postgres/src/backend/optimizer/plan/analyzejoins.c

/*-------------------------------------------------------------------------
 *
 * analyzejoins.c
 *	  Routines for simplifying joins after initial query analysis
 *
 * While we do a great deal of join simplification in prep/prepjointree.c,
 * certain optimizations cannot be performed at that stage for lack of
 * detailed information about the query.  The routines here are invoked
 * after initsplan.c has done its work, and can do additional join removal
 * and simplification steps based on the information extracted.  The penalty
 * is that we have to work harder to clean up after ourselves when we modify
 * the query, since the derived data structures have to be updated too.
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  src/backend/optimizer/plan/analyzejoins.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "nodes/nodeFuncs.h"
#include "optimizer/clauses.h"
#include "optimizer/joininfo.h"
#include "optimizer/pathnode.h"
#include "optimizer/paths.h"
#include "optimizer/planmain.h"
#include "optimizer/tlist.h"
#include "optimizer/var.h"
#include "utils/lsyscache.h"

/* local functions */
static bool join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo);
static void remove_rel_from_query(PlannerInfo *root, int relid,
					  Relids joinrelids);
static List *remove_rel_from_joinlist(List *joinlist, int relid, int *nremoved);
static bool rel_supports_distinctness(PlannerInfo *root, RelOptInfo *rel);
static bool rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel,
					List *clause_list);
static Oid	distinct_col_search(int colno, List *colnos, List *opids);


/*
 * remove_useless_joins
 *		Check for relations that don't actually need to be joined at all,
 *		and remove them from the query.
 *
 * We are passed the current joinlist and return the updated list.  Other
 * data structures that have to be updated are accessible via "root".
 */
List *
remove_useless_joins(PlannerInfo *root, List *joinlist)
{
	ListCell   *lc;

	/*
	 * We are only interested in relations that are left-joined to, so we can
	 * scan the join_info_list to find them easily.
	 */
restart:
	foreach(lc, root->join_info_list)
	{
		SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(lc);
		int			innerrelid;
		int			nremoved;

		/* Skip if not removable */
		if (!join_is_removable(root, sjinfo))
			continue;

		/*
		 * Currently, join_is_removable can only succeed when the sjinfo's
		 * righthand is a single baserel.  Remove that rel from the query and
		 * joinlist.
		 */
		innerrelid = bms_singleton_member(sjinfo->min_righthand);

		remove_rel_from_query(root, innerrelid,
							  bms_union(sjinfo->min_lefthand,
										sjinfo->min_righthand));

		/* We verify that exactly one reference gets removed from joinlist */
		nremoved = 0;
		joinlist = remove_rel_from_joinlist(joinlist, innerrelid, &nremoved);
		if (nremoved != 1)
			elog(ERROR, "failed to find relation %d in joinlist", innerrelid);

		/*
		 * We can delete this SpecialJoinInfo from the list too, since it's no
		 * longer of interest.
		 */
		root->join_info_list = list_delete_ptr(root->join_info_list, sjinfo);

		/*
		 * Restart the scan.  This is necessary to ensure we find all
		 * removable joins independently of ordering of the join_info_list
		 * (note that removal of attr_needed bits may make a join appear
		 * removable that did not before).  Also, since we just deleted the
		 * current list cell, we'd have to have some kluge to continue the
		 * list scan anyway.
		 */
		goto restart;
	}

	return joinlist;
}

/*
 * clause_sides_match_join
 *	  Determine whether a join clause is of the right form to use in this join.
 *
 * We already know that the clause is a binary opclause referencing only the
 * rels in the current join.  The point here is to check whether it has the
 * form "outerrel_expr op innerrel_expr" or "innerrel_expr op outerrel_expr",
 * rather than mixing outer and inner vars on either side.  If it matches,
 * we set the transient flag outer_is_left to identify which side is which.
 */
static inline bool
clause_sides_match_join(RestrictInfo *rinfo, Relids outerrelids,
						Relids innerrelids)
{
	if (bms_is_subset(rinfo->left_relids, outerrelids) &&
		bms_is_subset(rinfo->right_relids, innerrelids))
	{
		/* lefthand side is outer */
		rinfo->outer_is_left = true;
		return true;
	}
	else if (bms_is_subset(rinfo->left_relids, innerrelids) &&
			 bms_is_subset(rinfo->right_relids, outerrelids))
	{
		/* righthand side is outer */
		rinfo->outer_is_left = false;
		return true;
	}
	return false;				/* no good for these input relations */
}

