postgresql/src/backend/optimizer/path/joinpath.c

/*-------------------------------------------------------------------------
 *
 * joinpath.c
 *	  Routines to find all possible paths for processing a set of joins
 *
 * Portions Copyright (c) 1996-2000, PostgreSQL, Inc
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/optimizer/path/joinpath.c,v 1.52 2000/02/15 20:49:17 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
#include <sys/types.h>
#include <math.h>

#include "postgres.h"

#include "access/htup.h"
#include "catalog/pg_attribute.h"
#include "optimizer/clauses.h"
#include "optimizer/cost.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/restrictinfo.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);
static void match_unsorted_outer(Query *root, RelOptInfo *joinrel,
								 RelOptInfo *outerrel, RelOptInfo *innerrel,
								 List *restrictlist, List *mergeclause_list);
#ifdef NOT_USED
static void match_unsorted_inner(Query *root, RelOptInfo *joinrel,
								 RelOptInfo *outerrel, RelOptInfo *innerrel,
								 List *restrictlist, List *mergeclause_list);
#endif
static void hash_inner_and_outer(Query *root, RelOptInfo *joinrel,
								 RelOptInfo *outerrel, RelOptInfo *innerrel,
								 List *restrictlist);
static Path *best_innerjoin(List *join_paths, List *outer_relid);
static Selectivity estimate_disbursion(Query *root, Var *var);
static List *select_mergejoin_clauses(RelOptInfo *joinrel,
									  RelOptInfo *outerrel,
									  RelOptInfo *innerrel,
									  List *restrictlist);


/*
 * 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,
					 List *restrictlist)
{
	List	   *mergeclause_list = NIL;

	/*
	 * Find potential mergejoin clauses.
	 */
	if (enable_mergejoin)
		mergeclause_list = select_mergejoin_clauses(joinrel,
													outerrel,
													innerrel,
													restrictlist);

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

	/*
	 * 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);

#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);
#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);
}

/*
 * 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
 */
static void
sort_inner_and_outer(Query *root,
					 RelOptInfo *joinrel,
					 RelOptInfo *outerrel,
					 RelOptInfo *innerrel,
					 List *restrictlist,
					 List *mergeclause_list)
{
	List	   *i;

	/*
	 * 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.  Generating a path here
	 * for *every* permutation of mergejoin clauses doesn't seem like
	 * a winning strategy, however; the cost in planning time is too high.
	 *
	 * For now, we generate one path for each mergejoin clause, listing that
	 * clause 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 clauses, 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.)
	 */
	foreach(i, mergeclause_list)
	{
		RestrictInfo   *restrictinfo = lfirst(i);
		List		   *curclause_list;
		List		   *outerkeys;
		List		   *innerkeys;
		List		   *merge_pathkeys;

		/* Make a mergeclause list with this guy first. */
		curclause_list = lcons(restrictinfo,
							   lremove(restrictinfo,
									   listCopy(mergeclause_list)));
		/* 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,
												   curclause_list,
												   outerrel->targetlist);
		innerkeys = make_pathkeys_for_mergeclauses(root,
												   curclause_list,
												   innerrel->targetlist);
		/* Build pathkeys representing output sort order. */
		merge_pathkeys = build_join_pathkeys(outerkeys,
											 joinrel->targetlist,
											 root->equi_key_list);

		/*
		 * And now we can make the path.  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.
		 */
		add_path(joinrel, (Path *)
				 create_mergejoin_path(joinrel,
									   outerrel->cheapest_total_path,
									   innerrel->cheapest_total_path,
									   restrictlist,
									   merge_pathkeys,
									   get_actual_clauses(curclause_list),
									   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 three: one on the cheapest-total-cost
 * inner path, 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
 */
static void
match_unsorted_outer(Query *root,
					 RelOptInfo *joinrel,
					 RelOptInfo *outerrel,
					 RelOptInfo *innerrel,
					 List *restrictlist,
					 List *mergeclause_list)
{
	Path	   *bestinnerjoin;
	List	   *i;

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

	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			clausecnt;

		/*
		 * 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(outerpath->pathkeys,
											 joinrel->targetlist,
											 root->equi_key_list);

