postgresql/src/backend/optimizer/util/tlist.c

/*-------------------------------------------------------------------------
 *
 * tlist.c
 *	  Target list manipulation routines
 *
 * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  src/backend/optimizer/util/tlist.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "nodes/makefuncs.h"
#include "nodes/nodeFuncs.h"
#include "optimizer/cost.h"
#include "optimizer/optimizer.h"
#include "optimizer/tlist.h"


/* Test if an expression node represents a SRF call.  Beware multiple eval! */
#define IS_SRF_CALL(node) \
	((IsA(node, FuncExpr) && ((FuncExpr *) (node))->funcretset) || \
	 (IsA(node, OpExpr) && ((OpExpr *) (node))->opretset))

/*
 * Data structures for split_pathtarget_at_srfs().  To preserve the identity
 * of sortgroupref items even if they are textually equal(), what we track is
 * not just bare expressions but expressions plus their sortgroupref indexes.
 */
typedef struct
{
	Node	   *expr;			/* some subexpression of a PathTarget */
	Index		sortgroupref;	/* its sortgroupref, or 0 if none */
} split_pathtarget_item;

typedef struct
{
	/* This is a List of bare expressions: */
	List	   *input_target_exprs; /* exprs available from input */
	/* These are Lists of Lists of split_pathtarget_items: */
	List	   *level_srfs;		/* SRF exprs to evaluate at each level */
	List	   *level_input_vars;	/* input vars needed at each level */
	List	   *level_input_srfs;	/* input SRFs needed at each level */
	/* These are Lists of split_pathtarget_items: */
	List	   *current_input_vars; /* vars needed in current subexpr */
	List	   *current_input_srfs; /* SRFs needed in current subexpr */
	/* Auxiliary data for current split_pathtarget_walker traversal: */
	int			current_depth;	/* max SRF depth in current subexpr */
	Index		current_sgref;	/* current subexpr's sortgroupref, or 0 */
} split_pathtarget_context;

static bool split_pathtarget_walker(Node *node,
						split_pathtarget_context *context);
static void add_sp_item_to_pathtarget(PathTarget *target,
						  split_pathtarget_item *item);
static void add_sp_items_to_pathtarget(PathTarget *target, List *items);


/*****************************************************************************
 *		Target list creation and searching utilities
 *****************************************************************************/

/*
 * tlist_member
 *	  Finds the (first) member of the given tlist whose expression is
 *	  equal() to the given expression.  Result is NULL if no such member.
 */
TargetEntry *
tlist_member(Expr *node, List *targetlist)
{
	ListCell   *temp;

	foreach(temp, targetlist)
	{
		TargetEntry *tlentry = (TargetEntry *) lfirst(temp);

		if (equal(node, tlentry->expr))
			return tlentry;
	}
	return NULL;
}

/*
 * tlist_member_ignore_relabel
 *	  Same as above, except that we ignore top-level RelabelType nodes
 *	  while checking for a match.  This is needed for some scenarios
 *	  involving binary-compatible sort operations.
 */
TargetEntry *
tlist_member_ignore_relabel(Expr *node, List *targetlist)
{
	ListCell   *temp;

	while (node && IsA(node, RelabelType))
		node = ((RelabelType *) node)->arg;

	foreach(temp, targetlist)
	{
		TargetEntry *tlentry = (TargetEntry *) lfirst(temp);
		Expr	   *tlexpr = tlentry->expr;

		while (tlexpr && IsA(tlexpr, RelabelType))
			tlexpr = ((RelabelType *) tlexpr)->arg;

		if (equal(node, tlexpr))
			return tlentry;
	}
	return NULL;
}

/*
 * tlist_member_match_var
 *	  Same as above, except that we match the provided Var on the basis
 *	  of varno/varattno/varlevelsup/vartype only, rather than full equal().
 *
 * This is needed in some cases where we can't be sure of an exact typmod
 * match.  For safety, though, we insist on vartype match.
 */
static TargetEntry *
tlist_member_match_var(Var *var, List *targetlist)
{
	ListCell   *temp;

	foreach(temp, targetlist)
	{
		TargetEntry *tlentry = (TargetEntry *) lfirst(temp);
		Var		   *tlvar = (Var *) tlentry->expr;

		if (!tlvar || !IsA(tlvar, Var))
			continue;
		if (var->varno == tlvar->varno &&
			var->varattno == tlvar->varattno &&
			var->varlevelsup == tlvar->varlevelsup &&
			var->vartype == tlvar->vartype)
			return tlentry;
	}
	return NULL;
}

