PostgreSQL在何处处理 sql查询之五十六

之五十六"/>

PostgreSQL在何处处理 sql查询之五十六

接前面，继续分析 :

    cheapest_startup_path = cheapest_total_path = NULL;have_parameterized_paths = false;foreach(p, parent_rel->pathlist){Path       *path = (Path *) lfirst(p);int            cmp;/* We only consider unparameterized paths in this step */if (path->param_info){have_parameterized_paths = true;continue;}if (cheapest_total_path == NULL){cheapest_startup_path = cheapest_total_path = path;fprintf(stderr,"Here cheapest_total_path is set\n");continue;}/** If we find two paths of identical costs, try to keep the* better-sorted one.  The paths might have unrelated sort orderings,* in which case we can only guess which might be better to keep, but* if one is superior then we definitely should keep that one.*/cmp = compare_path_costs(cheapest_startup_path, path, STARTUP_COST);if (cmp > 0 ||(cmp == 0 &&compare_pathkeys(cheapest_startup_path->pathkeys,path->pathkeys) == PATHKEYS_BETTER2))cheapest_startup_path = path;cmp = compare_path_costs(cheapest_total_path, path, TOTAL_COST);if (cmp > 0 ||(cmp == 0 &&compare_pathkeys(cheapest_total_path->pathkeys,path->pathkeys) == PATHKEYS_BETTER2))cheapest_total_path = path;}

由于我执行的是简单查询，所以  parent_rel->pathlist 的长度是1，故此：

上面的for 循环可以简化为：

    cheapest_startup_path = cheapest_total_path = NULL;have_parameterized_paths = false;foreach(p, parent_rel->pathlist){Path       *path = (Path *) lfirst(p);int            cmp;/* We only consider unparameterized paths in this step */if (path->param_info){have_parameterized_paths = true;continue;}if (cheapest_total_path == NULL){cheapest_startup_path = cheapest_total_path = path;
continue;}...}
        ...
    }

因为只有一个path，所以这个就是最cheapest 的了。后面一小段代码就无视了：

    if (cheapest_total_path == NULL)elog(ERROR, "could not devise a query plan for the given query");parent_rel->cheapest_startup_path = cheapest_startup_path;parent_rel->cheapest_total_path = cheapest_total_path;parent_rel->cheapest_unique_path = NULL;    /* computed only if needed *//* Seed the parameterized-paths list with the cheapest total */parent_rel->cheapest_parameterized_paths = list_make1(cheapest_total_path);/* And, if there are any parameterized paths, add them in one at a time */if (have_parameterized_paths){foreach(p, parent_rel->pathlist){Path       *path = (Path *) lfirst(p);if (path->param_info)add_parameterized_path(parent_rel, path);}}

 可以说，还应当向上回溯，找到真正何处开始真正设置path。

cheapest_startup_path = cheapest_total_path = path; 
也就是说，要查 ->pathlist