You see a query plan with Bitmap Index Scan followed by Bitmap Heap Scan, but the query is running very slowly. What could be a likely cause?

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PostgreSQL - Performance Tuning

You see a query plan with Bitmap Index Scan followed by Bitmap Heap Scan, but the query is running very slowly. What could be a likely cause?

AThe index does not exist on the queried column

BThe table is empty, so no rows are fetched

CThe query is missing a WHERE clause

DThe bitmap is too large because the condition matches too many rows, causing inefficient heap fetch

Step-by-Step Solution

Solution:

Step 1: Analyze Bitmap Index Scan and Bitmap Heap Scan behavior
Bitmap Index Scan creates a bitmap of matching rows; Bitmap Heap Scan fetches rows using that bitmap.
Step 2: Understand performance impact of large bitmap
If too many rows match, the bitmap is large, causing many random disk accesses and slowing the query.
Final Answer:
The bitmap is too large because the condition matches too many rows, causing inefficient heap fetch -> Option D
Quick Check:
Large bitmap = slow Bitmap Heap Scan [OK]

Quick Trick: Large bitmap means many rows matched, slowing fetch [OK]

Common Mistakes:

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