Re: Multiple-Table-Spanning Joins with ORs in WHERE Clause

"Sven R. Kunze" <srkunze@xxxxxxx> · Thu, 29 Sep 2016 14:20:31 +0200



    On 23.09.2016 11:00, Pavel Stehule
      wrote:

    
      2016-09-23 8:35 GMT+02:00 Sven R. Kunze <srkunze@xxxxxxx>:

        
            I was
              wondering: would it be possible for PostgreSQL to rewrite
              the query to generate the UNION (or subquery plan if it's
              also fast) on it's own?

            
            It depends on real data. On your specific data the
              UNION variant is pretty fast, on different set, the UNION
              can be pretty slow. It is related to difficult OR
              predicate estimation.
          
        
    I figure that the UNION is fast if the sub-results are small (which
    they are in our case). On the contrary, when they are huge, the
    OUTER JOIN variant might be preferable.

    
    Is there something I can do to help here?

    
    Or do you think it's naturally application-dependent and thus should
    be solved with application logic just as we did?

    
    Cheers,

    Sven