10 — How real systems compare

Andy shows a table the CMU group assembled for a survey paper comparing MVCC implementations across production systems 1:08:29.

Version storage across systems

System	Version storage	Chain order	Notes
PostgreSQL	Append-only	Oldest → newest	The standout outlier; HOT updates avoid index churn ~40% of the time 1:08:29 56:17 1:25:03
MS SQL Server Hekaton	Append-only	—	Append-only chosen because it was grafted onto SQL Server to add in-memory storage 1:09:00
MySQL / InnoDB	Delta (rollback segment)	Newest → oldest	Secondary indexes store primary keys, so updates never touch them 40:10 57:09
Oracle	Delta	—	Its "serializable" level is really snapshot isolation 26:48 29:35
MS SQL Server (classic)	Time-travel	Newest → oldest	Retrofit onto an originally single-version 2PL engine 39:04 39:32
SAP HANA	Time-travel	Oldest → newest	GC/compaction promotes the latest version into the main table 39:32
HyPer / Hyrise	Delta	—	Hyrise abandoned append-only and switched to deltas 1:09:00

Headline conclusions

• Append-only is a bad idea for most systems. The systems still using it either predate the consensus or adopted it for a specific in-memory/distributed reason 1:09:00.
• Every other modern system, unless it forks PostgreSQL, does it the MySQL/Oracle (delta) way 29:35.