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2 stories

Banner: scattered random inserts across a wide index tree compared with neat appends at its right edge

Databases Bug Report

Random UUIDv4 primary keys slowed our inserts 6x; switching to UUIDv7 fixed it

John Nguyen · Apr 24, 2026 · 10 min read

Insert throughput on our events table fell by a factor of six as it passed 400 million rows, while CPU stayed idle. The cause was random UUIDv4 keys scattering writes across the whole primary key index. Postgres 18 uuidv7() brought it back, without changing the column type.

Banner: Postgres primary disk filled overnight because an abandoned CDC replication slot kept every WAL file

Databases Bug Report

Postgres primary disk filled overnight: an abandoned CDC replication slot kept every WAL file

Le Minh Khoa · Jan 26, 2026 · 7 min read

Our PostgreSQL primary went read-only at 4 a.m. when its disk filled. The pg_wal directory held 380 GB, because a logical replication slot created for a CDC trial weeks earlier was inactive and still retained WAL. How we found the slot, recovered safely, and capped WAL retention with max_slot_wal_keep_size.