pathnode.c

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

/*-------------------------------------------------------------------------
 *
 * pathnode.c
 *	  Routines to manipulate pathlists and create path nodes
 *
 * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/backend/optimizer/util/pathnode.c,v 1.125 2005/10/15 02:49:21 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <math.h>

#include "catalog/pg_operator.h"
#include "executor/executor.h"
#include "miscadmin.h"
#include "nodes/plannodes.h"
#include "optimizer/clauses.h"
#include "optimizer/cost.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/restrictinfo.h"
#include "optimizer/tlist.h"
#include "parser/parse_expr.h"
#include "parser/parse_oper.h"
#include "parser/parsetree.h"
#include "utils/memutils.h"
#include "utils/selfuncs.h"
#include "utils/syscache.h"


static List *translate_sub_tlist(List *tlist, int relid);
static bool query_is_distinct_for(Query *query, List *colnos);
static bool hash_safe_tlist(List *tlist);


/*****************************************************************************
 *		MISC. PATH UTILITIES
 *****************************************************************************/

/*
 * compare_path_costs
 *	  Return -1, 0, or +1 according as path1 is cheaper, the same cost,
 *	  or more expensive than path2 for the specified criterion.
 */
int
compare_path_costs(Path *path1, Path *path2, CostSelector criterion)
{
	if (criterion == STARTUP_COST)
	{
		if (path1->startup_cost < path2->startup_cost)
			return -1;
		if (path1->startup_cost > path2->startup_cost)
			return +1;

		/*
		 * If paths have the same startup cost (not at all unlikely), order
		 * them by total cost.
		 */
		if (path1->total_cost < path2->total_cost)
			return -1;
		if (path1->total_cost > path2->total_cost)
			return +1;
	}
	else
	{
		if (path1->total_cost < path2->total_cost)
			return -1;
		if (path1->total_cost > path2->total_cost)
			return +1;

		/*
		 * If paths have the same total cost, order them by startup cost.
		 */
		if (path1->startup_cost < path2->startup_cost)
			return -1;
		if (path1->startup_cost > path2->startup_cost)
			return +1;
	}
	return 0;
}

/*
 * compare_fuzzy_path_costs
 *	  Return -1, 0, or +1 according as path1 is cheaper, the same cost,
 *	  or more expensive than path2 for the specified criterion.
 *
 * This differs from compare_path_costs in that we consider the costs the
 * same if they agree to within a "fuzz factor".  This is used by add_path
 * to avoid keeping both of a pair of paths that really have insignificantly
 * different cost.
 */
static int
compare_fuzzy_path_costs(Path *path1, Path *path2, CostSelector criterion)
{
	/*
	 * We use a fuzz factor of 1% of the smaller cost.
	 *
	 * XXX does this percentage need to be user-configurable?
	 */
	if (criterion == STARTUP_COST)
	{
		if (path1->startup_cost > path2->startup_cost * 1.01)
			return +1;
		if (path2->startup_cost > path1->startup_cost * 1.01)
			return -1;

		/*
		 * If paths have the same startup cost (not at all unlikely), order
		 * them by total cost.
		 */
		if (path1->total_cost > path2->total_cost * 1.01)
			return +1;
		if (path2->total_cost > path1->total_cost * 1.01)
			return -1;
	}
	else
	{
		if (path1->total_cost > path2->total_cost * 1.01)
			return +1;
		if (path2->total_cost > path1->total_cost * 1.01)
			return -1;

		/*
		 * If paths have the same total cost, order them by startup cost.
		 */
		if (path1->startup_cost > path2->startup_cost * 1.01)
			return +1;
		if (path2->startup_cost > path1->startup_cost * 1.01)
			return -1;
	}
	return 0;
}

