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Complex SQL Query Nesting Tips

Oracle Tips by Burleson Consulting

May 14, 2010

Question:  Have you ever seen or written a query like this?

select round(io/(select sysdate-startup_time p from v$instance),1) as "Gb/day since instance startup",name from ( select sum(value)*8192/1024/1024/1024 io,name from v$sysstat where name in ('physical reads','physical writes') group by name) union (select round(sysdate-startup_time,1), 'days since startup'  from v$instance) union (select round(sum(bytes)/1024/1024/1024,1), 'Gb db size' from ( select sum(bytes) bytes from v$datafile union select sum(bytes) from v$log)) order by 2

It compares the size of the DB with the I/O (in Gb) per day since instance startup.
 
46.7  Gb db size
60.8  days since startup
68    physical reads
2.3   physical writes

Have you ever seen or written a query like this?
 
Answer:  Yes, I have seen queries like these, they give me brain cramps!  This mish-mosh of SQL is so complex that Einstein would have trouble understand it, and it is it very difficult to evaluate if it’s accurate, with all of the nesting of SQL statements!

Your query is a perfect example of hard to maintain SQL.  Whoever wrote it was trying to take a simple query language and turn it into a full-blown program!  Above I have tried to format below, with with all of the queries within queries, it's quite difficult to understand it without all of the nesting:

select
   round(io/(
   select
      sysdate-startup_time p
   from
      v$instance),1) as "Gb/day since instance startup",
   name

from
   (select
       sum(value)*8192/1024/1024/1024 io,
       name
    from
       v$sysstat
    where name in ('physical reads','physical writes') group by name)
 union
   (select
       round(sysdate-startup_time,1), 'days since startup
    from
       v$instance)
  union
    (select
        round(sum(bytes)/1024/1024/1024,1), 'Gb db size'
     from
        (select
            sum(bytes) bytes
         from
            v$datafile
         union
            select sum(bytes) from v$log)) order by 2

If I were writing this, I would make it four distinct SQL statements using an in-line view, something like this:

select
--******************************
-- Query 1
-- ****************************
   (select
          round(sysdate-startup_time,1), 'days since startup
       from
          v$instance
   as days_since_startup)

-- *******************
-- Query 2
-- ****************************
   (select
           round(sum(bytes)/1024/1024/1024,1), '
Gb db size'
        from
           (select
               sum(bytes) bytes
            from
               v$datafile
            union
               select sum(bytes) from v$log)) order by 2
   as
database_size_in_gig)
--******************************
-- Query 3
-- ****************************

   (select
          sum(value)*8192/1024/1024/1024 io,
          name
       from
          v$sysstat
       where
          name in ('physical reads','physical writes')
       group by
          name      as
total_reads_n_writes)
from from
   dual;

Rather than write my own code for this, I would use pre-written and tested Oracle scripts from my Oracle Script Download with over 600 pre-tested scripts!

 
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