The Real Cost of a Single Insert in Postgres

PostgreSQL write amplification occurs when a small logical insert produces substantially more physical storage and WAL activity due to MVCC tuple headers, page line pointers, index entries, and durability logging. Using a 40-byte sensor reading in a PostgreSQL 16 table with three indexes, the discussion estimates that each insert can generate four WAL records and roughly 345 bytes of WAL, with full-page images after checkpoints causing additional temporary spikes. It provides a SQL-based measurement approach using `pg_current_wal_lsn()` and `pg_stat_wal`, showing sample amplification of about 8.73–10.74 times with three indexes and 2.44 times after removing them, suggesting index maintenance accounts for much of the logging overhead. Recommended reductions include batching writes with COPY or multi-row inserts, removing rarely used indexes, enabling WAL compression, and adjusting checkpoint settings, while noting tradeoffs such as CPU use, recovery time, and WAL disk consumption.

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