- PostgreSQL uses a 'database cluster' to refer to a single instance managing multiple databases, not multiple servers.
- Databases and other objects are identified by OIDs (unsigned integers), with user-created objects starting from OID 16384.
- System catalogs like pg_class and pg_database store metadata and are regular tables that can be queried.
- The physical structure centers on the data directory ($PGDATA), containing subdirectories for each database (named by OID) and tables.
- Tables and indexes are stored in files under base/{database_oid}/{relfilenode}, with forks for free space and visibility maps.
- VACUUM FULL can change a table's relfilenode, decoupling it from its OID.
- Tablespaces allow storing database objects in custom directories, with symlinks in $PGDATA/pg_tblspc/.
- Heap tables consist of 8KB pages containing headers, line pointers, and tuples stored from the page end.
- TOAST handles large values by storing them externally in a separate table, chunked and indexed.
- Writing tuples involves updating page headers and line pointers, with alignment considerations.
- Reading tuples can use sequential scans or index scans (e.g., B-tree), depending on the query.