PostgreSQL在何处处理 sql查询之五十五

/*
 * set_cheapest
 *      Find the minimum-cost paths from among a relation's paths,
 *      and save them in the rel's cheapest-path fields.
 *
 * Only unparameterized paths are considered candidates for cheapest_startup
 * and cheapest_total.    The cheapest_parameterized_paths list collects paths
 * that are cheapest-total for their parameterization (i.e., there is no
 * cheaper path with the same or weaker parameterization).    This list always
 * includes the unparameterized cheapest-total path, too.
 *
 * This is normally called only after we've finished constructing the path
 * list for the rel node.
 */
void
set_cheapest(RelOptInfo *parent_rel)
{
    fprintf(stderr,"In set_cheapest ........\n");

    Path       *cheapest_startup_path;
    Path       *cheapest_total_path;
    bool        have_parameterized_paths;
    ListCell   *p;

    Assert(IsA(parent_rel, RelOptInfo));

    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;
        }

        /*
         * 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;
    }

    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);
        }
    }
}