MemoryContextAlloc: invalid request size 1969649011

hi everybody,

this query...:

my $tsql = qq{ insert into  user_month (select '$currentmonth'::date as
month,'*nopleid*',count(distinct uuid) from user_log where day<=now()::date
and day>(now()-interval '30 days')::date) };
my $tsth = $tdbh->prepare( $tsql );
$tsth->execute();

sometimes drops this error:
DBD::Pg::st execute failed: ERROR:  MemoryContextAlloc: invalid request size
1969649011

the table user_log has about 2.000.000 entries and the following structure:

 Column |          Type          | Modifiers
--------+------------------------+-----------
 day    | date                   |
 uuid   | character varying(31)  |
 pleid  | character varying(200) |
Indexes: user_log_day_idx,
         user_log_pleid_idx,
         user_log_uuid_idx

The system is postgresql-7.2-1PGDG on a redhat 7.2 system.
Looks like the postgresql asks the system for some 2gb ram... thats quiet a
lot (and i have read somewhere this is a magic border on intel systems).
The query gets executed while entrys are inserted into user_log by other
clients.
Do i have to lock the table before against inserts?

ideas?

thanks

Thilo Hille
thilo@resourcery.de