[Solved] Why is the UNION ALL keyword used in recursive CTEs for graph traversal instead of UNION? — Ans: Because UNION ALL is faster and duplicates are handled by recursion logic | PostgreSQL

PostgreSQL - Common Table Expressions

Why is the UNION ALL keyword used in recursive CTEs for graph traversal instead of UNION?

ABecause UNION ALL is faster and duplicates are handled by recursion logic

BBecause UNION ALL removes duplicates automatically

CBecause UNION ALL sorts the results

DBecause UNION ALL limits recursion depth

Step-by-Step Solution

Solution:

Step 1: Understand difference between UNION and UNION ALL
UNION removes duplicates but is slower; UNION ALL keeps duplicates and is faster.
Step 2: Recognize recursion handles duplicates
Recursive CTEs often handle duplicates via logic, so UNION ALL is preferred for performance.
Final Answer:
Because UNION ALL is faster and duplicates are handled by recursion logic -> Option A
Quick Check:
UNION ALL preferred for speed; recursion manages duplicates [OK]

Quick Trick: Use UNION ALL for speed; recursion logic manages duplicates [OK]

Common Mistakes:

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Why is the UNION ALL keyword used in recursive CTEs for graph traversal instead of UNION?