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