Apologies in advance if this is not the best list for this. Appreciate a redirection if there is something more appropriate. In the document is mentions that: "Technically, PRIMARY KEY is merely a combination of UNIQUE and NOT NULL" I wanted to clarify if that was, technically, true. I had a table where I had used "UNIQUE NOT NULL" rather than primary key (not recommended by the docs, probably not best practise, but I thought, at the time, if they are equivalent, it should work). I then had need to use the "Group by can guess some missing columns" feature described here: https://wiki.postgresql.org/wiki/What%27s_new_in_PostgreSQL_9.1#SQL_and_PL.2FPgSQL_features Unfortunately this feature does not seem to work when using a column that is merely "UNIQUE NOT NULL", however it does seem to work when I change the columns to "PRIMARY KEY". My questions are: 1. Is this intended behaviour. 2. Does this mean, technically, that PRIMARY KEY is not merely a combination of UNIQUE and NOT NULL? The documentation does clarify with: "identifying a set of columns as primary key also provides metadata about the design of the schema, as a primary key implies that other tables can rely on this set of columns as a unique identifier for rows." I had assumed that the "metadata" was schema information that may be used by external tools or the users of the database, however it seems that this meta-data is also used at some point in the query engine, so my assumption that "meta-data" was only for external use seems incorrect. Are there any other cases that take advantage of PRIMARY KEY? (Maybe NATURAL joins?). Thanks, Ben -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
