deprecating =>, take two

By consensus, we have removed the new-to-9.0 operator text[] => text[]
and renamed the hstore => text[] operator. (The current name is "%",
but there is some discussion of "%>", some yet other name, or getting
rid of it altogether; please comment on that thread if you wish to
weigh in.) This means that the only remaining => operator in CVS is
the text => text operator which constructs a single-element hstore,
which has been around since 8.2. In lieu of providing a substitute
operator, Tom Lane proposed that we simply encourage people to use the
hstore(text, text) function which does the same thing:

http://archives.postgresql.org/pgsql-hackers/2010-06/msg00711.php

Per that email, and subsequent concurrence, here is a series of
patches which does the following:

1. In CVS HEAD, document the hstore(text, text) function and adjust
CREATE OPERATOR to throw a warning when => is used as an operator
name, using the wording previously suggested by Tom.
2. In the back branches, add an hstore(text, text) function. These
branches already have a tconvert(text, text) function which does the
same thing, but the consensus seemed to be that we do not want to go
back to the name tconvert() for this functionality, and that
back-patching the new name was preferable.
3. In 8.4 and 8.3, also add hstore(text, text) to the documentation.
8.2 appears to have no contrib documentation.

Barring vigorous objections, I will apply these tomorrow so that we
can consider deprecating => as an operator name in 9.1, for better
compliance with the SQL standard.

http://archives.postgresql.org/pgsql-hackers/2010-05/msg01501.php

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company