A query plan includes a Bitmap Index Scan on an index but the query executes slowly. Which of the following is a plausible reason?

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PostgreSQL - Performance Tuning

A query plan includes a Bitmap Index Scan on an index but the query executes slowly. Which of the following is a plausible reason?

AThe table has no indexes at all

BThe index is corrupted and cannot be used

CThe query optimizer always prefers sequential scans over Bitmap Index Scans

DThe Bitmap Index Scan matches a very large number of rows causing many random heap fetches

Step-by-Step Solution

Solution:

Step 1: Analyze Bitmap Index Scan performance
Bitmap Index Scan is efficient when matching a moderate number of rows.
Step 2: Large matches cause slow heap fetches
If many rows match, random heap fetches increase, slowing the query.
Final Answer:
The Bitmap Index Scan matches a very large number of rows causing many random heap fetches -> Option D
Quick Check:
Corrupted index would cause errors, not slow scans [OK]

Quick Trick: Large matches cause slow Bitmap Index Scans [OK]

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