/*
 * join_is_removable
 *	  Check whether we need not perform this special join at all, because
 *	  it will just duplicate its left input.
 *
 * This is true for a left join for which the join condition cannot match
 * more than one inner-side row.  (There are other possibly interesting
 * cases, but we don't have the infrastructure to prove them.)  We also
 * have to check that the inner side doesn't generate any variables needed
 * above the join.
 */
static bool
join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
{
	int			innerrelid;
	RelOptInfo *innerrel;
	Relids		joinrelids;
	List	   *clause_list = NIL;
	ListCell   *l;
	int			attroff;

	/*
	 * Must be a non-delaying left join to a single baserel, else we aren't
	 * going to be able to do anything with it.
	 */
	if (sjinfo->jointype != JOIN_LEFT ||
		sjinfo->delay_upper_joins)
		return false;

	if (!bms_get_singleton_member(sjinfo->min_righthand, &innerrelid))
		return false;

	innerrel = find_base_rel(root, innerrelid);

	/*
	 * Before we go to the effort of checking whether any innerrel variables
	 * are needed above the join, make a quick check to eliminate cases in
	 * which we will surely be unable to prove uniqueness of the innerrel.
	 */
	if (!rel_supports_distinctness(root, innerrel))
		return false;

	/* Compute the relid set for the join we are considering */
	joinrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);

	/*
	 * We can't remove the join if any inner-rel attributes are used above the
	 * join.
	 *
	 * Note that this test only detects use of inner-rel attributes in higher
	 * join conditions and the target list.  There might be such attributes in
	 * pushed-down conditions at this join, too.  We check that case below.
	 *
	 * As a micro-optimization, it seems better to start with max_attr and
	 * count down rather than starting with min_attr and counting up, on the
	 * theory that the system attributes are somewhat less likely to be wanted
	 * and should be tested last.
	 */
	for (attroff = innerrel->max_attr - innerrel->min_attr;
		 attroff >= 0;
		 attroff--)
	{
		if (!bms_is_subset(innerrel->attr_needed[attroff], joinrelids))
			return false;
	}

	/*
	 * Similarly check that the inner rel isn't needed by any PlaceHolderVars
	 * that will be used above the join.  We only need to fail if such a PHV
	 * actually references some inner-rel attributes; but the correct check
	 * for that is relatively expensive, so we first check against ph_eval_at,
	 * which must mention the inner rel if the PHV uses any inner-rel attrs as
	 * non-lateral references.  Note that if the PHV's syntactic scope is just
	 * the inner rel, we can't drop the rel even if the PHV is variable-free.
	 */
	foreach(l, root->placeholder_list)
	{
		PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(l);

		if (bms_overlap(phinfo->ph_lateral, innerrel->relids))
			return false;		/* it references innerrel laterally */
		if (bms_is_subset(phinfo->ph_needed, joinrelids))
			continue;			/* PHV is not used above the join */
		if (!bms_overlap(phinfo->ph_eval_at, innerrel->relids))
			continue;			/* it definitely doesn't reference innerrel */
		if (bms_is_subset(phinfo->ph_eval_at, innerrel->relids))
			return false;		/* there isn't any other place to eval PHV */
		if (bms_overlap(pull_varnos((Node *) phinfo->ph_var->phexpr),
						innerrel->relids))
			return false;		/* it does reference innerrel */
	}

	/*
	 * Search for mergejoinable clauses that constrain the inner rel against
	 * either the outer rel or a pseudoconstant.  If an operator is
	 * mergejoinable then it behaves like equality for some btree opclass, so
	 * it's what we want.  The mergejoinability test also eliminates clauses
	 * containing volatile functions, which we couldn't depend on.
	 */
	foreach(l, innerrel->joininfo)
	{
		RestrictInfo *restrictinfo = (RestrictInfo *) lfirst(l);

