Memory exhausted in AllocSetAlloc

[Postgresql 7.0.3, intel Linux, 4 cpu, 2 GB RAM ]

create table stepparams(step_proc text, name text, txt text, units text, step_ver int2, width int2,
 xpos int2, param_edit int2, xlevel int2, mandatory int2, primary key(step_proc));
 

The table stepparams has 15799 rows, 387 distinct values of step_proc.

For each value stepparams.step_proc e.g. 'foo', there should be a postgres table named "s_foo".
I want to check this for consistancy, i.e. find any stepparams.step_proc that does not have a corresponding table.

I thought this was a straight forward query: select distinct step_proc    from stepparams sp    where not exists
(select* from pg_class where text(relname)= 's_' || sp.step_proc);
 
but, after a while I get an error:  FATAL 1: Memory exhausted in AllocSetAlloc() 
pqReadData() -- backend closed the channel unexpectedly.
[indeed the backend had grown to 400MB, 173MB resident!]

So, I tried another formulation: select distinct sp.step_proc   from stepparams sp    where substring(sp.step_proc from
2)not in (select sp.step_proc from pg_class);
 
but the same error results.

I tried: select distinct step_proc    from stepparams sp    where not exists (select * from pg_class where
text(relname)='s_' || sp.step_proc);
 
but again the same error.

What am I doing wrong?


Here appended are the explain for these queries:

explain select distinct sp.step_proc from stepparams sp where substring(sp.step_proc from 2) not in (select
sp.step_procfrom pg_class);
 
NOTICE:  QUERY PLAN:

Unique  (cost=0.00..198558.04 rows=1580 width=12) ->  Index Scan using stepparams_idx on stepparams sp
(cost=0.00..198518.54rows=15799 width=12)       SubPlan         ->  Seq Scan on pg_class  (cost=0.00..24.96 rows=896
width=4)

explain select distinct step_proc from stepparams sp where sp.step_proc not in (select 's_' || sp.step_proc from
pg_class);
NOTICE:  QUERY PLAN:

Unique  (cost=0.00..198518.54 rows=1580 width=12) ->  Index Scan using stepparams_idx on stepparams sp
(cost=0.00..198479.04rows=15799 width=12)       SubPlan         ->  Seq Scan on pg_class  (cost=0.00..24.96 rows=896
width=4)
explain select distinct step_proc from stepparams sp where not exists (select * from pg_class where text(relname)= 's_'
||sp.step_proc);
 
NOTICE:  QUERY PLAN:

Unique  (cost=56244.75..56244.75 rows=0 width=12) ->  Sort  (cost=56244.75..56244.75 rows=1 width=12)       ->  Seq
Scanon stepparams sp  (cost=0.00..56244.74 rows=1 width=12)             SubPlan               ->  Seq Scan on pg_class
(cost=0.00..31.68rows=9 width=85)
 


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