Question

I have a query where not all conditions are necessary. Here’s an example of what it looks like when all conditions are used:

select num
from (select distinct q.num
       from cqqv q
       where q.bcode = '1234567' --this is variable
             and q.lb = 'AXCT' --this is variable
             and q.type = 'privt' --this is variable
             and q.edate > sysdate - 30 --this is variable
       order by dbms_random.value()) subq
where rownum <= 10; --this is variable

The parts marked as --this is variable are the parts that, well, vary! If a condition is NOT specified, then there is no default value. For example, if the input specifies “*” for q.type (but leaves everything else the same), then the query should match everything for type, and execute as:

select num
from (select distinct q.num
       from cqqv q
       where q.bcode = '1234567' --this is variable
             and q.lb = 'AXCT' --this is variable
             --and q.type = 'privt' --this condition ignored because of "type=*" in input
             and q.edate > sysdate - 30 --this is variable
       order by dbms_random.value()) subq
where rownum <= 10; --this is variable

I know it is possible to use dynamic sql to build this query on the fly, but I am wondering what sort of performance problems this could cause, and if there is a better way to do this.

Answer 1

The solution I’ve settled on is one that generates an dynamic SQL query that may look like this:

select num
from (select distinct q.NUM
       from cqqv q 
       where  (q.bcode = :bcode) 
                  and  (1=1 or :lb is null) 
                  and  (1=1 or :type is null) 
                  and  (q.edate> :edate) 
                order by dbms_random.value()) subq 
where rownum <= :numrows

(in this example, the bcode and edate conditions were NOT optional, but the lb and type were)

I think this is (or is very similar to) what Michal Pravda was suggesting, and our DBA here prefers this solution over the context variable solution. Thanks for all that helped and offered advice!

A link our DBA found which details this solution is here:

Ask Tom: On Popularity and Natural Selection