		/*
		 * If it's not a join clause for this outer join, we can't use it.
		 * Note that if the clause is pushed-down, then it is logically from
		 * above the outer join, even if it references no other rels (it might
		 * be from WHERE, for example).
		 */
		if (restrictinfo->is_pushed_down ||
			!bms_equal(restrictinfo->required_relids, joinrelids))
		{
			/*
			 * If such a clause actually references the inner rel then join
			 * removal has to be disallowed.  We have to check this despite
			 * the previous attr_needed checks because of the possibility of
			 * pushed-down clauses referencing the rel.
			 */
			if (bms_is_member(innerrelid, restrictinfo->clause_relids))
				return false;
			continue;			/* else, ignore; not useful here */
		}

		/* Ignore if it's not a mergejoinable clause */
		if (!restrictinfo->can_join ||
			restrictinfo->mergeopfamilies == NIL)
			continue;			/* not mergejoinable */

		/*
		 * Check if clause has the form "outer op inner" or "inner op outer",
		 * and if so mark which side is inner.
		 */
		if (!clause_sides_match_join(restrictinfo, sjinfo->min_lefthand,
									 innerrel->relids))
			continue;			/* no good for these input relations */

		/* OK, add to list */
		clause_list = lappend(clause_list, restrictinfo);
	}

	/*
	 * Now that we have the relevant equality join clauses, try to prove the
	 * innerrel distinct.
	 */
	if (rel_is_distinct_for(root, innerrel, clause_list))
		return true;

	/*
	 * Some day it would be nice to check for other methods of establishing
	 * distinctness.
	 */
	return false;
}


/*
 * Remove the target relid from the planner's data structures, having
 * determined that there is no need to include it in the query.
 *
 * We are not terribly thorough here.  We must make sure that the rel is
 * no longer treated as a baserel, and that attributes of other baserels
 * are no longer marked as being needed at joins involving this rel.
 * Also, join quals involving the rel have to be removed from the joininfo
 * lists, but only if they belong to the outer join identified by joinrelids.
 */
static void
remove_rel_from_query(PlannerInfo *root, int relid, Relids joinrelids)
{
	RelOptInfo *rel = find_base_rel(root, relid);
	List	   *joininfos;
	Index		rti;
	ListCell   *l;
	ListCell   *nextl;

	/*
	 * Mark the rel as "dead" to show it is no longer part of the join tree.
	 * (Removing it from the baserel array altogether seems too risky.)
	 */
	rel->reloptkind = RELOPT_DEADREL;

	/*
	 * Remove references to the rel from other baserels' attr_needed arrays.
	 */
	for (rti = 1; rti < root->simple_rel_array_size; rti++)
	{
		RelOptInfo *otherrel = root->simple_rel_array[rti];
		int			attroff;

		/* there may be empty slots corresponding to non-baserel RTEs */
		if (otherrel == NULL)
			continue;

		Assert(otherrel->relid == rti); /* sanity check on array */

		/* no point in processing target rel itself */
		if (otherrel == rel)
			continue;

		for (attroff = otherrel->max_attr - otherrel->min_attr;
			 attroff >= 0;
			 attroff--)
		{
			otherrel->attr_needed[attroff] =
				bms_del_member(otherrel->attr_needed[attroff], relid);
		}
	}

	/*
	 * Likewise remove references from SpecialJoinInfo data structures.
	 *
	 * This is relevant in case the outer join we're deleting is nested inside
	 * other outer joins: the upper joins' relid sets have to be adjusted. The
	 * RHS of the target outer join will be made empty here, but that's OK
	 * since caller will delete that SpecialJoinInfo entirely.
	 */
	foreach(l, root->join_info_list)
	{
		SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(l);

		sjinfo->min_lefthand = bms_del_member(sjinfo->min_lefthand, relid);
		sjinfo->min_righthand = bms_del_member(sjinfo->min_righthand, relid);
		sjinfo->syn_lefthand = bms_del_member(sjinfo->syn_lefthand, relid);
		sjinfo->syn_righthand = bms_del_member(sjinfo->syn_righthand, relid);
	}

