Posts tagged “Databases”
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PostgreSQL transaction ID wraparound: why VACUUM freezes rows and how to read age(datfrozenxid)
What happens when PostgreSQL's 32-bit transaction ID wraps: freezing, relfrozenxid/datfrozenxid, anti-wraparound autovacuum, warnings, write refusal and recovery, per the PostgreSQL 18 docs.
What PostgreSQL data checksums catch, and what they don't
PostgreSQL data checksums detect corrupted pages on read. Covers when checks run, enabling them offline with pg_checksums, pg_stat_database counters, and what amcheck and backups still cover.
When a PostgreSQL UPDATE needs no new index entries: HOT updates and fillfactor
PostgreSQL HOT (heap-only tuple) updates avoid new index entries when no indexed column changes and the page has room. Covers both rules, fillfactor, and pg_stat_user_tables counters.
Using a PostgreSQL table as a job queue: FOR UPDATE SKIP LOCKED and when the lock is released
How FOR UPDATE SKIP LOCKED lets several workers share jobs in one PostgreSQL table, per the official docs: lock lifetime, LIMIT and OFFSET, ORDER BY, and locking clauses inside WITH.
Why your SQLite -wal file keeps growing: checkpoints and read transactions that never end
Why an SQLite -wal file keeps growing: the 1000-page auto-checkpoint, open read transactions that stall checkpoints, PASSIVE/FULL/RESTART/TRUNCATE, and journal_size_limit, per the official docs.
Why two SELECTs in one transaction can disagree: PostgreSQL READ COMMITTED and snapshots
Under PostgreSQL's default READ COMMITTED level, two SELECTs in one transaction can see different data. Learn how statement snapshots, row rechecks, and transaction retries work.
Why NOT IN returns no rows: SQL NULL and three-valued logic
One NULL in a subquery makes NOT IN return zero rows. Why, per the PostgreSQL, MySQL and SQLite docs: three-valued logic, how WHERE keeps rows, and fixing it with NOT EXISTS.
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