Sure, of course QUEL still worked... but Postgres was one of the last databases to use QUEL. It was pretty clear SQL won even when Postgre was just getting started (mid-80s). The Ingres project (that developed QUEL) ended by the time Postgres started (obviously, since the name is Postgres = Post Ingres). Ingres influenced Sybase, which switched to SQL in the late 80s. Ingres Corp even switched to SQL in the late 80s. What other databases using QUEL existed in the mid-90s?

POSTQUEL was definitely the primary interface to postgres. The navigational interface was an early Postgres feature. See page 3 and page 10 ("fast path") of Stonebraker's 1990 paper on the implementation of postgres: http://db.cs.berkeley.edu/papers/ERL-M90-34.pdf

I agree that commercially SQL was winning by the time postgres got started, but I think it was not yet obvious there was room for only one query language. I also suspect that the early implementation shared a lot of Ingres code as the focus was on the "post-relational" topics. Building SQL would have been a distraction. Perhaps also Stonebraker did not want to Osborne Effect Ingres. I'm sure even early Oracle marketing would have had fun with "even the founder of Ingres agrees QUEL is obsolete". Finally there was a lot to like in QUEL and SQL has quite a few warts.

Thanks for the link. Fast path was mostly about the function manager and only incidentally about data access. As in, "if you call all these functions that are part of the infrastructure, you can get at data". But that's not really a navigational interface (get first of set), more a side effect of exposing the internals via the function manager.

I started at Sybase in 1988, there were a lot Britton Lee alums, who had also worked on the ingres project as well. I ran in the the rest at Illustra in the 90s.