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The commit log (clog/pg_xact): where a transaction's outcome is recorded

Simple terms

Postgres has to remember the fate of every transaction that ever ran: did it commit, or did it abort? That answer is stored in two bits. Not two bytes, two bits, in a commit log on disk. So when your query looks at a row and asks whether it can see it, the real question underneath is whether the transaction that wrote that row committed. The commit log is where it goes to find out.

You might be asked

When a transaction commits, where exactly is 'this xid committed' recorded, and how does another transaction later find out whether some xid committed or aborted? How big is that per-transaction record, how many transactions fit on one page, and what happens to that information once the xid is frozen?

TopicThe commit log (clog/pg_xact): how transaction outcomes are stored as 2 bits per xid in an SLRU
PostgreSQL9.x-17 (measured on 17.10; pg_xact_status 10+, pg_stat_slru 13+)
Tools usedDocker pgi17 (PostgreSQL 17.10), pg_xact_status, pg_stat_slru, source @REL_17_10
Last reviewed2026-06-19

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