Notes for the problems that show up after launch: bad plans, awkward migrations, index debt, vacuum pressure, replica lag, and the small decisions that make PostgreSQL easier to operate.
Setting MAXDOP to 1 because of CXPACKET waits amputates a feature to treat a symptom. NUMA-aware MAXDOP guidance, why the default threshold of 5 is ancient history, and where each setting should actually live.
ORDER BY tenant_id, created_at DESC LIMIT 50 took 14 seconds as an external merge sort spilling 1.7 GB — and 180 ms with incremental sort. How the presorted-prefix trick works, and when it backfires.
MariaDB can store optimizer statistics as ordinary data in mysql.table_stats, column_stats and index_stats — histograms included. Here is how to turn it on, keep it fresh, and stop guessing why a plan changed.
full_page_writes protects you from torn pages but can dominate WAL volume. How checkpoints set the FPI rate, what wal_compression with lz4 or zstd buys you, and how to measure WAL with pg_stat_wal.
Updating one key in an 18KB JSON document should not rewrite 18KB into the binlog. MySQL 8.0 can patch documents in place and log just the diff — when the change qualifies, and when it silently does not.
A crash mid-ALTER on 5.7 left orphaned #sql tables and dictionary mismatches you fixed with file surgery. MySQL 8.0's transactional data dictionary ended that era — and quietly retired a few other habits.
A four-value status column over 300 million rows carried an 11.8 GB index before PostgreSQL 13. One REINDEX later it was 1.9 GB, same queries, same plans — posting lists did that, and the bloat math has not been the same since.
Seconds_Behind_Source is the most quoted and least trusted replication metric in MySQL. This deep dive covers heartbeat-based lag measurement, multi-threaded replica tuning, and the big-transaction lag pattern.
Replication filters diverge replicas without raising a single error, and source-side filters quietly gut your point-in-time recovery. Here is how each layer actually fails.
innodb_io_capacity is a background I/O budget, not a speed dial. Crank it past your redo-driven need and the page cleaner spends the surplus evicting warm pages you were about to read.
MySQL 8.0 replaced block nested loop with a real hash join, and half the join-tuning advice from the 5.7 era is now wrong. When the optimizer hashes, when an index beats it, and when it loses.
ORDER BY col1 ASC, col2 DESC forced a filesort for two decades of MySQL. Since 8.0, a composite with a real descending part resolves it straight from the index — if the directions match exactly.