/*
 * add_to_flat_tlist
 *		Add more items to a flattened tlist (if they're not already in it)
 *
 * 'tlist' is the flattened tlist
 * 'exprs' is a list of expressions (usually, but not necessarily, Vars)
 *
 * Returns the extended tlist.
 */
List *
add_to_flat_tlist(List *tlist, List *exprs)
{
	int			next_resno = list_length(tlist) + 1;
	ListCell   *lc;

	foreach(lc, exprs)
	{
		Expr	   *expr = (Expr *) lfirst(lc);

		if (!tlist_member(expr, tlist))
		{
			TargetEntry *tle;

			tle = makeTargetEntry(copyObject(expr), /* copy needed?? */
								  next_resno++,
								  NULL,
								  false);
			tlist = lappend(tlist, tle);
		}
	}
	return tlist;
}


/*
 * get_tlist_exprs
 *		Get just the expression subtrees of a tlist
 *
 * Resjunk columns are ignored unless includeJunk is true
 */
List *
get_tlist_exprs(List *tlist, bool includeJunk)
{
	List	   *result = NIL;
	ListCell   *l;

	foreach(l, tlist)
	{
		TargetEntry *tle = (TargetEntry *) lfirst(l);

		if (tle->resjunk && !includeJunk)
			continue;

		result = lappend(result, tle->expr);
	}
	return result;
}


/*
 * count_nonjunk_tlist_entries
 *		What it says ...
 */
int
count_nonjunk_tlist_entries(List *tlist)
{
	int			len = 0;
	ListCell   *l;

	foreach(l, tlist)
	{
		TargetEntry *tle = (TargetEntry *) lfirst(l);

		if (!tle->resjunk)
			len++;
	}
	return len;
}


/*
 * tlist_same_exprs
 *		Check whether two target lists contain the same expressions
 *
 * Note: this function is used to decide whether it's safe to jam a new tlist
 * into a non-projection-capable plan node.  Obviously we can't do that unless
 * the node's tlist shows it already returns the column values we want.
 * However, we can ignore the TargetEntry attributes resname, ressortgroupref,
 * resorigtbl, resorigcol, and resjunk, because those are only labelings that
 * don't affect the row values computed by the node.  (Moreover, if we didn't
 * ignore them, we'd frequently fail to make the desired optimization, since
 * the planner tends to not bother to make resname etc. valid in intermediate
 * plan nodes.)  Note that on success, the caller must still jam the desired
 * tlist into the plan node, else it won't have the desired labeling fields.
 */
bool
tlist_same_exprs(List *tlist1, List *tlist2)
{
	ListCell   *lc1,
			   *lc2;

	if (list_length(tlist1) != list_length(tlist2))
		return false;			/* not same length, so can't match */

	forboth(lc1, tlist1, lc2, tlist2)
	{
		TargetEntry *tle1 = (TargetEntry *) lfirst(lc1);
		TargetEntry *tle2 = (TargetEntry *) lfirst(lc2);

		if (!equal(tle1->expr, tle2->expr))
			return false;
	}

	return true;
}


/*
 * Does tlist have same output datatypes as listed in colTypes?
 *
 * Resjunk columns are ignored if junkOK is true; otherwise presence of
 * a resjunk column will always cause a 'false' result.
 *
 * Note: currently no callers care about comparing typmods.
 */
bool
tlist_same_datatypes(List *tlist, List *colTypes, bool junkOK)
{
	ListCell   *l;
	ListCell   *curColType = list_head(colTypes);

	foreach(l, tlist)
	{
		TargetEntry *tle = (TargetEntry *) lfirst(l);

		if (tle->resjunk)
		{
			if (!junkOK)
				return false;
		}
		else
		{
			if (curColType == NULL)
				return false;	/* tlist longer than colTypes */
			if (exprType((Node *) tle->expr) != lfirst_oid(curColType))
				return false;
			curColType = lnext(curColType);
		}
	}
	if (curColType != NULL)
		return false;			/* tlist shorter than colTypes */
	return true;
}

/*
 * Does tlist have same exposed collations as listed in colCollations?
 *
 * Identical logic to the above, but for collations.
 */
bool
tlist_same_collations(List *tlist, List *colCollations, bool junkOK)
{
	ListCell   *l;
	ListCell   *curColColl = list_head(colCollations);

	foreach(l, tlist)
	{
		TargetEntry *tle = (TargetEntry *) lfirst(l);

		if (tle->resjunk)
		{
			if (!junkOK)
				return false;
		}
		else
		{
			if (curColColl == NULL)
				return false;	/* tlist longer than colCollations */
			if (exprCollation((Node *) tle->expr) != lfirst_oid(curColColl))
				return false;
			curColColl = lnext(curColColl);
		}
	}
	if (curColColl != NULL)
		return false;			/* tlist shorter than colCollations */
	return true;
}

/*
 * apply_tlist_labeling
 *		Apply the TargetEntry labeling attributes of src_tlist to dest_tlist
 *
 * This is useful for reattaching column names etc to a plan's final output
 * targetlist.
 */
void
apply_tlist_labeling(List *dest_tlist, List *src_tlist)
{
	ListCell   *ld,
			   *ls;

