Query
            ↓
     [Parsing aur Translation]
            ↓
     [Logical Query Plan]
            ↓
     [Query Optimization]
            ↓
     [Multiple Physical Plans]
            ↓
     [Cost Estimation]
            ↓
     [Best Plan Select]
            ↓
     [Query Execute hoti hai]

     Types of Query Optimization

    Type Description
    Heuristic Optimization Simple rules use karta hai (jaise selection pehle lagao). Fast hota hai but always best nahi hota.
    Cost-Based Optimization Sab plans ka cost calculate karta hai aur sabse cheapest plan choose karta hai. Accurate hota hai.

     Full Process Summary:

    • Query likhi gayi → DBMS usse samjhta hai

    • Query ka internal tree banata hai

    • Us tree ko optimize karta hai

    • Fir alag-alag execution plans banata hai

    • Har plan ka cost estimate karta hai

    • Jo plan sabse cheap (fast) ho, usse run karta hai

     Conclusion

    Query Optimization ek smart feature hai jo DBMS ko fast aur efficient banata hai. Ye large database me performance boost karta hai aur ensure karta hai ki queries jaldi execute ho.

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