Re: psql UPDATE field [tab] expands to DEFAULT?

I wrote:
> I took a closer look and realized that this isn't some magic behavior of
> arcane parts of libreadline; it's more like self-inflicted damage. It
> happens because tab-complete.c's complete_from_const() is doing exactly
> what its comment says it does:

> /*
> * This function returns one fixed string the first time even if it doesn't
> * match what's there, and nothing the second time. This should be used if
> * there is only one possibility that can appear at a certain spot, so
> * misspellings will be overwritten. The string to be passed must be in
> * completion_charp.
> */

> This is unlike complete_from_list(), which will only return completions
> that match the text-string-so-far.

> I have to wonder whether complete_from_const()'s behavior is really
> a good idea; I think there might be an argument for getting rid of it
> and using complete_from_list() even for one-element lists.

I experimented with ripping out complete_from_const() altogether, and
soon found that there's still one place where we need it: down at the
end of psql_completion, where we've failed to find any useful completion.
If that instance of COMPLETE_WITH("") is implemented by complete_from_list
then readline will happily try to do filename completion :-(.
(I don't quite understand why we don't get the wiping-out behavior there;
maybe an empty-string result is treated differently from
not-empty-string?)

So I propose the attached instead, which doesn't get rid of
complete_from_const but ensures that it's only used in that one place.

This is independent of the other patch shown upthread. I'm proposing
this one for HEAD only but would back-patch the other.

regards, tom lane