	Assert(list_length(dest_tlist) == list_length(src_tlist));
	forboth(ld, dest_tlist, ls, src_tlist)
	{
		TargetEntry *dest_tle = (TargetEntry *) lfirst(ld);
		TargetEntry *src_tle = (TargetEntry *) lfirst(ls);

		Assert(dest_tle->resno == src_tle->resno);
		dest_tle->resname = src_tle->resname;
		dest_tle->ressortgroupref = src_tle->ressortgroupref;
		dest_tle->resorigtbl = src_tle->resorigtbl;
		dest_tle->resorigcol = src_tle->resorigcol;
		dest_tle->resjunk = src_tle->resjunk;
	}
}


/*
 * get_sortgroupref_tle
 *		Find the targetlist entry matching the given SortGroupRef index,
 *		and return it.
 */
TargetEntry *
get_sortgroupref_tle(Index sortref, List *targetList)
{
	ListCell   *l;

	foreach(l, targetList)
	{
		TargetEntry *tle = (TargetEntry *) lfirst(l);

		if (tle->ressortgroupref == sortref)
			return tle;
	}

	elog(ERROR, "ORDER/GROUP BY expression not found in targetlist");
	return NULL;				/* keep compiler quiet */
}

/*
 * get_sortgroupclause_tle
 *		Find the targetlist entry matching the given SortGroupClause
 *		by ressortgroupref, and return it.
 */
TargetEntry *
get_sortgroupclause_tle(SortGroupClause *sgClause,
						List *targetList)
{
	return get_sortgroupref_tle(sgClause->tleSortGroupRef, targetList);
}

/*
 * get_sortgroupclause_expr
 *		Find the targetlist entry matching the given SortGroupClause
 *		by ressortgroupref, and return its expression.
 */
Node *
get_sortgroupclause_expr(SortGroupClause *sgClause, List *targetList)
{
	TargetEntry *tle = get_sortgroupclause_tle(sgClause, targetList);

	return (Node *) tle->expr;
}

/*
 * get_sortgrouplist_exprs
 *		Given a list of SortGroupClauses, build a list
 *		of the referenced targetlist expressions.
 */
List *
get_sortgrouplist_exprs(List *sgClauses, List *targetList)
{
	List	   *result = NIL;
	ListCell   *l;

	foreach(l, sgClauses)
	{
		SortGroupClause *sortcl = (SortGroupClause *) lfirst(l);
		Node	   *sortexpr;

		sortexpr = get_sortgroupclause_expr(sortcl, targetList);
		result = lappend(result, sortexpr);
	}
	return result;
}


/*****************************************************************************
 *		Functions to extract data from a list of SortGroupClauses
 *
 * These don't really belong in tlist.c, but they are sort of related to the
 * functions just above, and they don't seem to deserve their own file.
 *****************************************************************************/

/*
 * get_sortgroupref_clause
 *		Find the SortGroupClause matching the given SortGroupRef index,
 *		and return it.
 */
SortGroupClause *
get_sortgroupref_clause(Index sortref, List *clauses)
{
	ListCell   *l;

	foreach(l, clauses)
	{
		SortGroupClause *cl = (SortGroupClause *) lfirst(l);

		if (cl->tleSortGroupRef == sortref)
			return cl;
	}

	elog(ERROR, "ORDER/GROUP BY expression not found in list");
	return NULL;				/* keep compiler quiet */
}

/*
 * get_sortgroupref_clause_noerr
 *		As above, but return NULL rather than throwing an error if not found.
 */
SortGroupClause *
get_sortgroupref_clause_noerr(Index sortref, List *clauses)
{
	ListCell   *l;

	foreach(l, clauses)
	{
		SortGroupClause *cl = (SortGroupClause *) lfirst(l);

		if (cl->tleSortGroupRef == sortref)
			return cl;
	}

	return NULL;
}

/*
 * extract_grouping_ops - make an array of the equality operator OIDs
 *		for a SortGroupClause list
 */
Oid *
extract_grouping_ops(List *groupClause)
{
	int			numCols = list_length(groupClause);
	int			colno = 0;
	Oid		   *groupOperators;
	ListCell   *glitem;

	groupOperators = (Oid *) palloc(sizeof(Oid) * numCols);

	foreach(glitem, groupClause)
	{
		SortGroupClause *groupcl = (SortGroupClause *) lfirst(glitem);

		groupOperators[colno] = groupcl->eqop;
		Assert(OidIsValid(groupOperators[colno]));
		colno++;
	}

	return groupOperators;
}

/*
 * extract_grouping_collations - make an array of the grouping column collations
 *		for a SortGroupClause list
 */
Oid *
extract_grouping_collations(List *groupClause, List *tlist)
{
	int			numCols = list_length(groupClause);
	int			colno = 0;
	Oid		   *grpCollations;
	ListCell   *glitem;

