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The reason a modern DBMS prefers declarative SQL over hand-written RA is that
ASQL supports more data types than RA
BRA cannot express joins, so SQL is strictly more expressive
CSQL is mandatory on all NoSQL stores by ANSI standard
Dthe optimiser picks the cheapest equivalent RA tree using table stats
Answer & Solution
Correct answer: D. the optimiser picks the cheapest equivalent RA tree using table stats
RA can express joins (the symbol ⋈ literally is the join). The win is that declarative SQL hands the ordering decision to the optimiser, which uses live statistics to pick the cheapest equivalent expression. Hand-tuned RA bakes one ordering in and rots as data shifts.
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