In Postgres, a unique constraint is implemented with a unique B-tree index. The manual:
Adding a unique constraint will automatically create a unique B-tree
index on the column or group of columns used in the constraint.
Indexes use the same basic storing mechanisms as tables: arrays of data pages. Indexes have some minor additional features. The manual:
In a table, an item is a row; in an index, an item is an index entry.
Every table and index is stored as an array of pages of a fixed size
(usually 8 kB, although a different page size can be selected when
compiling the server).
The maximum size of an index entry is roughly a third of a data page. See:
The maximum size was reduced to 2704 bytes in Postgres 12. Still the same in Postgres 15. Meaning your
UNIQUE constraint is not possible.
Consider a unique index on hash value, either after adding a redundant column with the hash, or just an index on the hash expression.
You could use the built-in function
md5() or something more efficient for huge values. Related: