joinpath.c

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/*-------------------------------------------------------------------------
 *
 * joinpath.c
 *	  Routines to find all possible paths for processing a set of joins
 *
 * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/optimizer/path/joinpath.c,v 1.82.2.1 2004/04/06 18:46:25 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <math.h>

#include "optimizer/clauses.h"
#include "optimizer/cost.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "parser/parsetree.h"
#include "utils/lsyscache.h"


static void sort_inner_and_outer(Query *root, RelOptInfo *joinrel,
					 RelOptInfo *outerrel, RelOptInfo *innerrel,
					 List *restrictlist, List *mergeclause_list,
					 JoinType jointype);
static void match_unsorted_outer(Query *root, RelOptInfo *joinrel,
					 RelOptInfo *outerrel, RelOptInfo *innerrel,
					 List *restrictlist, List *mergeclause_list,
					 JoinType jointype);
static void hash_inner_and_outer(Query *root, RelOptInfo *joinrel,
					 RelOptInfo *outerrel, RelOptInfo *innerrel,
					 List *restrictlist, JoinType jointype);
static List *select_mergejoin_clauses(RelOptInfo *joinrel,
						 RelOptInfo *outerrel,
						 RelOptInfo *innerrel,
						 List *restrictlist,
						 JoinType jointype);


/*
 * add_paths_to_joinrel
 *	  Given a join relation and two component rels from which it can be made,
 *	  consider all possible paths that use the two component rels as outer
 *	  and inner rel respectively.  Add these paths to the join rel's pathlist
 *	  if they survive comparison with other paths (and remove any existing
 *	  paths that are dominated by these paths).
 *
 * Modifies the pathlist field of the joinrel node to contain the best
 * paths found so far.
 */
void
add_paths_to_joinrel(Query *root,
					 RelOptInfo *joinrel,
					 RelOptInfo *outerrel,
					 RelOptInfo *innerrel,
					 JoinType jointype,
					 List *restrictlist)
{
	List	   *mergeclause_list = NIL;

	/*
	 * Find potential mergejoin clauses.  We can skip this if we are not
	 * interested in doing a mergejoin.  However, mergejoin is currently
	 * our only way of implementing full outer joins, so override
	 * mergejoin disable if it's a full join.
	 */
	if (enable_mergejoin || jointype == JOIN_FULL)
		mergeclause_list = select_mergejoin_clauses(joinrel,
													outerrel,
													innerrel,
													restrictlist,
													jointype);

	/*
	 * 1. Consider mergejoin paths where both relations must be explicitly
	 * sorted.
	 */
	sort_inner_and_outer(root, joinrel, outerrel, innerrel,
						 restrictlist, mergeclause_list, jointype);

	/*
	 * 2. Consider paths where the outer relation need not be explicitly
	 * sorted. This includes both nestloops and mergejoins where the outer
	 * path is already ordered.
	 */
	match_unsorted_outer(root, joinrel, outerrel, innerrel,
						 restrictlist, mergeclause_list, jointype);

#ifdef NOT_USED

	/*
	 * 3. Consider paths where the inner relation need not be explicitly
	 * sorted.	This includes mergejoins only (nestloops were already
	 * built in match_unsorted_outer).
	 *
	 * Diked out as redundant 2/13/2000 -- tgl.  There isn't any really
	 * significant difference between the inner and outer side of a
	 * mergejoin, so match_unsorted_inner creates no paths that aren't
	 * equivalent to those made by match_unsorted_outer when
	 * add_paths_to_joinrel() is invoked with the two rels given in the
	 * other order.
	 */
	match_unsorted_inner(root, joinrel, outerrel, innerrel,
						 restrictlist, mergeclause_list, jointype);
#endif

	/*
	 * 4. Consider paths where both outer and inner relations must be
	 * hashed before being joined.
	 */
	if (enable_hashjoin)
		hash_inner_and_outer(root, joinrel, outerrel, innerrel,
							 restrictlist, jointype);
}

/*
 * sort_inner_and_outer
 *	  Create mergejoin join paths by explicitly sorting both the outer and
 *	  inner join relations on each available merge ordering.
 *
 * 'joinrel' is the join relation
 * 'outerrel' is the outer join relation
 * 'innerrel' is the inner join relation
 * 'restrictlist' contains all of the RestrictInfo nodes for restriction
 *		clauses that apply to this join
 * 'mergeclause_list' is a list of RestrictInfo nodes for available
 *		mergejoin clauses in this join
 * 'jointype' is the type of join to do
 */
static void
sort_inner_and_outer(Query *root,
					 RelOptInfo *joinrel,
					 RelOptInfo *outerrel,
					 RelOptInfo *innerrel,
					 List *restrictlist,
					 List *mergeclause_list,
					 JoinType jointype)
{
	bool		useallclauses;
	Path	   *outer_path;
	Path	   *inner_path;
	List	   *all_pathkeys;
	List	   *i;

