PostgreSQL's `default_table_access_method` GUC controls which storage engine new tables use when no `USING` clause is specified. Introduced alongside the table access method (TAM) API in PostgreSQL 12, it enables pluggable storage engines via ~45 callback functions registered in `pg_am`. While heap remains the only built-in AM, notable alternatives include Citus columnar (compressed column-oriented storage), OrioleDB (an undo-log engine aiming to eventually replace heap), and tde_heap from Percona's pg_tde for transparent data encryption. The parameter's practical value is as a deployment mechanism: setting it cluster-wide or per-database enforces a storage policy (e.g., encryption by default) without relying on per-table DDL discipline. Per-session use is also valid for bulk migration scripts. The TID-based tuple addressing constraint in the API has historically limited alternative AM adoption, but OrioleDB's shrinking patch set suggests the landscape may shift in coming releases.