		/*
		 * Always consider a nestloop join with this outer and cheapest-
		 * total-cost inner.  Consider nestloops using the cheapest-
		 * startup-cost inner as well, and the best innerjoin indexpath.
		 */
		add_path(joinrel, (Path *)
				 create_nestloop_path(joinrel,
									  outerpath,
									  innerrel->cheapest_total_path,
									  restrictlist,
									  merge_pathkeys));
		if (innerrel->cheapest_startup_path != innerrel->cheapest_total_path)
			add_path(joinrel, (Path *)
					 create_nestloop_path(joinrel,
										  outerpath,
										  innerrel->cheapest_startup_path,
										  restrictlist,
										  merge_pathkeys));
		if (bestinnerjoin != NULL)
			add_path(joinrel, (Path *)
					 create_nestloop_path(joinrel,
										  outerpath,
										  bestinnerjoin,
										  restrictlist,
										  merge_pathkeys));

		/* Look for useful mergeclauses (if any) */
		mergeclauses = find_mergeclauses_for_pathkeys(outerpath->pathkeys,
													  mergeclause_list);

		/* Done with this outer path if no chance for a mergejoin */
		if (mergeclauses == NIL)
			continue;

		/* Compute the required ordering of the inner path */
		innersortkeys = make_pathkeys_for_mergeclauses(root,
													   mergeclauses,
													   innerrel->targetlist);

		/*
		 * Generate a mergejoin on the basis of sorting the cheapest inner.
		 * Since a sort will be needed, only cheapest total cost matters.
		 */
		add_path(joinrel, (Path *)
				 create_mergejoin_path(joinrel,
									   outerpath,
									   innerrel->cheapest_total_path,
									   restrictlist,
									   merge_pathkeys,
									   get_actual_clauses(mergeclauses),
									   NIL,
									   innersortkeys));

		/*
		 * Look for presorted inner paths that satisfy the mergeclause list
		 * or any truncation thereof.  Here, we consider both cheap startup
		 * cost and cheap total cost.
		 */
		trialsortkeys = listCopy(innersortkeys); /* modifiable copy */
		cheapest_startup_inner = NULL;
		cheapest_total_inner = NULL;

		for (clausecnt = length(mergeclauses); clausecnt > 0; clausecnt--)
		{
			Path	   *innerpath;

			/* Look for an inner path ordered well enough to merge with
			 * the first 'clausecnt' mergeclauses.  NB: trialsortkeys list
			 * is modified destructively, which is why we made a copy...
			 */
			trialsortkeys = ltruncate(clausecnt, trialsortkeys);
			innerpath = get_cheapest_path_for_pathkeys(innerrel->pathlist,
													   trialsortkeys,
													   TOTAL_COST);
			if (innerpath != NULL &&
				(cheapest_total_inner == NULL ||
				 compare_path_costs(innerpath, cheapest_total_inner,
									TOTAL_COST) < 0))
			{
				/* Found a cheap (or even-cheaper) sorted path */
				List   *newclauses;

				newclauses = ltruncate(clausecnt,
									   get_actual_clauses(mergeclauses));
				add_path(joinrel, (Path *)
						 create_mergejoin_path(joinrel,
											   outerpath,
											   innerpath,
											   restrictlist,
											   merge_pathkeys,
											   newclauses,
											   NIL,
											   NIL));
				cheapest_total_inner = innerpath;
			}
			/* Same on the basis of cheapest startup cost ... */
			innerpath = get_cheapest_path_for_pathkeys(innerrel->pathlist,
													   trialsortkeys,
													   STARTUP_COST);
			if (innerpath != NULL &&
				(cheapest_startup_inner == NULL ||
				 compare_path_costs(innerpath, cheapest_startup_inner,
									STARTUP_COST) < 0))
			{
				/* Found a cheap (or even-cheaper) sorted path */
				if (innerpath != cheapest_total_inner)
				{
					List   *newclauses;

					newclauses = ltruncate(clausecnt,
										   get_actual_clauses(mergeclauses));
					add_path(joinrel, (Path *)
							 create_mergejoin_path(joinrel,
												   outerpath,
												   innerpath,
												   restrictlist,
												   merge_pathkeys,
												   newclauses,
												   NIL,
												   NIL));
				}
				cheapest_startup_inner = innerpath;
			}
		}
	}
}

#ifdef NOT_USED

/*
 * match_unsorted_inner
 *	  Generate mergejoin paths that use an explicit sort of the outer path
 *	  with an already-ordered inner path.
 *
 * 'joinrel' is the join result 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
 */
static void
match_unsorted_inner(Query *root,
					 RelOptInfo *joinrel,
					 RelOptInfo *outerrel,
					 RelOptInfo *innerrel,
					 List *restrictlist,
					 List *mergeclause_list)
{
	List	   *i;

	foreach(i, innerrel->pathlist)
	{
		Path	   *innerpath = (Path *) lfirst(i);
		List	   *mergeclauses;
		List	   *outersortkeys;
		List	   *merge_pathkeys;
		Path	   *totalouterpath;
		Path	   *startupouterpath;

