create or replace function test.test_create_tbl() returns void as
$$
begin
	if not exists(
		select * from information_schema.tables 
		where table_schema='test' 
			and table_name='test'	
			and table_type='BASE TABLE') then
		create table test.test(id serial primary key,name text,age int check(age> 0  and age< 150 ));
	end if;
end
$$ language plpgsql;

--drop function test.exec_if_not_exists(text,text)
create or replace function test.exec_if_not_exists(p_query_exists text,p_query_exec text) returns bool as 
$$
declare
	i int;
begin
	execute p_query_exists;
	get diagnostics i=row_count;
	--raise notice 'row_count=%',i;
	if i> 0  then 
		return false;
	else
		execute(p_query_exec); 
		return true;
	end if; 
end
$$ language plpgsql;
comment on function test.exec_if_not_exists(text,text) is 'если выполнение запроса, текст которого даётся первым параметром не 
возвращает строк - то выполняется строка sql задаваемая вторым параметром.';

select test.exec_if_not_exists(
'select * from information_schema.tables where table_type=''BASE TABLE'' and table_catalog=''cito'' and table_name=''test_if_exists'''::text,
	'create table test_if_exists(id serial primary key,descr text unique not null)'::text);
/*
drop table test_if_exists;
select * from test_if_exists;
select * from information_schema.tables where table_type='BASE TABLE' and table_catalog='cito' and table_name='test_if_exists';
*/