	grpCollations = (Oid *) palloc(sizeof(Oid) * numCols);

	foreach(glitem, groupClause)
	{
		SortGroupClause *groupcl = (SortGroupClause *) lfirst(glitem);
		TargetEntry *tle = get_sortgroupclause_tle(groupcl, tlist);

		grpCollations[colno++] = exprCollation((Node *) tle->expr);
	}

	return grpCollations;
}

/*
 * extract_grouping_cols - make an array of the grouping column resnos
 *		for a SortGroupClause list
 */
AttrNumber *
extract_grouping_cols(List *groupClause, List *tlist)
{
	AttrNumber *grpColIdx;
	int			numCols = list_length(groupClause);
	int			colno = 0;
	ListCell   *glitem;

	grpColIdx = (AttrNumber *) palloc(sizeof(AttrNumber) * numCols);

	foreach(glitem, groupClause)
	{
		SortGroupClause *groupcl = (SortGroupClause *) lfirst(glitem);
		TargetEntry *tle = get_sortgroupclause_tle(groupcl, tlist);

		grpColIdx[colno++] = tle->resno;
	}

	return grpColIdx;
}

/*
 * grouping_is_sortable - is it possible to implement grouping list by sorting?
 *
 * This is easy since the parser will have included a sortop if one exists.
 */
bool
grouping_is_sortable(List *groupClause)
{
	ListCell   *glitem;

	foreach(glitem, groupClause)
	{
		SortGroupClause *groupcl = (SortGroupClause *) lfirst(glitem);

		if (!OidIsValid(groupcl->sortop))
			return false;
	}
	return true;
}

/*
 * grouping_is_hashable - is it possible to implement grouping list by hashing?
 *
 * We rely on the parser to have set the hashable flag correctly.
 */
bool
grouping_is_hashable(List *groupClause)
{
	ListCell   *glitem;

	foreach(glitem, groupClause)
	{
		SortGroupClause *groupcl = (SortGroupClause *) lfirst(glitem);

		if (!groupcl->hashable)
			return false;
	}
	return true;
}


/*****************************************************************************
 *		PathTarget manipulation functions
 *
 * PathTarget is a somewhat stripped-down version of a full targetlist; it
 * omits all the TargetEntry decoration except (optionally) sortgroupref data,
 * and it adds evaluation cost and output data width info.
 *****************************************************************************/

/*
 * make_pathtarget_from_tlist
 *	  Construct a PathTarget equivalent to the given targetlist.
 *
 * This leaves the cost and width fields as zeroes.  Most callers will want
 * to use create_pathtarget(), so as to get those set.
 */
PathTarget *
make_pathtarget_from_tlist(List *tlist)
{
	PathTarget *target = makeNode(PathTarget);
	int			i;
	ListCell   *lc;

	target->sortgrouprefs = (Index *) palloc(list_length(tlist) * sizeof(Index));

	i = 0;
	foreach(lc, tlist)
	{
		TargetEntry *tle = (TargetEntry *) lfirst(lc);

		target->exprs = lappend(target->exprs, tle->expr);
		target->sortgrouprefs[i] = tle->ressortgroupref;
		i++;
	}

	return target;
}

/*
 * make_tlist_from_pathtarget
 *	  Construct a targetlist from a PathTarget.
 */
List *
make_tlist_from_pathtarget(PathTarget *target)
{
	List	   *tlist = NIL;
	int			i;
	ListCell   *lc;

	i = 0;
	foreach(lc, target->exprs)
	{
		Expr	   *expr = (Expr *) lfirst(lc);
		TargetEntry *tle;

		tle = makeTargetEntry(expr,
							  i + 1,
							  NULL,
							  false);
		if (target->sortgrouprefs)
			tle->ressortgroupref = target->sortgrouprefs[i];
		tlist = lappend(tlist, tle);
		i++;
	}

	return tlist;
}

/*
 * copy_pathtarget
 *	  Copy a PathTarget.
 *
 * The new PathTarget has its own List cells, but shares the underlying
 * target expression trees with the old one.  We duplicate the List cells
 * so that items can be added to one target without damaging the other.
 */
PathTarget *
copy_pathtarget(PathTarget *src)
{
	PathTarget *dst = makeNode(PathTarget);

	/* Copy scalar fields */
	memcpy(dst, src, sizeof(PathTarget));
	/* Shallow-copy the expression list */
	dst->exprs = list_copy(src->exprs);
	/* Duplicate sortgrouprefs if any (if not, the memcpy handled this) */
	if (src->sortgrouprefs)
	{
		Size		nbytes = list_length(src->exprs) * sizeof(Index);

		dst->sortgrouprefs = (Index *) palloc(nbytes);
		memcpy(dst->sortgrouprefs, src->sortgrouprefs, nbytes);
	}
	return dst;
}

