PostgreSQL - Performance Tuning
A query using Merge Join runs very slowly. Which of these is a likely cause?
A query using Merge Join runs very slowly. Which of these is a likely cause?
medium📝 Debug Q7 of 15
Step-by-Step Solution
Solution:
Step 1: Check Merge Join requirements
Merge Join requires sorted inputs; unsorted tables cause slow performance.Step 2: Review other options
Disabling Hash Join or forcing Nested Loop does not explain slow Merge Join; indexes do not slow Merge Join directly.Final Answer:
Input tables are not sorted on join keys -> Option DQuick Check:
Merge Join slow = unsorted inputs [OK]
Quick Trick: Merge Join needs sorted inputs for speed [OK]
Common Mistakes:
- Blaming disabled Hash Join for Merge Join slowness
- Assuming indexes slow Merge Join
- Confusing forced Nested Loop with Merge Join speed
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