Interview Q&A / PostgreSQL
PostgreSQL interview questions
Indexes, EXPLAIN, transactions and isolation, JSONB and the Postgres specifics backend interviews probe.
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- What is a HOT update in PostgreSQL and how does fillfactor help?
A HOT (heap-only tuple) update happens when no indexed column changes and the new row version fits on the same page, so no new index entries are needed. Lowering fillfactor leaves free space on each page to make that more likely.
AdvancedQuick answer1 min read - What is transaction ID wraparound in PostgreSQL and how is it prevented?
Transaction IDs are 32-bit numbers, so after about two billion, old ones would look like the future and rows would vanish. VACUUM freezes old row versions to prevent this, and PostgreSQL forces aggressive vacuums and finally stops accepting writes if it falls too far behind.
AdvancedQuick answer1 min read