PostgreSQL在何处处理 sql查询之十四

/*
 * query_planner
 *      Generate a path (that is, a simplified plan) for a basic query,
 *      which may involve joins but not any fancier features.
 *
 * Since query_planner does not handle the toplevel processing (grouping,
 * sorting, etc) it cannot select the best path by itself.    It selects
 * two paths: the cheapest path that produces all the required tuples,
 * independent of any ordering considerations, and the cheapest path that
 * produces the expected fraction of the required tuples in the required
 * ordering, if there is a path that is cheaper for this than just sorting
 * the output of the cheapest overall path.  The caller (grouping_planner)
 * will make the final decision about which to use.
 *
 * Input parameters:
 * root describes the query to plan
 * tlist is the target list the query should produce
 *        (this is NOT necessarily root->parse->targetList!)
 * tuple_fraction is the fraction of tuples we expect will be retrieved
 * limit_tuples is a hard limit on number of tuples to retrieve,
 *        or -1 if no limit
 *
 * Output parameters:
 * *cheapest_path receives the overall-cheapest path for the query
 * *sorted_path receives the cheapest presorted path for the query,
 *                if any (NULL if there is no useful presorted path)
 * *num_groups receives the estimated number of groups, or 1 if query
 *                does not use grouping
 *
 * Note: the PlannerInfo node also includes a query_pathkeys field, which is
 * both an input and an output of query_planner().    The input value signals
 * query_planner that the indicated sort order is wanted in the final output
 * plan.  But this value has not yet been "canonicalized", since the needed
 * info does not get computed until we scan the qual clauses.  We canonicalize
 * it as soon as that task is done.  (The main reason query_pathkeys is a
 * PlannerInfo field and not a passed parameter is that the low-level routines
 * in indxpath.c need to see it.)
 *
 * Note: the PlannerInfo node includes other pathkeys fields besides
 * query_pathkeys, all of which need to be canonicalized once the info is
 * available.  See canonicalize_all_pathkeys.
 *
 * tuple_fraction is interpreted as follows:
 *      0: expect all tuples to be retrieved (normal case)
 *      0 < tuple_fraction < 1: expect the given fraction of tuples available
 *        from the plan to be retrieved
 *      tuple_fraction >= 1: tuple_fraction is the absolute number of tuples
 *        expected to be retrieved (ie, a LIMIT specification)
 * Note that a nonzero tuple_fraction could come from outer context; it is
 * therefore not redundant with limit_tuples.  We use limit_tuples to determine
 * whether a bounded sort can be used at runtime.
 */
void
query_planner(PlannerInfo *root, List *tlist,
              double tuple_fraction, double limit_tuples,
              Path **cheapest_path, Path **sorted_path,
              double *num_groups)
{
    ...

    /*
     * Construct RelOptInfo nodes for all base relations in query, and
     * indirectly for all appendrel member relations ("other rels").  This
     * will give us a RelOptInfo for every "simple" (non-join) rel involved in
     * the query.
     *
     * Note: the reason we find the rels by searching the jointree and
     * appendrel list, rather than just scanning the rangetable, is that the
     * rangetable may contain RTEs for rels not actively part of the query,
     * for example views.  We don't want to make RelOptInfos for them.
     */
    add_base_rels_to_query(root, (Node *) parse->jointree);

    /*
     * Examine the targetlist and join tree, adding entries to baserel
     * targetlists for all referenced Vars, and generating PlaceHolderInfo
     * entries for all referenced PlaceHolderVars.    Restrict and join clauses
     * are added to appropriate lists belonging to the mentioned relations. We
     * also build EquivalenceClasses for provably equivalent expressions. The
     * SpecialJoinInfo list is also built to hold information about join order
     * restrictions.  Finally, we form a target joinlist for make_one_rel() to
     * work from.
     */
    build_base_rel_tlists(root, tlist);

    find_placeholders_in_jointree(root);

    joinlist = deconstruct_jointree(root);
    ...
}