Why might PostgreSQL choose a sequential scan over an index scan even if an index exists and the query filters on the indexed column?

hard📝 Conceptual Q10 of 15

PostgreSQL - Performance Tuning

ABecause the index is only used for sorting, not filtering.

BBecause PostgreSQL does not support index scans on columns with NULL values.

CBecause sequential scans always use less memory than index scans.

DBecause the planner estimates that reading the whole table is cheaper than many random index lookups.

Step-by-Step Solution

Solution:

Step 1: Understand planner cost estimation
The planner estimates costs for sequential vs index scans based on data distribution and expected rows.
Step 2: Explain scan choice
If many rows match, sequential scan can be cheaper than many random index lookups, so planner chooses it.
Final Answer:
Because the planner estimates that reading the whole table is cheaper than many random index lookups. -> Option D
Quick Check:
Planner cost estimation drives scan choice = D [OK]

Quick Trick: Planner picks cheapest scan based on estimated cost [OK]

Common Mistakes:

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