	/*
	 * Likewise remove references from PlaceHolderVar data structures,
	 * removing any no-longer-needed placeholders entirely.
	 *
	 * Removal is a bit tricker than it might seem: we can remove PHVs that
	 * are used at the target rel and/or in the join qual, but not those that
	 * are used at join partner rels or above the join.  It's not that easy to
	 * distinguish PHVs used at partner rels from those used in the join qual,
	 * since they will both have ph_needed sets that are subsets of
	 * joinrelids.  However, a PHV used at a partner rel could not have the
	 * target rel in ph_eval_at, so we check that while deciding whether to
	 * remove or just update the PHV.  There is no corresponding test in
	 * join_is_removable because it doesn't need to distinguish those cases.
	 */
	for (l = list_head(root->placeholder_list); l != NULL; l = nextl)
	{
		PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(l);

		nextl = lnext(l);
		Assert(!bms_is_member(relid, phinfo->ph_lateral));
		if (bms_is_subset(phinfo->ph_needed, joinrelids) &&
			bms_is_member(relid, phinfo->ph_eval_at))
			root->placeholder_list = list_delete_ptr(root->placeholder_list,
													 phinfo);
		else
		{
			phinfo->ph_eval_at = bms_del_member(phinfo->ph_eval_at, relid);
			Assert(!bms_is_empty(phinfo->ph_eval_at));
			phinfo->ph_needed = bms_del_member(phinfo->ph_needed, relid);
		}
	}

	/*
	 * Remove any joinquals referencing the rel from the joininfo lists.
	 *
	 * In some cases, a joinqual has to be put back after deleting its
	 * reference to the target rel.  This can occur for pseudoconstant and
	 * outerjoin-delayed quals, which can get marked as requiring the rel in
	 * order to force them to be evaluated at or above the join.  We can't
	 * just discard them, though.  Only quals that logically belonged to the
	 * outer join being discarded should be removed from the query.
	 *
	 * We must make a copy of the rel's old joininfo list before starting the
	 * loop, because otherwise remove_join_clause_from_rels would destroy the
	 * list while we're scanning it.
	 */
	joininfos = list_copy(rel->joininfo);
	foreach(l, joininfos)
	{
		RestrictInfo *rinfo = (RestrictInfo *) lfirst(l);

		remove_join_clause_from_rels(root, rinfo, rinfo->required_relids);

		if (rinfo->is_pushed_down ||
			!bms_equal(rinfo->required_relids, joinrelids))
		{
			/* Recheck that qual doesn't actually reference the target rel */
			Assert(!bms_is_member(relid, rinfo->clause_relids));

			/*
			 * The required_relids probably aren't shared with anything else,
			 * but let's copy them just to be sure.
			 */
			rinfo->required_relids = bms_copy(rinfo->required_relids);
			rinfo->required_relids = bms_del_member(rinfo->required_relids,
													relid);
			distribute_restrictinfo_to_rels(root, rinfo);
		}
	}

	/*
	 * There may be references to the rel in root->fkey_list, but if so,
	 * match_foreign_keys_to_quals() will get rid of them.
	 */
}

/*
 * Remove any occurrences of the target relid from a joinlist structure.
 *
 * It's easiest to build a whole new list structure, so we handle it that
 * way.  Efficiency is not a big deal here.
 *
 * *nremoved is incremented by the number of occurrences removed (there
 * should be exactly one, but the caller checks that).
 */
static List *
remove_rel_from_joinlist(List *joinlist, int relid, int *nremoved)
{
	List	   *result = NIL;
	ListCell   *jl;

	foreach(jl, joinlist)
	{
		Node	   *jlnode = (Node *) lfirst(jl);

		if (IsA(jlnode, RangeTblRef))
		{
			int			varno = ((RangeTblRef *) jlnode)->rtindex;