/*
 * compare_path_fractional_costs
 *	  Return -1, 0, or +1 according as path1 is cheaper, the same cost,
 *	  or more expensive than path2 for fetching the specified fraction
 *	  of the total tuples.
 *
 * If fraction is <= 0 or > 1, we interpret it as 1, ie, we select the
 * path with the cheaper total_cost.
 */
int
compare_fractional_path_costs(Path *path1, Path *path2,
							  double fraction)
{
	Cost		cost1,
				cost2;

	if (fraction <= 0.0 || fraction >= 1.0)
		return compare_path_costs(path1, path2, TOTAL_COST);
	cost1 = path1->startup_cost +
		fraction * (path1->total_cost - path1->startup_cost);
	cost2 = path2->startup_cost +
		fraction * (path2->total_cost - path2->startup_cost);
	if (cost1 < cost2)
		return -1;
	if (cost1 > cost2)
		return +1;
	return 0;
}

/*
 * set_cheapest
 *	  Find the minimum-cost paths from among a relation's paths,
 *	  and save them in the rel's cheapest-path fields.
 *
 * This is normally called only after we've finished constructing the path
 * list for the rel node.
 *
 * 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 it.
 */
void
set_cheapest(RelOptInfo *parent_rel)
{
	List	   *pathlist = parent_rel->pathlist;
	ListCell   *p;
	Path	   *cheapest_startup_path;
	Path	   *cheapest_total_path;

	Assert(IsA(parent_rel, RelOptInfo));

	if (pathlist == NIL)
		elog(ERROR, "could not devise a query plan for the given query");

	cheapest_startup_path = cheapest_total_path = (Path *) linitial(pathlist);

	for_each_cell(p, lnext(list_head(pathlist)))
	{
		Path	   *path = (Path *) lfirst(p);
		int			cmp;

		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;
	}

	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 */
}

/*
 * add_path
 *	  Consider a potential implementation path for the specified parent rel,
 *	  and add it to the rel's pathlist if it is worthy of consideration.
 *	  A path is worthy if it has either a better sort order (better pathkeys)
 *	  or cheaper cost (on either dimension) than any of the existing old paths.
 *
 *	  We also remove from the rel's pathlist any old paths that are dominated
 *	  by new_path --- that is, new_path is both cheaper and at least as well
 *	  ordered.
 *
 *	  The pathlist is kept sorted by TOTAL_COST metric, with cheaper paths
 *	  at the front.  No code depends on that for correctness; it's simply
 *	  a speed hack within this routine.  Doing it that way makes it more
 *	  likely that we will reject an inferior path after a few comparisons,
 *	  rather than many comparisons.
 *
 *	  NOTE: discarded Path objects are immediately pfree'd to reduce planner
 *	  memory consumption.  We dare not try to free the substructure of a Path,
 *	  since much of it may be shared with other Paths or the query tree itself;
 *	  but just recycling discarded Path nodes is a very useful savings in
 *	  a large join tree.  We can recycle the List nodes of pathlist, too.
 *
 *	  BUT: we do not pfree IndexPath objects, since they may be referenced as
 *	  children of BitmapHeapPaths as well as being paths in their own right.
 *
 * 'parent_rel' is the relation entry to which the path corresponds.
 * 'new_path' is a potential path for parent_rel.
 *
 * Returns nothing, but modifies parent_rel->pathlist.
 */
void
add_path(RelOptInfo *parent_rel, Path *new_path)
{
	bool		accept_new = true;		/* unless we find a superior old path */
	ListCell   *insert_after = NULL;	/* where to insert new item */
	ListCell   *p1_prev = NULL;
	ListCell   *p1;

	/*
	 * This is a convenient place to check for query cancel --- no part of the
	 * planner goes very long without calling add_path().
	 */
	CHECK_FOR_INTERRUPTS();

	/*
	 * Loop to check proposed new path against old paths.  Note it is possible
	 * for more than one old path to be tossed out because new_path dominates
	 * it.
	 */
	p1 = list_head(parent_rel->pathlist);		/* cannot use foreach here */
	while (p1 != NULL)
	{
		Path	   *old_path = (Path *) lfirst(p1);
		bool		remove_old = false; /* unless new proves superior */
		int			costcmp;