	/*
	 * If we are doing a right or full join, we must use *all* the
	 * mergeclauses as join clauses, else we will not have a valid plan.
	 */
	switch (jointype)
	{
		case JOIN_INNER:
		case JOIN_LEFT:
		case JOIN_IN:
		case JOIN_UNIQUE_OUTER:
		case JOIN_UNIQUE_INNER:
			useallclauses = false;
			break;
		case JOIN_RIGHT:
		case JOIN_FULL:
			useallclauses = true;
			break;
		default:
			elog(ERROR, "unrecognized join type: %d",
				 (int) jointype);
			useallclauses = false;		/* keep compiler quiet */
			break;
	}

	/*
	 * We only consider the cheapest-total-cost input paths, since we are
	 * assuming here that a sort is required.  We will consider
	 * cheapest-startup-cost input paths later, and only if they don't
	 * need a sort.
	 *
	 * If unique-ification is requested, do it and then handle as a plain
	 * inner join.
	 */
	outer_path = outerrel->cheapest_total_path;
	inner_path = innerrel->cheapest_total_path;
	if (jointype == JOIN_UNIQUE_OUTER)
	{
		outer_path = (Path *) create_unique_path(root, outerrel, outer_path);
		jointype = JOIN_INNER;
	}
	else if (jointype == JOIN_UNIQUE_INNER)
	{
		inner_path = (Path *) create_unique_path(root, innerrel, inner_path);
		jointype = JOIN_INNER;
	}

	/*
	 * Each possible ordering of the available mergejoin clauses will
	 * generate a differently-sorted result path at essentially the same
	 * cost.  We have no basis for choosing one over another at this level
	 * of joining, but some sort orders may be more useful than others for
	 * higher-level mergejoins, so it's worth considering multiple
	 * orderings.
	 *
	 * Actually, it's not quite true that every mergeclause ordering will
	 * generate a different path order, because some of the clauses may be
	 * redundant.  Therefore, what we do is convert the mergeclause list
	 * to a list of canonical pathkeys, and then consider different
	 * orderings of the pathkeys.
	 *
	 * Generating a path for *every* permutation of the pathkeys doesn't seem
	 * like a winning strategy; the cost in planning time is too high. For
	 * now, we generate one path for each pathkey, listing that pathkey
	 * first and the rest in random order.	This should allow at least a
	 * one-clause mergejoin without re-sorting against any other possible
	 * mergejoin partner path.	But if we've not guessed the right
	 * ordering of secondary keys, we may end up evaluating clauses as
	 * qpquals when they could have been done as mergeclauses. We need to
	 * figure out a better way.  (Two possible approaches: look at all the
	 * relevant index relations to suggest plausible sort orders, or make
	 * just one output path and somehow mark it as having a sort-order
	 * that can be rearranged freely.)
	 */
	all_pathkeys = make_pathkeys_for_mergeclauses(root,
												  mergeclause_list,
												  outerrel);

	foreach(i, all_pathkeys)
	{
		List	   *front_pathkey = lfirst(i);
		List	   *cur_pathkeys;
		List	   *cur_mergeclauses;
		List	   *outerkeys;
		List	   *innerkeys;
		List	   *merge_pathkeys;

		/* Make a pathkey list with this guy first. */
		if (i != all_pathkeys)
			cur_pathkeys = lcons(front_pathkey,
								 lremove(front_pathkey,
										 listCopy(all_pathkeys)));
		else
			cur_pathkeys = all_pathkeys;		/* no work at first one... */

		/*
		 * Select mergeclause(s) that match this sort ordering.  If we had
		 * redundant merge clauses then we will get a subset of the
		 * original clause list.  There had better be some match,
		 * however...
		 */
		cur_mergeclauses = find_mergeclauses_for_pathkeys(root,
														  cur_pathkeys,
													   mergeclause_list);
		Assert(cur_mergeclauses != NIL);

		/* Forget it if can't use all the clauses in right/full join */
		if (useallclauses &&
			length(cur_mergeclauses) != length(mergeclause_list))
			continue;

		/*
		 * Build sort pathkeys for both sides.
		 *
		 * Note: it's possible that the cheapest paths will already be sorted
		 * properly.  create_mergejoin_path will detect that case and
		 * suppress an explicit sort step, so we needn't do so here.
		 */
		outerkeys = make_pathkeys_for_mergeclauses(root,
												   cur_mergeclauses,
												   outerrel);
		innerkeys = make_pathkeys_for_mergeclauses(root,
												   cur_mergeclauses,
												   innerrel);
		/* Build pathkeys representing output sort order. */
		merge_pathkeys = build_join_pathkeys(root, joinrel, outerkeys);

		/*
		 * And now we can make the path.
		 */
		add_path(joinrel, (Path *)
				 create_mergejoin_path(root,
									   joinrel,
									   jointype,
									   outer_path,
									   inner_path,
									   restrictlist,
									   merge_pathkeys,
									   cur_mergeclauses,
									   outerkeys,
									   innerkeys));
	}
}

