+ Indexing and{" "} + Hashing are techniques used to + speed up data retrieval in large databases without scanning every row. +
+ +CREATE INDEX idx_name ON table(column);Dense index is faster to search but uses more space. Sparse index saves space but requires the file to be sorted.
+B+ Tree Structure
+{`Internal nodes: [10 | 20 | 30]
+ / | | \\
+Leaf nodes: [5,8] [12,15] [22,25] [35,40]
+ ↔ ↔ ↔ ↔ (linked)`}
+ Use indexing (B+ Tree) for range queries. Use hashing for fast exact lookups.
++ Query Processing is how the DBMS + takes a SQL query, understands it, and executes it efficiently.{" "} + Query Optimization selects the + most efficient execution plan. +
+ +Query Processing Pipeline
+{`SQL Query
+ ↓ Parser
+Parse Tree
+ ↓ Translator
+Relational Algebra Expression
+ ↓ Optimizer
+Execution Plan
+ ↓ Evaluator
+Query Result`}
+ Hash Join is generally the fastest for large datasets. Merge Join is best when data is already sorted.
+Heuristic Example
+{`-- Before optimization (filter happens after join):
+Students ⋈ Enrollments WHERE Students.dept = 'CSE'
+
+-- After optimization (filter before join):
+σ(dept='CSE')(Students) ⋈ Enrollments`}
+ + Recovery restores the database to + a consistent state after a failure.{" "} + Security ensures only authorized + users can access or modify data. +
+ +WAL is the golden rule of recovery — always log before you change.
+Recovery After Crash
+{`Checkpoint at T=10
+T1 committed at T=8 → already safe, skip
+T2 committed at T=12 → redo (may not be on disk)
+T3 active at crash → undo (was never committed)`}
+ GRANT SELECT ON students TO user1;REVOKE SELECT ON students FROM user1;SQL Injection Example
+{`-- Vulnerable query:
+"SELECT * FROM users WHERE name = '" + input + "'"
+
+-- Attacker enters: ' OR '1'='1
+-- Resulting query (returns all users!):
+SELECT * FROM users WHERE name = '' OR '1'='1'
+
+-- Safe fix (prepared statement):
+SELECT * FROM users WHERE name = ?`}
+