		/* Look for useful mergeclauses (if any) */
		mergeclauses = find_mergeclauses_for_pathkeys(innerpath->pathkeys,
													  mergeclause_list);
		if (mergeclauses == NIL)
			continue;

		/* Compute the required ordering of the outer path */
		outersortkeys = make_pathkeys_for_mergeclauses(root,
													   mergeclauses,
													   outerrel->targetlist);

		/*
		 * Generate a mergejoin on the basis of sorting the cheapest outer.
		 * Since a sort will be needed, only cheapest total cost matters.
		 */
		merge_pathkeys = build_join_pathkeys(outersortkeys,
											 joinrel->targetlist,
											 root->equi_key_list);
		add_path(joinrel, (Path *)
				 create_mergejoin_path(joinrel,
									   outerrel->cheapest_total_path,
									   innerpath,
									   restrictlist,
									   merge_pathkeys,
									   get_actual_clauses(mergeclauses),
									   outersortkeys,
									   NIL));
		/*
		 * Now generate mergejoins based on already-sufficiently-ordered
		 * outer paths.  There's likely to be some redundancy here with paths
		 * already generated by merge_unsorted_outer ... but since
		 * merge_unsorted_outer doesn't consider all permutations of the
		 * mergeclause list, it may fail to notice that this particular
		 * innerpath could have been used with this outerpath.
		 */
		totalouterpath = get_cheapest_path_for_pathkeys(outerrel->pathlist,
														outersortkeys,
														TOTAL_COST);
		if (totalouterpath == NULL)
			continue;			/* there won't be a startup-cost path either */

		merge_pathkeys = build_join_pathkeys(totalouterpath->pathkeys,
											 joinrel->targetlist,
											 root->equi_key_list);
		add_path(joinrel, (Path *)
				 create_mergejoin_path(joinrel,
									   totalouterpath,
									   innerpath,
									   restrictlist,
									   merge_pathkeys,
									   get_actual_clauses(mergeclauses),
									   NIL,
									   NIL));

		startupouterpath = get_cheapest_path_for_pathkeys(outerrel->pathlist,
														  outersortkeys,
														  STARTUP_COST);
		if (startupouterpath != NULL && startupouterpath != totalouterpath)
		{
			merge_pathkeys = build_join_pathkeys(startupouterpath->pathkeys,
												 joinrel->targetlist,
												 root->equi_key_list);
			add_path(joinrel, (Path *)
					 create_mergejoin_path(joinrel,
										   startupouterpath,
										   innerpath,
										   restrictlist,
										   merge_pathkeys,
										   get_actual_clauses(mergeclauses),
										   NIL,
										   NIL));
		}
	}
}

#endif

/*
 * hash_inner_and_outer
 *	  Create hashjoin join paths by explicitly hashing both the outer and
 *	  inner join relations of each available hash clause.
 *
 * '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
 */
static void
hash_inner_and_outer(Query *root,
					 RelOptInfo *joinrel,
					 RelOptInfo *outerrel,
					 RelOptInfo *innerrel,
					 List *restrictlist)
{
	Relids		outerrelids = outerrel->relids;
	Relids		innerrelids = innerrel->relids;
	List	   *i;

	/*
	 * Scan the join's restrictinfo list to find hashjoinable clauses
	 * that are usable with this pair of sub-relations.  Since we currently
	 * accept only var-op-var clauses as hashjoinable, we need only check
	 * the membership of the vars to determine whether a particular clause
	 * can be used with this pair of sub-relations.  This code would need
	 * to be upgraded if we wanted to allow more-complex expressions in
	 * hash joins.
	 */
	foreach(i, restrictlist)
	{
		RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(i);
		Expr	   *clause;
		Var		   *left,
				   *right,
				   *inner;
		Selectivity	innerdisbursion;

		if (restrictinfo->hashjoinoperator == InvalidOid)
			continue;			/* not hashjoinable */

		clause = restrictinfo->clause;
		/* these must be OK, since check_hashjoinable accepted the clause */
		left = get_leftop(clause);
		right = get_rightop(clause);

		/* check if clause is usable with these sub-rels, find inner var */
		if (intMember(left->varno, outerrelids) &&
			intMember(right->varno, innerrelids))
			inner = right;
		else if (intMember(left->varno, innerrelids) &&
				 intMember(right->varno, outerrelids))
			inner = left;
		else
			continue;			/* no good for these input relations */

