Answer
Index-only scans succeed when the visibility map confirms all tuples on a page are visible. If pages contain dead tuples, PostgreSQL must recheck the heap, making the scan not fully index-only.
Quick Answer
Index-only scans work by reading all needed column values from the index itself, skipping the heap fetch. The visibility map must show the page as all-visible (VACUUM has run recently). They fail when: the query needs columns not in the index, the visibility map bit is not set (heap fetch needed to verify visibility), or the table has recent unfrozen changes. Maximize by using covering indexes and running VACUUM.
S
SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