		/*
		 * As of Postgres 8.0, we use fuzzy cost comparison to avoid wasting
		 * cycles keeping paths that are really not significantly different in
		 * cost.
		 */
		costcmp = compare_fuzzy_path_costs(new_path, old_path, TOTAL_COST);

		/*
		 * If the two paths compare differently for startup and total cost,
		 * then we want to keep both, and we can skip the (much slower)
		 * comparison of pathkeys.	If they compare the same, proceed with the
		 * pathkeys comparison.  Note: this test relies on the fact that
		 * compare_fuzzy_path_costs will only return 0 if both costs are
		 * effectively equal (and, therefore, there's no need to call it twice
		 * in that case).
		 */
		if (costcmp == 0 ||
			costcmp == compare_fuzzy_path_costs(new_path, old_path,
												STARTUP_COST))
		{
			switch (compare_pathkeys(new_path->pathkeys, old_path->pathkeys))
			{
				case PATHKEYS_EQUAL:
					if (costcmp < 0)
						remove_old = true;		/* new dominates old */
					else if (costcmp > 0)
						accept_new = false;		/* old dominates new */
					else
					{
						/*
						 * Same pathkeys, and fuzzily the same cost, so keep
						 * just one --- but we'll do an exact cost comparison
						 * to decide which.
						 */
						if (compare_path_costs(new_path, old_path,
											   TOTAL_COST) < 0)
							remove_old = true;	/* new dominates old */
						else
							accept_new = false; /* old equals or dominates new */
					}
					break;
				case PATHKEYS_BETTER1:
					if (costcmp <= 0)
						remove_old = true;		/* new dominates old */
					break;
				case PATHKEYS_BETTER2:
					if (costcmp >= 0)
						accept_new = false;		/* old dominates new */
					break;
				case PATHKEYS_DIFFERENT:
					/* keep both paths, since they have different ordering */
					break;
			}
		}

		/*
		 * Remove current element from pathlist if dominated by new.
		 */
		if (remove_old)
		{
			parent_rel->pathlist = list_delete_cell(parent_rel->pathlist,
													p1, p1_prev);

			/*
			 * Delete the data pointed-to by the deleted cell, if possible
			 */
			if (!IsA(old_path, IndexPath))
				pfree(old_path);
			/* Advance list pointer */
			if (p1_prev)
				p1 = lnext(p1_prev);
			else
				p1 = list_head(parent_rel->pathlist);
		}
		else
		{
			/* new belongs after this old path if it has cost >= old's */
			if (costcmp >= 0)
				insert_after = p1;
			/* Advance list pointers */
			p1_prev = p1;
			p1 = lnext(p1);
		}

		/*
		 * If we found an old path that dominates new_path, we can quit
		 * scanning the pathlist; we will not add new_path, and we assume
		 * new_path cannot dominate any other elements of the pathlist.
		 */
		if (!accept_new)
			break;
	}

	if (accept_new)
	{
		/* Accept the new path: insert it at proper place in pathlist */
		if (insert_after)
			lappend_cell(parent_rel->pathlist, insert_after, new_path);
		else
			parent_rel->pathlist = lcons(new_path, parent_rel->pathlist);
	}
	else
	{
		/* Reject and recycle the new path */
		if (!IsA(new_path, IndexPath))
			pfree(new_path);
	}
}


/*****************************************************************************
 *		PATH NODE CREATION ROUTINES
 *****************************************************************************/

/*
 * create_seqscan_path
 *	  Creates a path corresponding to a sequential scan, returning the
 *	  pathnode.
 */
Path *
create_seqscan_path(PlannerInfo *root, RelOptInfo *rel)
{
	Path	   *pathnode = makeNode(Path);

	pathnode->pathtype = T_SeqScan;
	pathnode->parent = rel;
	pathnode->pathkeys = NIL;	/* seqscan has unordered result */

	cost_seqscan(pathnode, root, rel);

	return pathnode;
}

/*
 * create_index_path
 *	  Creates a path node for an index scan.
 *
 * 'index' is a usable index.