		/* estimate disbursion of inner var for costing purposes */
		innerdisbursion = estimate_disbursion(root, inner);

		/*
		 * We consider both the cheapest-total-cost and cheapest-startup-cost
		 * outer paths.  There's no need to consider any but the cheapest-
		 * total-cost inner path, however.
		 */
		add_path(joinrel, (Path *)
				 create_hashjoin_path(joinrel,
									  outerrel->cheapest_total_path,
									  innerrel->cheapest_total_path,
									  restrictlist,
									  lcons(clause, NIL),
									  innerdisbursion));
		if (outerrel->cheapest_startup_path != outerrel->cheapest_total_path)
			add_path(joinrel, (Path *)
					 create_hashjoin_path(joinrel,
										  outerrel->cheapest_startup_path,
										  innerrel->cheapest_total_path,
										  restrictlist,
										  lcons(clause, NIL),
										  innerdisbursion));
	}
}

/*
 * best_innerjoin
 *	  Find the cheapest index path that has already been identified by
 *	  indexable_joinclauses() as being a possible inner path for the given
 *	  outer relation(s) in a nestloop join.
 *
 * We compare indexpaths on total_cost only, assuming that they will all have
 * zero or negligible startup_cost.  We might have to think harder someday...
 *
 * 'join_paths' is a list of potential inner indexscan join paths
 * 'outer_relids' is the relid list of the outer join relation
 *
 * Returns the pathnode of the best path, or NULL if there's no
 * usable path.
 */
static Path *
best_innerjoin(List *join_paths, Relids outer_relids)
{
	Path	   *cheapest = (Path *) NULL;
	List	   *join_path;

	foreach(join_path, join_paths)
	{
		Path	   *path = (Path *) lfirst(join_path);

		Assert(IsA(path, IndexPath));

		/* path->joinrelids is the set of base rels that must be part of
		 * outer_relids in order to use this inner path, because those
		 * rels are used in the index join quals of this inner path.
		 */
		if (is_subseti(((IndexPath *) path)->joinrelids, outer_relids) &&
			(cheapest == NULL ||
			 compare_path_costs(path, cheapest, TOTAL_COST) < 0))
			cheapest = path;
	}
	return cheapest;
}

/*
 * Estimate disbursion of the specified Var
 *
 * We use a default of 0.1 if we can't figure out anything better.
 * This will typically discourage use of a hash rather strongly,
 * if the inner relation is large.  We do not want to hash unless
 * we know that the inner rel is well-dispersed (or the alternatives
 * seem much worse).
 */
static Selectivity
estimate_disbursion(Query *root, Var *var)
{
	Oid			relid;

	if (! IsA(var, Var))
		return 0.1;

	relid = getrelid(var->varno, root->rtable);

	return (Selectivity) get_attdisbursion(relid, var->varattno, 0.1);
}

/*
 * select_mergejoin_clauses
 *	  Select mergejoin clauses that are usable for a particular join.
 *	  Returns a list of RestrictInfo nodes for those clauses.
 *
 * We examine each restrictinfo clause known for the join to see
 * if it is mergejoinable and involves vars from the two sub-relations
 * currently of interest.
 *
 * Since we currently allow only plain Vars as the left and right sides
 * of mergejoin clauses, this test is relatively simple.  This routine
 * would need to be upgraded to support more-complex expressions
 * as sides of mergejoins.  In theory, we could allow arbitrarily complex
 * expressions in mergejoins, so long as one side uses only vars from one
 * sub-relation and the other side uses only vars from the other.
 */
static List *
select_mergejoin_clauses(RelOptInfo *joinrel,
						 RelOptInfo *outerrel,
						 RelOptInfo *innerrel,
						 List *restrictlist)
{
	List	   *result_list = NIL;
	Relids		outerrelids = outerrel->relids;
	Relids		innerrelids = innerrel->relids;
	List	   *i;

	foreach(i, restrictlist)
	{
		RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(i);
		Expr	   *clause;
		Var		   *left,
				   *right;

		if (restrictinfo->mergejoinoperator == InvalidOid)
			continue;			/* not mergejoinable */

		clause = restrictinfo->clause;
		/* these must be OK, since check_mergejoinable accepted the clause */
		left = get_leftop(clause);
		right = get_rightop(clause);

		if ((intMember(left->varno, outerrelids) &&
			 intMember(right->varno, innerrelids)) ||
			(intMember(left->varno, innerrelids) &&
			 intMember(right->varno, outerrelids)))
			result_list = lcons(restrictinfo, result_list);
	}

	return result_list;
}