/*
 * create_empty_pathtarget
 *	  Create an empty (zero columns, zero cost) PathTarget.
 */
PathTarget *
create_empty_pathtarget(void)
{
	/* This is easy, but we don't want callers to hard-wire this ... */
	return makeNode(PathTarget);
}

/*
 * add_column_to_pathtarget
 *		Append a target column to the PathTarget.
 *
 * As with make_pathtarget_from_tlist, we leave it to the caller to update
 * the cost and width fields.
 */
void
add_column_to_pathtarget(PathTarget *target, Expr *expr, Index sortgroupref)
{
	/* Updating the exprs list is easy ... */
	target->exprs = lappend(target->exprs, expr);
	/* ... the sortgroupref data, a bit less so */
	if (target->sortgrouprefs)
	{
		int			nexprs = list_length(target->exprs);

		/* This might look inefficient, but actually it's usually cheap */
		target->sortgrouprefs = (Index *)
			repalloc(target->sortgrouprefs, nexprs * sizeof(Index));
		target->sortgrouprefs[nexprs - 1] = sortgroupref;
	}
	else if (sortgroupref)
	{
		/* Adding sortgroupref labeling to a previously unlabeled target */
		int			nexprs = list_length(target->exprs);

		target->sortgrouprefs = (Index *) palloc0(nexprs * sizeof(Index));
		target->sortgrouprefs[nexprs - 1] = sortgroupref;
	}
}

/*
 * add_new_column_to_pathtarget
 *		Append a target column to the PathTarget, but only if it's not
 *		equal() to any pre-existing target expression.
 *
 * The caller cannot specify a sortgroupref, since it would be unclear how
 * to merge that with a pre-existing column.
 *
 * As with make_pathtarget_from_tlist, we leave it to the caller to update
 * the cost and width fields.
 */
void
add_new_column_to_pathtarget(PathTarget *target, Expr *expr)
{
	if (!list_member(target->exprs, expr))
		add_column_to_pathtarget(target, expr, 0);
}

/*
 * add_new_columns_to_pathtarget
 *		Apply add_new_column_to_pathtarget() for each element of the list.
 */
void
add_new_columns_to_pathtarget(PathTarget *target, List *exprs)
{
	ListCell   *lc;

	foreach(lc, exprs)
	{
		Expr	   *expr = (Expr *) lfirst(lc);

		add_new_column_to_pathtarget(target, expr);
	}
}

/*
 * apply_pathtarget_labeling_to_tlist
 *		Apply any sortgrouprefs in the PathTarget to matching tlist entries
 *
 * Here, we do not assume that the tlist entries are one-for-one with the
 * PathTarget.  The intended use of this function is to deal with cases
 * where createplan.c has decided to use some other tlist and we have
 * to identify what matches exist.
 */
void
apply_pathtarget_labeling_to_tlist(List *tlist, PathTarget *target)
{
	int			i;
	ListCell   *lc;

	/* Nothing to do if PathTarget has no sortgrouprefs data */
	if (target->sortgrouprefs == NULL)
		return;

	i = 0;
	foreach(lc, target->exprs)
	{
		Expr	   *expr = (Expr *) lfirst(lc);
		TargetEntry *tle;

		if (target->sortgrouprefs[i])
		{
			/*
			 * For Vars, use tlist_member_match_var's weakened matching rule;
			 * this allows us to deal with some cases where a set-returning
			 * function has been inlined, so that we now have more knowledge
			 * about what it returns than we did when the original Var was
			 * created.  Otherwise, use regular equal() to find the matching
			 * TLE.  (In current usage, only the Var case is actually needed;
			 * but it seems best to have sane behavior here for non-Vars too.)
			 */
			if (expr && IsA(expr, Var))
				tle = tlist_member_match_var((Var *) expr, tlist);
			else
				tle = tlist_member(expr, tlist);

			/*
			 * Complain if noplace for the sortgrouprefs label, or if we'd
			 * have to label a column twice.  (The case where it already has
			 * the desired label probably can't happen, but we may as well
			 * allow for it.)
			 */
			if (!tle)
				elog(ERROR, "ORDER/GROUP BY expression not found in targetlist");
			if (tle->ressortgroupref != 0 &&
				tle->ressortgroupref != target->sortgrouprefs[i])
				elog(ERROR, "targetlist item has multiple sortgroupref labels");

