[HACKERS] dubious error message from partition.c

In the original table partitioning commit
(f0e44751d7175fa3394da2c8f85e3ceb3cdbfe630), we introduced the
following code, which hasn't changed materially since then:

+                if (partition_rbound_cmp(key, lower->datums,
lower->content, true,
+                                         upper) >= 0)
+                    ereport(ERROR,
+                            (errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+                    errmsg("cannot create range partition with empty range"),
+                             parser_errposition(pstate, spec->location)));

In retrospect, I'm not thrilled by this error message, for two reasons:

1. It gives no details, as other nearby messages do.  For example,
further down in the function, we have a message "partition \"%s\"
would overlap partition \"%s\", which tells you the names of the old
and new partitions.  But this message has no %-escapes at all.  It
tells you neither the name of the partition that you're trying to
create nor the problematic values.  You might object that you can only
be creating one partition at a time and should therefore know its name
but (a) you might be running a script and be uncertain which command
failed and (b) we've talked repeatedly about introducing convenience
syntax for creating a bunch of partitions along with the parent table,
and if we do that at some point, then this will become more of an
issue.

2. The message text is, in my opinion, not as clear as it could be.
The most logical interpretation of that message, ISTM, would be that
you've specified a lower bound that is equal to the upper bound - and
you would indeed get this message in that case.  However, you'd also
get it if you specified a lower bound that is GREATER than the upper
bound, and I think that it's possible somebody might get confused.
For example:

rhaas=# create table example (a citext) partition by range (a);
CREATE TABLE
rhaas=# create table example1 partition of example for values from
('Bob') to ('alice');
ERROR:  cannot create range partition with empty range

A user who fails to realize what the comparison semantics of citext
are might scratch their head at this message.  Surely 'Bob' precedes
'alice' since 'B' = 66 and 'a' = 97, so what's the matter?  This could
also come up with plain old text if the partition bounds have a
different collation than the user is expecting.

So, I suggest something like:

"lower bound %s for partition \"%s\" must precede upper bound %s"

e.g. lower bound (Bob) for partition "example1" must precede upper bound (alice)

You could still be confused about why Bob comes before alice (sexism?)
but you'll at least know which partition is the problem and hopefully
at least have a clearer notion that the problem is that the system's
idea about how those bounds are ordered differs from your own.

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