Why does UNION remove duplicates internally, and how does this affect performance compared to UNION ALL?

hard📝 Conceptual Q10 of 15

PostgreSQL - Set Operations and Advanced Queries

Why does UNION remove duplicates internally, and how does this affect performance compared to UNION ALL?

AUNION and UNION ALL have identical performance because they do the same work

BUNION removes duplicates by joining tables, making it faster than UNION ALL

CUNION removes duplicates by sorting or hashing, which adds overhead; UNION ALL is faster as it skips this step

DUNION removes duplicates by filtering rows before combining, which is faster than UNION ALL

Step-by-Step Solution

Solution:

Step 1: Understand how UNION removes duplicates
UNION removes duplicates by sorting or hashing the combined result, which requires extra processing.
Step 2: Compare performance with UNION ALL
UNION ALL skips duplicate removal, so it runs faster because it just concatenates results.
Final Answer:
UNION removes duplicates by sorting or hashing, which adds overhead; UNION ALL is faster as it skips this step -> Option C
Quick Check:
UNION slower due to duplicate removal; UNION ALL faster [OK]

Quick Trick: UNION removes duplicates with sorting/hashing; UNION ALL is faster [OK]

Common Mistakes:

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