What will happen to the primary key if you create a table without specifying PRIMARY KEY?

Question

PostgreSQL

I have created tables with explicit indication of the primary key for the column, and without specifying the primary key at all. But the primary key is mandatory for every table. Is it set automatically? If yes, then where? For the first column?

Answer 1

But primary key is required for every table

From the point of view of relational theory and normal forms in particular.
From the point of view of SQL - it is not obligatory. A table may not declare a primary key, or even some unique key. Next are the implementation details.
In PostgreSQL, even a surrogate primary key will not be created in this case. Row addressing in PostgreSQL is performed by TID (tuple ID) - the physical address of the row in the datafile (and therefore the TID is unique in the table), including when searching by the primary key. But TID is not an index, has nothing to do with the stored data and does not impose any restrictions on the data.