Troubles with mysql union - mysql

Question

I'm having some troubles (both technical and conceptual) optimizating some very slow queries.
This is my original query:
select Con_Progr,
Pre_Progr
from contribuente,
preavviso_ru,
comune,
via
where [lots of where clauses]
and ((Via_Progr = Con_Via_Sec) or (Via_Progr = Con_Via_Res and (Con_Via_Sec is null or Con_Via_Sec = '0')))
order by Con_Cognome,
Con_Nome asc;
This query takes about 38secs to execute, which is a really slow time. I manipulated it a bit and managed to speed up to about 0.1sec and now the query looks like this:
(select Con_Progr,
Pre_Progr
from preavviso_ru
join contribuente
on Pre_Contribuente = Con_Progr
join via
on Via_Progr = Con_Via_Sec
join comune
on Via_Comune = Com_CC
where [lots of where clauses]
order by Con_Cognome,
Con_Nome asc
)
union
(
select Con_Progr,
Pre_Progr
from preavviso_ru
join contribuente
on Pre_Contribuente = Con_Progr
join via
on Via_Progr = Con_Via_Res
join comune
on Via_Comune = Com_CC
where [lots of where clauses]
and (Con_Via_Sec is null or Con_Via_Sec = '0')
order by Con_Cognome,
Con_Nome asc
)
As you can see I split up the where clause in the original query that used an OR operator in two different subqueries and then merged them. That resolved the speed problem. The result though is not perfect, 'cause I've lost the ordering. I tried to select the columns in the subqueries and then perform the ordering on that result, like this:
select Con_Progr,
Pre_Progr
from (
[FIRST SUBQUERY]
) as T1 union (
[SECOND SUBQUERY]
) as T2
order by Con_Cognome,
Con_Nome asc
but I get a syntax error near 'union'. Any suggestion?
This was the technical issue. Now for the conceptual, I reckon that the two subqueries are VERY similar (they only differ by a join clause and a where clause), is there a way to rearrange the second query (the fast one) in a more elegant way?

Answer 1

I resolved the techincal issue, I misplaced the parenthesis:
select Con_Progr,
Pre_Progr
from (
[FIRST SUBQUERY]
union
[SECOND SUBQUERY]
) as T
order by Con_Cognome,
Con_Nome asc
Now it works almost perfectly (there's still a bit of discrepancy in the ordering but it's not a problem.
About the efficiency of the query, I found that the problem is the OR condition on two different columns, since MySQL 4.0 (which I'm using for backwards compatibility issues) allows only one index for table. But still, why doesn't it use at least one of those indexes. I'll test a bit more...