Re: [BUGS] ON CONFLICT with constraint name doesn't work

To me, it now seems to be correct as well. 

From what I see experimenting with unique indexes/constraints and looking to "pg_constraint" and "pg_indexes" catalogs: 

  a) if there is a unique constraint created by user, there is always the corresponding unique index defined, with the same name; and renaming of the index leads to implicit renaming of the constraint;

  b) in contrast, creation of a unique index does not automatically lead to creation of the corresponding unique constraint;

  c) any primary key is also a unique index by definition (in Postgres context, it's not a "unique constraint", it's a "unique index").

So violation of uniqueness is always a violation of a unique index, in all three cases. However, case (b) is very tricky and I suspect that many users will be consused -- just like I was today. Anyway, the proposed patches makes messaging and docs closer to the current implementation, minimizing the possible confusion.

Also, I assume that in the future, there is a possibility to distinguish cases "violates unique constraint", "violates primary key" and "violates unique index" – as I know, in Oracle, for example, you can have a *deferrable* unique constraint based on non-unique, regular index...

Anyway, attached are 2 separate patches: 

 1) version 2 of patch fixing the message, including regression tests;

 2) proposed change to the documentation https://www.postgresql.org/docs/current/static/sql-insert.html