Avoid compiler warning in non-assert builds.
After
3c90dcd03, try_partitionwise_join's child_joinrelids
variable is read only in an Assert, provoking a compiler
warning in non-assert builds. Rearrange code to avoid the
warning and eliminate unnecessary work in the non-assert case.
Per CI testing (via Jeff Davis and Bharath Rupireddy)
Discussion: https://postgr.es/m/
ef0de9713e605451f1b60b30648c5ee900b2394c.camel@j-davis.com