First attempt: support for "\dg" in psql

G'day folks,

Attached is my first attempt at providing "\du"-like functionality for
groups (i.e. a basic "\dg"). It's really just a copy of describeUsers().

It has one major limitation - members are listed by numeric id as I can't
think of a "cheap" way of turning the list into the corresponding list of
usernames.

I thought about having describeGroups() do a lookup on pg_user, but that
could become quite expensive if there is a large number of groups and/or
users (have I missed an obvious facility for caching this information?).

Also, the TODO mentions that the intention is to have \du list groups - I
assume that means output along the lines of the following is wanted?

List of database users
User name | User ID | Attributes | Groups
-----------+---------+----------------------------+---------------
test | 100 | superuser, create database | testg1, testg2

Getting the group names for \du or the usernames for \dg are both
relatively easy with the appropriate JOIN, but the result is always a set
of rows. How to turn that into a single array? What would be handy is a
a library function returning an array, which takes an array, a table and two
two columns from that table:
F(A,T,C1,C2) returning an array of T.C2 values, one for each
value of T.C1 found in A.

With regard to criticism, please be gentle - this is only my second
contribution (the first being a very minor update to to_char). ;-)

Ciao.

--
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Daniel Baldoni BAppSc, PGradDipCompSci | Technical Director
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