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DBMS → Normalization 1

Fix for EmpID→DeptID→DeptName (3NF violation) is
Create Dept(DeptID,DeptName) and Emp(EmpID,DeptID)
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DBMS → Introduction to DBMS 29

A relation with ONLY one attribute is always in
BCNF
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Isolation ensures
Concurrent transactions do not interfere with each other
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Dirty read occurs when
T1 reads data written by uncommitted T2 (invalid if T2 rolls back)
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Non-repeatable read occurs when
Same row read twice gives different values because another transaction modified it between reads
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Phantom read occurs when
New rows APPEAR in repeated range query inserted by another transaction
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2PL growing phase
Only acquires locks (no releasing allowed)
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2PL shrinking phase
Only releases locks (no new acquisitions allowed)
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2PL guarantees
Conflict serializability
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Strict 2PL holds exclusive locks until
Transaction COMMITS or ABORTS
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Two operations conflict when
Same data item, different transactions, at least one is a WRITE
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Wait-Die scheme: T_i requests lock held by T_j
Older T_i waits; younger T_i aborts (dies)
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Wound-Wait scheme: T_i requests lock held by T_j
Older T_i wounds (aborts) T_j; younger T_i waits
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MVCC keeps multiple versions to
Allow readers and writers to proceed without blocking each other
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READ UNCOMMITTED isolation
Allows ALL anomalies (dirty, non-repeatable, phantom) - lowest isolation, highest concurrency
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READ COMMITTED isolation
Prevents dirty reads but allows non-repeatable and phantom reads
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REPEATABLE READ isolation
Prevents dirty and non-repeatable reads but may allow phantom reads
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Shared (S) lock allows
Multiple concurrent readers no writers
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Exclusive (X) lock allows
ONE transaction to read or write blocking all others
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Write-Ahead Logging (WAL) requires
Log written to stable storage BEFORE data page written to disk
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Redo in recovery
Re-applies operations of committed transactions to ensure durability after crash
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Undo in recovery
Reverses operations of uncommitted transactions to restore consistency
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ARIES recovery passes in order
Analysis, Redo, Undo
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Checkpoint in recovery is used to
Reduce redo work needed after crash
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No-Force policy means
Pages need NOT be forced to disk at commit - requires REDO on recovery
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Steal policy means
Dirty pages of uncommitted transactions CAN be written to disk - requires UNDO on recovery
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Optimistic CC performs best when
Conflicts are RARE - mostly reads few conflicts
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Multiple granularity locking allows
Different hierarchy levels: database, table, page, row
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IS (Intention-Shared) lock signals
Intent to set S locks at FINER granularity levels
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IX (Intention-Exclusive) lock signals
Intent to set X locks at FINER granularity levels
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DBMS → Transactions 6

ACID stands for
Atomicity,Consistency,Isolation,Durability
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Atomicity ensures
Transaction happens in entirety or not at all
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Consistency ensures
DB transitions from one valid state to another maintaining all integrity constraints
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Durability ensures
Once committed changes persist permanently even after system failures
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Cascading rollback occurs when
Rolling back T1 forces other transactions that read T1's dirty data to also roll back
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Timestamp-based CC assigns each transaction
Unique timestamp at start time for ordering operations
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DBMS → Concurrency Control 2

A schedule is conflict serializable if
Its precedence graph is ACYCLIC (no cycles)
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SERIALIZABLE isolation
Prevents ALL anomalies - highest isolation, lowest concurrency
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DBMS → Deadlock 2

Deadlock occurs when
Two or more transactions wait indefinitely for each other to release locks (circular wait)
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Wait-for graph detects deadlock when
A CYCLE is detected in the graph
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