“PostgreSQL resolves uniqueness through heap tuple visibility”

I recently commented on Jonathan Lewis’s blog, Savepoint Funny , where I compared how PostgreSQL handles uniqueness differently: “PostgreSQL resolves uniqueness through heap tuple visibility". This deserves a more detailed explanation. In Oracle, unique indexes store unique entries because the B-tree key is the index key, preventing duplicates. Non-unique indexes add the ROWID to ensure that all entries are physically unique, even when indexed column values are duplicated. In PostgreSQL, all indexes, even unique ones, created explicitly by CREATE UNIQUE INDEX or implicitly to enforce a unique constraint, behave like non-unique indexes by appending the TID (tuple ID, similar to Oracle's ROWID) to the index key. This indicates that the index itself doesn't guarantee physical uniqueness, allowing multiple entries to have identical logical keys but point to different heap tuples. The actual uniqueness verification occurs at the heap level, not within the index entries. Initially, this might seem unusual—a unique index that permits duplicates. However, PostgreSQL requires this because of its MVCC system. MVCC allows duplicate entries to coexist in an index, since they can represent different versions of the same logical row. Still, PostgreSQL must guarantee that no MVCC snapshot views two rows with the same index key. Oracle doesn't face this issue because its MVCC implementation also versions index blocks, allowing a single index version to maintain unique keys. Let’s show that. Page inspect In PostgreSQL, the heap contains the table data, and index entries point to heap tuples. Visibility depends on the heap header, especially the transaction information. Index scans often visit the heap pages to check visibility, except for index-only scans, which use the heap's visibility maps as an optimization. B-tree indexes can store entries for multiple versions of the same logical row, including versions that are no longer visible to current snapshots. To ensure uniqueness, the