PostgreSQL - Common Table Expressions
Before PostgreSQL 12, what is a typical impact of using a CTE instead of a subquery on query planning and execution?
Before PostgreSQL 12, what is a typical impact of using a CTE instead of a subquery on query planning and execution?
easy📝 Conceptual Q1 of 15
Step-by-Step Solution
Solution:
Step 1: Understand CTE behavior pre-PostgreSQL 12
CTEs act as optimization fences, meaning the planner executes them independently and does not push filters inside.Step 2: Compare with subqueries
Subqueries can be inlined and optimized with filter pushdown, often resulting in better performance.Final Answer:
CTEs act as optimization fences, preventing planner from pushing down filters -> Option DQuick Check:
CTEs block optimization before v12 [OK]
Quick Trick: CTEs block optimization before v12 [OK]
Common Mistakes:
- Assuming CTEs are always inlined like subqueries
- Believing CTEs cache results automatically
- Thinking CTEs enable parallelism by default
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