/*
 * match_unsorted_outer
 *	  Creates possible join paths for processing a single join relation
 *	  'joinrel' by employing either iterative substitution or
 *	  mergejoining on each of its possible outer paths (considering
 *	  only outer paths that are already ordered well enough for merging).
 *
 * We always generate a nestloop path for each available outer path.
 * In fact we may generate as many as four: one on the cheapest-total-cost
 * inner path, one on the same with materialization, one on the
 * cheapest-startup-cost inner path (if different),
 * and one on the best inner-indexscan path (if any).
 *
 * We also consider mergejoins if mergejoin clauses are available.	We have
 * two ways to generate the inner path for a mergejoin: sort the cheapest
 * inner path, or use an inner path that is already suitably ordered for the
 * merge.  If we have several mergeclauses, it could be that there is no inner
 * path (or only a very expensive one) for the full list of mergeclauses, but
 * better paths exist if we truncate the mergeclause list (thereby discarding
 * some sort key requirements).  So, we consider truncations of the
 * mergeclause list as well as the full list.  (Ideally we'd consider all
 * subsets of the mergeclause list, but that seems way too expensive.)
 *
 * 'joinrel' is the join relation
 * 'outerrel' is the outer join relation
 * 'innerrel' is the inner join relation
 * 'restrictlist' contains all of the RestrictInfo nodes for restriction
 *		clauses that apply to this join
 * 'mergeclause_list' is a list of RestrictInfo nodes for available
 *		mergejoin clauses in this join
 * 'jointype' is the type of join to do
 */
static void
match_unsorted_outer(Query *root,
					 RelOptInfo *joinrel,
					 RelOptInfo *outerrel,
					 RelOptInfo *innerrel,
					 List *restrictlist,
					 List *mergeclause_list,
					 JoinType jointype)
{
	JoinType	save_jointype = jointype;
	bool		nestjoinOK;
	bool		useallclauses;
	Path	   *inner_cheapest_startup = innerrel->cheapest_startup_path;
	Path	   *inner_cheapest_total = innerrel->cheapest_total_path;
	Path	   *matpath = NULL;
	Path	   *bestinnerjoin = NULL;
	List	   *i;

	/*
	 * Nestloop only supports inner, left, and IN joins.  Also, if we are
	 * doing a right or full join, we must use *all* the mergeclauses as
	 * join clauses, else we will not have a valid plan.  (Although these
	 * two flags are currently inverses, keep them separate for clarity
	 * and possible future changes.)
	 */
	switch (jointype)
	{
		case JOIN_INNER:
		case JOIN_LEFT:
		case JOIN_IN:
		case JOIN_UNIQUE_OUTER:
		case JOIN_UNIQUE_INNER:
			nestjoinOK = true;
			useallclauses = false;
			break;
		case JOIN_RIGHT:
		case JOIN_FULL:
			nestjoinOK = false;
			useallclauses = true;
			break;
		default:
			elog(ERROR, "unrecognized join type: %d",
				 (int) jointype);
			nestjoinOK = false; /* keep compiler quiet */
			useallclauses = false;
			break;
	}

	/*
	 * If we need to unique-ify the inner path, we will consider only the
	 * cheapest inner.
	 */
	if (jointype == JOIN_UNIQUE_INNER)
	{
		inner_cheapest_total = (Path *)
			create_unique_path(root, innerrel, inner_cheapest_total);
		inner_cheapest_startup = inner_cheapest_total;
		jointype = JOIN_INNER;
	}
	else if (nestjoinOK)
	{
		/*
		 * If the cheapest inner path is a join or seqscan, we should
		 * consider materializing it.  (This is a heuristic: we could
		 * consider it always, but for inner indexscans it's probably a
		 * waste of time.)
		 */
		if (!(IsA(inner_cheapest_total, IndexPath) ||
			  IsA(inner_cheapest_total, TidPath)))
			matpath = (Path *)
				create_material_path(innerrel, inner_cheapest_total);

		/*
		 * Get the best innerjoin indexpath (if any) for this outer rel.
		 * It's the same for all outer paths.
		 */
		bestinnerjoin = best_inner_indexscan(root, innerrel,
											 outerrel->relids, jointype);
	}

	foreach(i, outerrel->pathlist)
	{
		Path	   *outerpath = (Path *) lfirst(i);
		List	   *merge_pathkeys;
		List	   *mergeclauses;
		List	   *innersortkeys;
		List	   *trialsortkeys;
		Path	   *cheapest_startup_inner;
		Path	   *cheapest_total_inner;
		int			num_sortkeys;
		int			sortkeycnt;

		/*
		 * If we need to unique-ify the outer path, it's pointless to
		 * consider any but the cheapest outer.
		 */
		if (save_jointype == JOIN_UNIQUE_OUTER)
		{
			if (outerpath != outerrel->cheapest_total_path)
				continue;
			outerpath = (Path *) create_unique_path(root, outerrel, outerpath);
			jointype = JOIN_INNER;
		}

		/*
		 * The result will have this sort order (even if it is implemented
		 * as a nestloop, and even if some of the mergeclauses are
		 * implemented by qpquals rather than as true mergeclauses):
		 */
		merge_pathkeys = build_join_pathkeys(root, joinrel,
											 outerpath->pathkeys);

		if (nestjoinOK)