tuplestore : write failed

What mean it ?
I have query :

SELECT DISTINCT n.nspname as pnspname,n2.nspname as fnspname, c.relname as prelname,
c2.relname as frelname, t.tgconstrname, a.attnum as keyseq, ic.relname as fkeyname,
t.tgdeferrable, t.tginitdeferred, t.tgnargs,t.tgargs, p1.proname as updaterule,
p2.proname as deleterule,con.conname as conname
FROM pg_catalog.pg_namespace n, pg_catalog.pg_namespace n2,
pg_catalog.pg_trigger t, pg_catalog.pg_trigger t1,
pg_catalog.pg_class c, pg_catalog.pg_class c2,
pg_catalog.pg_class ic, pg_catalog.pg_proc p1,
pg_catalog.pg_proc p2, pg_catalog.pg_index i,
pg_catalog.pg_attribute a,pg_catalog.pg_constraint con
WHERE (t.tgrelid=c.oid AND t.tgisconstraint
AND t.tgconstrrelid=c2.oid AND t.tgfoid=p1.oid AND p1.proname like 'RI\\_FKey\\_%\\_upd')
AND (t1.tgrelid=c.oid AND t1.tgisconstraint AND t1.tgconstrrelid=c2.oid
AND t1.tgfoid=p2.oid AND p2.proname like 'RI\\_FKey\\_%\\_del') AND i.indrelid=c.oid
AND i.indexrelid=ic.oid AND ic.oid=a.attrelid AND i.indisprimary AND c.relnamespace = n.oid
AND c2.relnamespace=n2.oid AND c2.relname='pg_class'
AND (c2.oid =con.conrelid AND n.oid=con.connamespace AND con.contype='f' AND c.oid=con.confrelid)
ORDER BY prelname,keyseq

I try with pgsql 7.3b[12345],7.3rc[12] and 7.3
Query work sometime ; timing is from 1 second to 170 second, but sometime return
tuplestore : write error
database is almost emty - I have 10 tables with 10 primary and foreign keys (72 rows in pg_constraint)

query work fine with emty database (empty pg_constraint) - 1-2 second

regards
Haris Peco