Query hanging when queries in the resultset of UNION returned no rows

Question

Hi all,
I have a query like this.
Select * from table a
UNION
select * from table b
UNION
select * from table c
UNION
select * from table d
table a, table b, table c has some rows of data.
table d does'nt have any data.
so,
when i do union
of results of table a,table b and table c, I will get the results.
But when I include the UNION of table d(which is not having rows), Then the query is hanging.
Please help me in this regard.
Thanks in advance,
Sreekeshava S

Answer 1

Please find the query below.
select nn.node_name as nn_node_name,
t.template_name as t_template_name,
decode(status_flag, 0, 'NO', 'YES')
from opc_node_names nn, opc_source_templ t,
opc_node_config c, opc_change_status s
where nn.node_id = c.node_id
and c.node_id = s.object_id
and c.template_id = t.template_id
and s.object_type = 1
and s.status_type = 1
and s.value != 4
and s.value != 5
and nn.node_id not in (select node_id from opc_nodes where is_virtual=1)
union
select nn.node_name as nn_node_name,
t.template_name as t_template_name,
'Yes (Force)'
from opc_node_names nn, opc_source_templ t,
opc_node_config c, opc_change_status s
where nn.node_id = c.node_id
and s.object_id = c.node_id
and c.template_id = t.template_id
and s.object_type = 1
and s.status_type = 1
and s.value in (4, 5)
and nn.node_id not in (select node_id from opc_nodes where is_virtual=1)
union
select nn.node_name as nn_node_name,
t.template_name as t_template_name,
'(Virtual Node)'
from opc_node_names nn, opc_source_templ t,
opc_node_config c, opc_change_status s
where nn.node_id = c.node_id
and s.object_id = c.node_id
and c.template_id = t.template_id
and s.object_type = 1
and s.status_type = 1
and nn.node_id in (select node_id from opc_nodes where is_virtual=1)
union
select 'Management Server' as nn_node_name,
t.template_name as t_template_name,
decode(status_flag, 0, 'NO', 'YES')
from opc_source_templ t, opc_node_config c, opc_change_status s
where c.node_id is null
and c.template_id = t.template_id
and s.object_type = 1
and s.status_type = 1
and s.object_id = '000000000000000000000000000000000000'
and s.value != 4
and s.value != 5
union
select 'Management Server' as nn_node_name,
t.template_name as t_template_name,
'Yes (Force)'
from opc_source_templ t, opc_node_config c, opc_change_status s
where c.node_id is null
and c.template_id = t.template_id
and s.object_type = 1
and s.status_type = 1
and s.object_id = '000000000000000000000000000000000000'
and s.value in (4, 5)
In the above query,
only first 2 select select statements is having some data.
Next select statements does'nt return any data.
If we execute the select queries from the above query independently, It will execute within no time.
as i told only first 2 queries are having the data.
other sql queries does'nt have any data.
But if i union all the queries, query is hanging.
I am executing the query through SQL developer connecting to the database located in the Remote location.