Ok, thanks... it is not a bug;
2008/10/27 hubert depesz lubaczewski <depesz@depesz.com>:
> On Mon, Oct 27, 2008 at 05:08:35PM +0300, Oleg Serov wrote:
>> I can't get hstore value by key. i have very interesting error
>
> the problem is that -> has very low priority, but you work with it
> anyway:
> # select tconvert('a', 'b')->'a' is null;
> ERROR: operator does not exist: hstore -> boolean
> LINE 1: select tconvert('a', 'b')->'a' is null;
> ^
> HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts.
> (depesz@[local]:5840) 15:14:03 [depesz]
> # select (tconvert('a', 'b')->'a') is null;
> ?column?
> ----------
> f
> (1 row)
>
> depesz
>
> --
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>