Fix overoptimistic assumptions in column width estimation for subqueries.
set_append_rel_pathlist supposed that, while computing per-column width estimates for the appendrel, it could ignore child rels for which the translated reltargetlist entry wasn't a Var. This gave rise to completely silly estimates in some common cases, such as constant outputs from some or all of the arms of a UNION ALL. Instead, fall back on get_typavgwidth to estimate from the value's datatype; which might be a poor estimate but at least it's not completely wacko. That problem was exposed by an Assert in set_subquery_size_estimates, which unfortunately was still overoptimistic even with that fix, since we don't compute attr_widths estimates for appendrels that are entirely excluded by constraints. So remove the Assert; we'll just fall back on get_typavgwidth in such cases. Also, since set_subquery_size_estimates calls set_baserel_size_estimates which calls set_rel_width, there's no need for set_subquery_size_estimates to call get_typavgwidth; set_rel_width will handle it for us if we just leave the estimate set to zero. Remove the unnecessary code. Per report from Erik Rijkers and subsequent investigation.
Showing
想要评论请 注册 或 登录