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Interviewer
Can you explain the difference between a B-tree and a hash index in PostgreSQL? When would you choose one over the other?
TL;DR
B-tree indexes support range queries and ordering; hash indexes are faster for simple equality lookups. Choose B-tree by default.
B-tree vs Hash Indexes
B-tree is the default index type in PostgreSQL and supports <, >, BETWEEN, and ORDER BY. Hash indexes only support equality comparisons.
| Feature | B-tree | Hash |
|---|---|---|
| Range queries | Yes | No |
| Equality | Yes | Yes |
| WAL support | Yes | v10+ |
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Transcript
"How does PostgreSQL handle MVCC and what are the implications for vacuum..."
TL;DR
MVCC uses transaction IDs (xmin/xmax) on each tuple. Dead tuples accumulate and require VACUUM to reclaim space.
MVCC in PostgreSQL
PostgreSQL implements MVCC by storing multiple physical versions of each row. Each transaction sees a snapshot...
Session Performance
Recommendation
Study PostgreSQL WAL internals and checkpoint tuning. Practice explaining replication lag scenarios.
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