Interview Q&A / PostgreSQL
PostgreSQL interview questions
Indexes, EXPLAIN, transactions and isolation, JSONB and the Postgres specifics backend interviews probe.
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- Why does PostgreSQL sometimes show Index Only Scan but still read the table?
An index does not store row visibility. PostgreSQL checks the visibility map, and for pages not marked all-visible it must fetch the heap row. EXPLAIN ANALYZE shows this as Heap Fetches, which VACUUM reduces.
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