			tle->ressortgroupref = target->sortgrouprefs[i];
		}
		i++;
	}
}

/*
 * split_pathtarget_at_srfs
 *		Split given PathTarget into multiple levels to position SRFs safely
 *
 * The executor can only handle set-returning functions that appear at the
 * top level of the targetlist of a ProjectSet plan node.  If we have any SRFs
 * that are not at top level, we need to split up the evaluation into multiple
 * plan levels in which each level satisfies this constraint.  This function
 * creates appropriate PathTarget(s) for each level.
 *
 * As an example, consider the tlist expression
 *		x + srf1(srf2(y + z))
 * This expression should appear as-is in the top PathTarget, but below that
 * we must have a PathTarget containing
 *		x, srf1(srf2(y + z))
 * and below that, another PathTarget containing
 *		x, srf2(y + z)
 * and below that, another PathTarget containing
 *		x, y, z
 * When these tlists are processed by setrefs.c, subexpressions that match
 * output expressions of the next lower tlist will be replaced by Vars,
 * so that what the executor gets are tlists looking like
 *		Var1 + Var2
 *		Var1, srf1(Var2)
 *		Var1, srf2(Var2 + Var3)
 *		x, y, z
 * which satisfy the desired property.
 *
 * Another example is
 *		srf1(x), srf2(srf3(y))
 * That must appear as-is in the top PathTarget, but below that we need
 *		srf1(x), srf3(y)
 * That is, each SRF must be computed at a level corresponding to the nesting
 * depth of SRFs within its arguments.
 *
 * In some cases, a SRF has already been evaluated in some previous plan level
 * and we shouldn't expand it again (that is, what we see in the target is
 * already meant as a reference to a lower subexpression).  So, don't expand
 * any tlist expressions that appear in input_target, if that's not NULL.
 *
 * It's also important that we preserve any sortgroupref annotation appearing
 * in the given target, especially on expressions matching input_target items.
 *
 * The outputs of this function are two parallel lists, one a list of
 * PathTargets and the other an integer list of bool flags indicating
 * whether the corresponding PathTarget contains any evaluatable SRFs.
 * The lists are given in the order they'd need to be evaluated in, with
 * the "lowest" PathTarget first.  So the last list entry is always the
 * originally given PathTarget, and any entries before it indicate evaluation
 * levels that must be inserted below it.  The first list entry must not
 * contain any SRFs (other than ones duplicating input_target entries), since
 * it will typically be attached to a plan node that cannot evaluate SRFs.
 *
 * Note: using a list for the flags may seem like overkill, since there
 * are only a few possible patterns for which levels contain SRFs.
 * But this representation decouples callers from that knowledge.
 */
void
split_pathtarget_at_srfs(PlannerInfo *root,
						 PathTarget *target, PathTarget *input_target,
						 List **targets, List **targets_contain_srfs)
{
	split_pathtarget_context context;
	int			max_depth;
	bool		need_extra_projection;
	List	   *prev_level_tlist;
	int			lci;
	ListCell   *lc,
			   *lc1,
			   *lc2,
			   *lc3;

	/*
	 * It's not unusual for planner.c to pass us two physically identical
	 * targets, in which case we can conclude without further ado that all
	 * expressions are available from the input.  (The logic below would
	 * arrive at the same conclusion, but much more tediously.)
	 */
	if (target == input_target)
	{
		*targets = list_make1(target);
		*targets_contain_srfs = list_make1_int(false);
		return;
	}

	/* Pass any input_target exprs down to split_pathtarget_walker() */
	context.input_target_exprs = input_target ? input_target->exprs : NIL;

	/*
	 * Initialize with empty level-zero lists, and no levels after that.
	 * (Note: we could dispense with representing level zero explicitly, since
	 * it will never receive any SRFs, but then we'd have to special-case that
	 * level when we get to building result PathTargets.  Level zero describes
	 * the SRF-free PathTarget that will be given to the input plan node.)
	 */
	context.level_srfs = list_make1(NIL);
	context.level_input_vars = list_make1(NIL);
	context.level_input_srfs = list_make1(NIL);

	/* Initialize data we'll accumulate across all the target expressions */
	context.current_input_vars = NIL;
	context.current_input_srfs = NIL;
	max_depth = 0;
	need_extra_projection = false;

	/* Scan each expression in the PathTarget looking for SRFs */
	lci = 0;
	foreach(lc, target->exprs)
	{
		Node	   *node = (Node *) lfirst(lc);

		/* Tell split_pathtarget_walker about this expr's sortgroupref */
		context.current_sgref = get_pathtarget_sortgroupref(target, lci);
		lci++;

		/*
		 * Find all SRFs and Vars (and Var-like nodes) in this expression, and
		 * enter them into appropriate lists within the context struct.
		 */
		context.current_depth = 0;
		split_pathtarget_walker(node, &context);

		/* An expression containing no SRFs is of no further interest */
		if (context.current_depth == 0)
			continue;

		/*
		 * Track max SRF nesting depth over the whole PathTarget.  Also, if
		 * this expression establishes a new max depth, we no longer care
		 * whether previous expressions contained nested SRFs; we can handle
		 * any required projection for them in the final ProjectSet node.
		 */
		if (max_depth < context.current_depth)
		{
			max_depth = context.current_depth;
			need_extra_projection = false;
		}