			if (varno == relid)
				(*nremoved)++;
			else
				result = lappend(result, jlnode);
		}
		else if (IsA(jlnode, List))
		{
			/* Recurse to handle subproblem */
			List	   *sublist;

			sublist = remove_rel_from_joinlist((List *) jlnode,
											   relid, nremoved);
			/* Avoid including empty sub-lists in the result */
			if (sublist)
				result = lappend(result, sublist);
		}
		else
		{
			elog(ERROR, "unrecognized joinlist node type: %d",
				 (int) nodeTag(jlnode));
		}
	}

	return result;
}


/*
 * rel_supports_distinctness
 *		Could the relation possibly be proven distinct on some set of columns?
 *
 * This is effectively a pre-checking function for rel_is_distinct_for().
 * It must return TRUE if rel_is_distinct_for() could possibly return TRUE
 * with this rel, but it should not expend a lot of cycles.  The idea is
 * that callers can avoid doing possibly-expensive processing to compute
 * rel_is_distinct_for()'s argument lists if the call could not possibly
 * succeed.
 */
static bool
rel_supports_distinctness(PlannerInfo *root, RelOptInfo *rel)
{
	/* We only know about baserels ... */
	if (rel->reloptkind != RELOPT_BASEREL)
		return false;
	if (rel->rtekind == RTE_RELATION)
	{
		/*
		 * For a plain relation, we only know how to prove uniqueness by
		 * reference to unique indexes.  Make sure there's at least one
		 * suitable unique index.  It must be immediately enforced, and if
		 * it's a partial index, it must match the query.  (Keep these
		 * conditions in sync with relation_has_unique_index_for!)
		 */
		ListCell   *lc;

		foreach(lc, rel->indexlist)
		{
			IndexOptInfo *ind = (IndexOptInfo *) lfirst(lc);

			if (ind->unique && ind->immediate &&
				(ind->indpred == NIL || ind->predOK))
				return true;
		}
	}
	else if (rel->rtekind == RTE_SUBQUERY)
	{
		Query	   *subquery = root->simple_rte_array[rel->relid]->subquery;

		/* Check if the subquery has any qualities that support distinctness */
		if (query_supports_distinctness(subquery))
			return true;
	}
	/* We have no proof rules for any other rtekinds. */
	return false;
}

/*
 * rel_is_distinct_for
 *		Does the relation return only distinct rows according to clause_list?
 *
 * clause_list is a list of join restriction clauses involving this rel and
 * some other one.  Return true if no two rows emitted by this rel could
 * possibly join to the same row of the other rel.
 *
 * The caller must have already determined that each condition is a
 * mergejoinable equality with an expression in this relation on one side, and
 * an expression not involving this relation on the other.  The transient
 * outer_is_left flag is used to identify which side references this relation:
 * left side if outer_is_left is false, right side if it is true.
 *
 * Note that the passed-in clause_list may be destructively modified!  This
 * is OK for current uses, because the clause_list is built by the caller for
 * the sole purpose of passing to this function.
 */
static bool
rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel, List *clause_list)
{
	/*
	 * We could skip a couple of tests here if we assume all callers checked
	 * rel_supports_distinctness first, but it doesn't seem worth taking any
	 * risk for.
	 */
	if (rel->reloptkind != RELOPT_BASEREL)
		return false;
	if (rel->rtekind == RTE_RELATION)
	{
		/*
		 * Examine the indexes to see if we have a matching unique index.
		 * relation_has_unique_index_for automatically adds any usable
		 * restriction clauses for the rel, so we needn't do that here.
		 */
		if (relation_has_unique_index_for(root, rel, clause_list, NIL, NIL))
			return true;
	}
	else if (rel->rtekind == RTE_SUBQUERY)
	{
		Index		relid = rel->relid;
		Query	   *subquery = root->simple_rte_array[relid]->subquery;
		List	   *colnos = NIL;
		List	   *opids = NIL;
		ListCell   *l;

