You set work_mem to '100MB' but notice queries still perform slow external sorts. What is a likely cause?

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PostgreSQL - Performance Tuning

You set work_mem to '100MB' but notice queries still perform slow external sorts. What is a likely cause?

AThe <code>effective_cache_size</code> is too high causing planner confusion

BThe <code>work_mem</code> setting was not applied because of missing quotes

CThe query uses multiple sorts, each limited to 100MB, but total memory is insufficient

DThe disk is too fast, causing PostgreSQL to prefer external sorts

Step-by-Step Solution

Solution:

Step 1: Analyze memory per sort
Each sort can use up to 100MB, but multiple sorts multiply memory needs.
Step 2: Understand total memory usage
If total memory for all sorts exceeds available RAM, external sorts still occur.
Final Answer:
The query uses multiple sorts, each limited to 100MB, but total memory is insufficient -> Option C
Quick Check:
Multiple sorts multiply work_mem needs [OK]

Quick Trick: Check total memory for all sorts, not just one [OK]

Common Mistakes:

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