		/*
		 * If any maximum-depth SRF is not at the top level of its expression,
		 * we'll need an extra Result node to compute the top-level scalar
		 * expression.
		 */
		if (max_depth == context.current_depth && !IS_SRF_CALL(node))
			need_extra_projection = true;
	}

	/*
	 * If we found no SRFs needing evaluation (maybe they were all present in
	 * input_target, or maybe they were all removed by const-simplification),
	 * then no ProjectSet is needed; fall out.
	 */
	if (max_depth == 0)
	{
		*targets = list_make1(target);
		*targets_contain_srfs = list_make1_int(false);
		return;
	}

	/*
	 * The Vars and SRF outputs needed at top level can be added to the last
	 * level_input lists if we don't need an extra projection step.  If we do
	 * need one, add a SRF-free level to the lists.
	 */
	if (need_extra_projection)
	{
		context.level_srfs = lappend(context.level_srfs, NIL);
		context.level_input_vars = lappend(context.level_input_vars,
										   context.current_input_vars);
		context.level_input_srfs = lappend(context.level_input_srfs,
										   context.current_input_srfs);
	}
	else
	{
		lc = list_nth_cell(context.level_input_vars, max_depth);
		lfirst(lc) = list_concat(lfirst(lc), context.current_input_vars);
		lc = list_nth_cell(context.level_input_srfs, max_depth);
		lfirst(lc) = list_concat(lfirst(lc), context.current_input_srfs);
	}

	/*
	 * Now construct the output PathTargets.  The original target can be used
	 * as-is for the last one, but we need to construct a new SRF-free target
	 * representing what the preceding plan node has to emit, as well as a
	 * target for each intermediate ProjectSet node.
	 */
	*targets = *targets_contain_srfs = NIL;
	prev_level_tlist = NIL;

	forthree(lc1, context.level_srfs,
			 lc2, context.level_input_vars,
			 lc3, context.level_input_srfs)
	{
		List	   *level_srfs = (List *) lfirst(lc1);
		PathTarget *ntarget;

		if (lnext(lc1) == NULL)
		{
			ntarget = target;
		}
		else
		{
			ntarget = create_empty_pathtarget();

			/*
			 * This target should actually evaluate any SRFs of the current
			 * level, and it needs to propagate forward any Vars needed by
			 * later levels, as well as SRFs computed earlier and needed by
			 * later levels.
			 */
			add_sp_items_to_pathtarget(ntarget, level_srfs);
			for_each_cell(lc, lnext(lc2))
			{
				List	   *input_vars = (List *) lfirst(lc);

				add_sp_items_to_pathtarget(ntarget, input_vars);
			}
			for_each_cell(lc, lnext(lc3))
			{
				List	   *input_srfs = (List *) lfirst(lc);
				ListCell   *lcx;

				foreach(lcx, input_srfs)
				{
					split_pathtarget_item *item = lfirst(lcx);

					if (list_member(prev_level_tlist, item->expr))
						add_sp_item_to_pathtarget(ntarget, item);
				}
			}
			set_pathtarget_cost_width(root, ntarget);
		}

		/*
		 * Add current target and does-it-compute-SRFs flag to output lists.
		 */
		*targets = lappend(*targets, ntarget);
		*targets_contain_srfs = lappend_int(*targets_contain_srfs,
											(level_srfs != NIL));

		/* Remember this level's output for next pass */
		prev_level_tlist = ntarget->exprs;
	}
}

/*
 * Recursively examine expressions for split_pathtarget_at_srfs.
 *
 * Note we make no effort here to prevent duplicate entries in the output
 * lists.  Duplicates will be gotten rid of later.
 */
static bool
split_pathtarget_walker(Node *node, split_pathtarget_context *context)
{
	if (node == NULL)
		return false;

	/*
	 * A subexpression that matches an expression already computed in
	 * input_target can be treated like a Var (which indeed it will be after
	 * setrefs.c gets done with it), even if it's actually a SRF.  Record it
	 * as being needed for the current expression, and ignore any
	 * substructure.  (Note in particular that this preserves the identity of
	 * any expressions that appear as sortgrouprefs in input_target.)
	 */
	if (list_member(context->input_target_exprs, node))
	{
		split_pathtarget_item *item = palloc(sizeof(split_pathtarget_item));

		item->expr = node;
		item->sortgroupref = context->current_sgref;
		context->current_input_vars = lappend(context->current_input_vars,
											  item);
		return false;
	}

	/*
	 * Vars and Var-like constructs are expected to be gotten from the input,
	 * too.  We assume that these constructs cannot contain any SRFs (if one
	 * does, there will be an executor failure from a misplaced SRF).
	 */
	if (IsA(node, Var) ||
		IsA(node, PlaceHolderVar) ||
		IsA(node, Aggref) ||
		IsA(node, GroupingFunc) ||
		IsA(node, WindowFunc))
	{
		split_pathtarget_item *item = palloc(sizeof(split_pathtarget_item));

		item->expr = node;
		item->sortgroupref = context->current_sgref;
		context->current_input_vars = lappend(context->current_input_vars,
											  item);
		return false;
	}