		/*
		 * Build the argument lists for query_is_distinct_for: a list of
		 * output column numbers that the query needs to be distinct over, and
		 * a list of equality operators that the output columns need to be
		 * distinct according to.
		 *
		 * (XXX we are not considering restriction clauses attached to the
		 * subquery; is that worth doing?)
		 */
		foreach(l, clause_list)
		{
			RestrictInfo *rinfo = castNode(RestrictInfo, lfirst(l));
			Oid			op;
			Var		   *var;

			/*
			 * Get the equality operator we need uniqueness according to.
			 * (This might be a cross-type operator and thus not exactly the
			 * same operator the subquery would consider; that's all right
			 * since query_is_distinct_for can resolve such cases.)  The
			 * caller's mergejoinability test should have selected only
			 * OpExprs.
			 */
			op = castNode(OpExpr, rinfo->clause)->opno;

			/* caller identified the inner side for us */
			if (rinfo->outer_is_left)
				var = (Var *) get_rightop(rinfo->clause);
			else
				var = (Var *) get_leftop(rinfo->clause);

			/*
			 * If inner side isn't a Var referencing a subquery output column,
			 * this clause doesn't help us.
			 */
			if (!var || !IsA(var, Var) ||
				var->varno != relid || var->varlevelsup != 0)
				continue;

			colnos = lappend_int(colnos, var->varattno);
			opids = lappend_oid(opids, op);
		}

		if (query_is_distinct_for(subquery, colnos, opids))
			return true;
	}
	return false;
}


/*
 * query_supports_distinctness - could the query possibly be proven distinct
 *		on some set of output columns?
 *
 * This is effectively a pre-checking function for query_is_distinct_for().
 * It must return TRUE if query_is_distinct_for() could possibly return TRUE
 * with this query, but it should not expend a lot of cycles.  The idea is
 * that callers can avoid doing possibly-expensive processing to compute
 * query_is_distinct_for()'s argument lists if the call could not possibly
 * succeed.
 */
bool
query_supports_distinctness(Query *query)
{
	/* we don't cope with SRFs, see comment below */
	if (query->hasTargetSRFs)
		return false;

	/* check for features we can prove distinctness with */
	if (query->distinctClause != NIL ||
		query->groupClause != NIL ||
		query->groupingSets != NIL ||
		query->hasAggs ||
		query->havingQual ||
		query->setOperations)
		return true;

	return false;
}

/*
 * query_is_distinct_for - does query never return duplicates of the
 *		specified columns?
 *
 * query is a not-yet-planned subquery (in current usage, it's always from
 * a subquery RTE, which the planner avoids scribbling on).
 *
 * colnos is an integer list of output column numbers (resno's).  We are
 * interested in whether rows consisting of just these columns are certain
 * to be distinct.  "Distinctness" is defined according to whether the
 * corresponding upper-level equality operators listed in opids would think
 * the values are distinct.  (Note: the opids entries could be cross-type
 * operators, and thus not exactly the equality operators that the subquery
 * would use itself.  We use equality_ops_are_compatible() to check
 * compatibility.  That looks at btree or hash opfamily membership, and so
 * should give trustworthy answers for all operators that we might need
 * to deal with here.)
 */
bool
query_is_distinct_for(Query *query, List *colnos, List *opids)
{
	ListCell   *l;
	Oid			opid;

	Assert(list_length(colnos) == list_length(opids));

	/*
	 * A set-returning function in the query's targetlist can result in
	 * returning duplicate rows, if the SRF is evaluated after the
	 * de-duplication step; so we play it safe and say "no" if there are any
	 * SRFs.  (We could be certain that it's okay if SRFs appear only in the
	 * specified columns, since those must be evaluated before de-duplication;
	 * but it doesn't presently seem worth the complication to check that.)
	 */
	if (query->hasTargetSRFs)
		return false;

