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GK One-Line Question & Answer

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DBMS → Introduction to DBMS 24

Table-valued function returns
A result set (table) usable in FROM clause
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Which constraint is validated at statement execution time by default
NOT DEFERRABLE (default)
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DEFERRABLE INITIALLY DEFERRED constraint
Is checked at end of transaction (COMMIT time) by default
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Which SQL creates a partial/filtered index
CREATE INDEX idx ON emp(salary) WHERE dept='IT'
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Composite index (A,B,C): which query CAN use this index
WHERE A=1 AND B=5 (leading columns)
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Index fragmentation above 30% should be addressed by
REBUILD INDEX (drops and recreates, eliminates all fragmentation)
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Which SQL checks index fragmentation in SQL Server
sys.dm_db_index_physical_stats
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UPDATE STATISTICS command purpose
Refresh the statistical metadata the optimizer uses for query planning
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Which SQL best finds all tables in current database
SHOW TABLES or SELECT * FROM information_schema.TABLES
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INFORMATION_SCHEMA is
A system database providing metadata about all database objects
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Which SQL lists all columns of the questions table
SHOW COLUMNS FROM questions OR DESCRIBE questions
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EXPLAIN SELECT * FROM questions WHERE q_level=3 shows
The execution plan: whether index is used, rows examined, join type
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Which helps identify slow queries in MySQL
EXPLAIN and slow_query_log
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Which lock type is compatible with another shared lock
Shared (S) lock
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X (exclusive) lock is compatible with
No other lock type
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Which CC protocol allows both readers and writers without blocking each other
MVCC (Multi-Version Concurrency Control)
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Optimistic CC three phases are
Read, Validate, Write (commit if valid; else restart)
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3PC (Three-Phase Commit) adds which phase to resolve 2PC blocking
PRE-COMMIT phase between PREPARE and COMMIT making protocol non-blocking
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RAID 0 provides
Striping only (improved performance, NO fault tolerance)
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RAID 6 provides
Double parity - can survive TWO simultaneous disk failures
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SSD vs HDD in databases: key advantage of SSD
Much faster random I/O: near-zero seek time and rotational latency
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Buffer pool hit ratio measures
Percentage of page requests served from memory vs disk (higher=better)
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Query rewrite: pushing projection down the operator tree
Reduces tuple width early limiting data carried through subsequent operations
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Which is a complete set of minimal Armstrong's axioms
Reflexivity, Augmentation, Transitivity
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DBMS → PL/SQL 3

Which SQL creates a stored procedure with output parameter
CREATE PROCEDURE p(OUT result INT) BEGIN...END
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Recursive stored procedure/function requires
A termination condition to prevent infinite recursion
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Which SQL creates trigger to log inserts on questions table
CREATE TRIGGER trg AFTER INSERT ON questions FOR EACH ROW INSERT INTO audit_log...
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DBMS → Transactions 4

Which isolation level is the SQL standard default in most databases
READ COMMITTED
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MySQL InnoDB default isolation level is
REPEATABLE READ
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Database transaction log is used for
Recovery: undoing incomplete transactions and redoing committed ones after crashes
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2PC blocking problem: coordinator fails between PREPARE and COMMIT causes
Participants that voted COMMIT block indefinitely waiting for coordinator decision
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DBMS → DDL Commands 1

Which DDL command modifies column data type
ALTER TABLE t MODIFY/ALTER COLUMN col newtype
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DBMS → Relational Model 1

PRIMARY KEY vs UNIQUE: key difference
UNIQUE allows one NULL; PRIMARY KEY does not allow any NULL
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DBMS → Deadlock 1

Deadlock detection frequency tradeoff
Higher frequency: detects deadlocks sooner but adds overhead; lower: less overhead but longer wait
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DBMS → Concurrency Control 1

Thomas Write Rule modification to timestamp ordering
Silently ignores an obsolete write when a newer transaction already wrote the item
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DBMS → Joins 2

Which join algorithm is best when inner relation has no index and doesn't fit in memory
Block nested-loop join (reads outer in blocks to reduce I/O)
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Cost of index-based nested-loop join (outer b_r blocks, inner has index)
b_r + n_r × (cost of index lookup)
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DBMS → Normalization 3

Which normal form specifically addresses redundancy due to functional dependencies
BCNF
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If R(A,B,C) is in BCNF is it also in 3NF
Yes - every BCNF relation is also in 3NF
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A relation in 4NF is also in
BCNF (since 4NF ⊂ BCNF ⊂ 3NF ⊂ 2NF ⊂ 1NF)
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