	/*
	 * If it's a SRF, recursively examine its inputs, determine its level, and
	 * make appropriate entries in the output lists.
	 */
	if (IS_SRF_CALL(node))
	{
		split_pathtarget_item *item = palloc(sizeof(split_pathtarget_item));
		List	   *save_input_vars = context->current_input_vars;
		List	   *save_input_srfs = context->current_input_srfs;
		int			save_current_depth = context->current_depth;
		int			srf_depth;
		ListCell   *lc;

		item->expr = node;
		item->sortgroupref = context->current_sgref;

		context->current_input_vars = NIL;
		context->current_input_srfs = NIL;
		context->current_depth = 0;
		context->current_sgref = 0; /* subexpressions are not sortgroup items */

		(void) expression_tree_walker(node, split_pathtarget_walker,
									  (void *) context);

		/* Depth is one more than any SRF below it */
		srf_depth = context->current_depth + 1;

		/* If new record depth, initialize another level of output lists */
		if (srf_depth >= list_length(context->level_srfs))
		{
			context->level_srfs = lappend(context->level_srfs, NIL);
			context->level_input_vars = lappend(context->level_input_vars, NIL);
			context->level_input_srfs = lappend(context->level_input_srfs, NIL);
		}

		/* Record this SRF as needing to be evaluated at appropriate level */
		lc = list_nth_cell(context->level_srfs, srf_depth);
		lfirst(lc) = lappend(lfirst(lc), item);

		/* Record its inputs as being needed at the same level */
		lc = list_nth_cell(context->level_input_vars, srf_depth);
		lfirst(lc) = list_concat(lfirst(lc), context->current_input_vars);
		lc = list_nth_cell(context->level_input_srfs, srf_depth);
		lfirst(lc) = list_concat(lfirst(lc), context->current_input_srfs);

		/*
		 * Restore caller-level state and update it for presence of this SRF.
		 * Notice we report the SRF itself as being needed for evaluation of
		 * surrounding expression.
		 */
		context->current_input_vars = save_input_vars;
		context->current_input_srfs = lappend(save_input_srfs, item);
		context->current_depth = Max(save_current_depth, srf_depth);

		/* We're done here */
		return false;
	}

	/*
	 * Otherwise, the node is a scalar (non-set) expression, so recurse to
	 * examine its inputs.
	 */
	context->current_sgref = 0; /* subexpressions are not sortgroup items */
	return expression_tree_walker(node, split_pathtarget_walker,
								  (void *) context);
}

/*
 * Add a split_pathtarget_item to the PathTarget, unless a matching item is
 * already present.  This is like add_new_column_to_pathtarget, but allows
 * for sortgrouprefs to be handled.  An item having zero sortgroupref can
 * be merged with one that has a sortgroupref, acquiring the latter's
 * sortgroupref.
 *
 * Note that we don't worry about possibly adding duplicate sortgrouprefs
 * to the PathTarget.  That would be bad, but it should be impossible unless
 * the target passed to split_pathtarget_at_srfs already had duplicates.
 * As long as it didn't, we can have at most one split_pathtarget_item with
 * any particular nonzero sortgroupref.
 */
static void
add_sp_item_to_pathtarget(PathTarget *target, split_pathtarget_item *item)
{
	int			lci;
	ListCell   *lc;

	/*
	 * Look for a pre-existing entry that is equal() and does not have a
	 * conflicting sortgroupref already.
	 */
	lci = 0;
	foreach(lc, target->exprs)
	{
		Node	   *node = (Node *) lfirst(lc);
		Index		sgref = get_pathtarget_sortgroupref(target, lci);

		if ((item->sortgroupref == sgref ||
			 item->sortgroupref == 0 ||
			 sgref == 0) &&
			equal(item->expr, node))
		{
			/* Found a match.  Assign item's sortgroupref if it has one. */
			if (item->sortgroupref)
			{
				if (target->sortgrouprefs == NULL)
				{
					target->sortgrouprefs = (Index *)
						palloc0(list_length(target->exprs) * sizeof(Index));
				}
				target->sortgrouprefs[lci] = item->sortgroupref;
			}
			return;
		}
		lci++;
	}

	/*
	 * No match, so add item to PathTarget.  Copy the expr for safety.
	 */
	add_column_to_pathtarget(target, (Expr *) copyObject(item->expr),
							 item->sortgroupref);
}

/*
 * Apply add_sp_item_to_pathtarget to each element of list.
 */
static void
add_sp_items_to_pathtarget(PathTarget *target, List *items)
{
	ListCell   *lc;

	foreach(lc, items)
	{
		split_pathtarget_item *item = lfirst(lc);

		add_sp_item_to_pathtarget(target, item);
	}
}