	/*
	 * DISTINCT (including DISTINCT ON) guarantees uniqueness if all the
	 * columns in the DISTINCT clause appear in colnos and operator semantics
	 * match.
	 */
	if (query->distinctClause)
	{
		foreach(l, query->distinctClause)
		{
			SortGroupClause *sgc = (SortGroupClause *) lfirst(l);
			TargetEntry *tle = get_sortgroupclause_tle(sgc,
													   query->targetList);

			opid = distinct_col_search(tle->resno, colnos, opids);
			if (!OidIsValid(opid) ||
				!equality_ops_are_compatible(opid, sgc->eqop))
				break;			/* exit early if no match */
		}
		if (l == NULL)			/* had matches for all? */
			return true;
	}

	/*
	 * Similarly, GROUP BY without GROUPING SETS guarantees uniqueness if all
	 * the grouped columns appear in colnos and operator semantics match.
	 */
	if (query->groupClause && !query->groupingSets)
	{
		foreach(l, query->groupClause)
		{
			SortGroupClause *sgc = (SortGroupClause *) lfirst(l);
			TargetEntry *tle = get_sortgroupclause_tle(sgc,
													   query->targetList);

			opid = distinct_col_search(tle->resno, colnos, opids);
			if (!OidIsValid(opid) ||
				!equality_ops_are_compatible(opid, sgc->eqop))
				break;			/* exit early if no match */
		}
		if (l == NULL)			/* had matches for all? */
			return true;
	}
	else if (query->groupingSets)
	{
		/*
		 * If we have grouping sets with expressions, we probably don't have
		 * uniqueness and analysis would be hard. Punt.
		 */
		if (query->groupClause)
			return false;

		/*
		 * If we have no groupClause (therefore no grouping expressions), we
		 * might have one or many empty grouping sets. If there's just one,
		 * then we're returning only one row and are certainly unique. But
		 * otherwise, we know we're certainly not unique.
		 */
		if (list_length(query->groupingSets) == 1 &&
			((GroupingSet *) linitial(query->groupingSets))->kind == GROUPING_SET_EMPTY)
			return true;
		else
			return false;
	}
	else
	{
		/*
		 * If we have no GROUP BY, but do have aggregates or HAVING, then the
		 * result is at most one row so it's surely unique, for any operators.
		 */
		if (query->hasAggs || query->havingQual)
			return true;
	}

	/*
	 * UNION, INTERSECT, EXCEPT guarantee uniqueness of the whole output row,
	 * except with ALL.
	 */
	if (query->setOperations)
	{
		SetOperationStmt *topop = castNode(SetOperationStmt, query->setOperations);

		Assert(topop->op != SETOP_NONE);

		if (!topop->all)
		{
			ListCell   *lg;

			/* We're good if all the nonjunk output columns are in colnos */
			lg = list_head(topop->groupClauses);
			foreach(l, query->targetList)
			{
				TargetEntry *tle = (TargetEntry *) lfirst(l);
				SortGroupClause *sgc;

				if (tle->resjunk)
					continue;	/* ignore resjunk columns */

				/* non-resjunk columns should have grouping clauses */
				Assert(lg != NULL);
				sgc = (SortGroupClause *) lfirst(lg);
				lg = lnext(lg);

				opid = distinct_col_search(tle->resno, colnos, opids);
				if (!OidIsValid(opid) ||
					!equality_ops_are_compatible(opid, sgc->eqop))
					break;		/* exit early if no match */
			}
			if (l == NULL)		/* had matches for all? */
				return true;
		}
	}

	/*
	 * XXX Are there any other cases in which we can easily see the result
	 * must be distinct?
	 *
	 * If you do add more smarts to this function, be sure to update
	 * query_supports_distinctness() to match.
	 */

	return false;
}

/*
 * distinct_col_search - subroutine for query_is_distinct_for
 *
 * If colno is in colnos, return the corresponding element of opids,
 * else return InvalidOid.  (Ordinarily colnos would not contain duplicates,
 * but if it does, we arbitrarily select the first match.)
 */
static Oid
distinct_col_search(int colno, List *colnos, List *opids)
{
	ListCell   *lc1,
			   *lc2;

	forboth(lc1, colnos, lc2, opids)
	{
		if (colno == lfirst_int(lc1))
			return lfirst_oid(lc2);
	}
	return InvalidOid;
}