deparse.c

/*-------------------------------------------------------------------------
 *
 * deparse.c
 *		  Query deparser for postgres_fdw
 *
 * This file includes functions that examine query WHERE clauses to see
 * whether they're safe to send to the remote server for execution, as
 * well as functions to construct the query text to be sent.  The latter
 * functionality is annoyingly duplicative of ruleutils.c, but there are
 * enough special considerations that it seems best to keep this separate.
 * One saving grace is that we only need deparse logic for node types that
 * we consider safe to send.
 *
 * We assume that the remote session's search_path is exactly "pg_catalog",
 * and thus we need schema-qualify all and only names outside pg_catalog.
 *
 * We do not consider that it is ever safe to send COLLATE expressions to
 * the remote server: it might not have the same collation names we do.
 * (Later we might consider it safe to send COLLATE "C", but even that would
 * fail on old remote servers.)  An expression is considered safe to send
 * only if all operator/function input collations used in it are traceable to
 * Var(s) of the foreign table.  That implies that if the remote server gets
 * a different answer than we do, the foreign table's columns are not marked
 * with collations that match the remote table's columns, which we can
 * consider to be user error.
 *
 * Portions Copyright (c) 2012-2016, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *		  contrib/postgres_fdw/deparse.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "postgres_fdw.h"

#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/sysattr.h"
#include "catalog/pg_collation.h"
#include "catalog/pg_namespace.h"
#include "catalog/pg_operator.h"
#include "catalog/pg_proc.h"
#include "catalog/pg_type.h"
#include "commands/defrem.h"
#include "nodes/makefuncs.h"
#include "nodes/nodeFuncs.h"
#include "nodes/plannodes.h"
#include "optimizer/clauses.h"
#include "optimizer/prep.h"
#include "optimizer/tlist.h"
#include "optimizer/var.h"
#include "parser/parsetree.h"
#include "utils/builtins.h"
#include "utils/lsyscache.h"
#include "utils/rel.h"
#include "utils/syscache.h"


/*
 * Global context for foreign_expr_walker's search of an expression tree.
 */
typedef struct foreign_glob_cxt
{
	PlannerInfo *root;			/* global planner state */
	RelOptInfo *foreignrel;		/* the foreign relation we are planning for */
} foreign_glob_cxt;

/*
 * Local (per-tree-level) context for foreign_expr_walker's search.
 * This is concerned with identifying collations used in the expression.
 */
typedef enum
{
	FDW_COLLATE_NONE,			/* expression is of a noncollatable type, or
								 * it has default collation that is not
								 * traceable to a foreign Var */
	FDW_COLLATE_SAFE,			/* collation derives from a foreign Var */
	FDW_COLLATE_UNSAFE			/* collation is non-default and derives from
								 * something other than a foreign Var */
} FDWCollateState;

typedef struct foreign_loc_cxt
{
	Oid			collation;		/* OID of current collation, if any */
	FDWCollateState state;		/* state of current collation choice */
} foreign_loc_cxt;

/*
 * Context for deparseExpr
 */
typedef struct deparse_expr_cxt
{
	PlannerInfo *root;			/* global planner state */
	RelOptInfo *foreignrel;		/* the foreign relation we are planning for */
	StringInfo	buf;			/* output buffer to append to */
	List	  **params_list;	/* exprs that will become remote Params */
} deparse_expr_cxt;

#define REL_ALIAS_PREFIX	"r"
/* Handy macro to add relation name qualification */
#define ADD_REL_QUALIFIER(buf, varno)	\
		appendStringInfo((buf), "%s%d.", REL_ALIAS_PREFIX, (varno))

/*
 * Functions to determine whether an expression can be evaluated safely on
 * remote server.
 */
static bool foreign_expr_walker(Node *node,
					foreign_glob_cxt *glob_cxt,
					foreign_loc_cxt *outer_cxt);
static char *deparse_type_name(Oid type_oid, int32 typemod);

/*
 * Functions to construct string representation of a node tree.
 */
static void deparseTargetList(StringInfo buf,
				  PlannerInfo *root,
				  Index rtindex,
				  Relation rel,
				  bool is_returning,
				  Bitmapset *attrs_used,
				  bool qualify_col,
				  List **retrieved_attrs);
static void deparseExplicitTargetList(List *tlist, List **retrieved_attrs,
						  deparse_expr_cxt *context);
static void deparseReturningList(StringInfo buf, PlannerInfo *root,
					 Index rtindex, Relation rel,
					 bool trig_after_row,
					 List *returningList,
					 List **retrieved_attrs);
static void deparseColumnRef(StringInfo buf, int varno, int varattno,
				 PlannerInfo *root, bool qualify_col);
static void deparseRelation(StringInfo buf, Relation rel);
static void deparseExpr(Expr *expr, deparse_expr_cxt *context);
static void deparseVar(Var *node, deparse_expr_cxt *context);
static void deparseConst(Const *node, deparse_expr_cxt *context);
static void deparseParam(Param *node, deparse_expr_cxt *context);
static void deparseArrayRef(ArrayRef *node, deparse_expr_cxt *context);
static void deparseFuncExpr(FuncExpr *node, deparse_expr_cxt *context);
static void deparseOpExpr(OpExpr *node, deparse_expr_cxt *context);
static void deparseOperatorName(StringInfo buf, Form_pg_operator opform);
static void deparseDistinctExpr(DistinctExpr *node, deparse_expr_cxt *context);
static void deparseScalarArrayOpExpr(ScalarArrayOpExpr *node,
						 deparse_expr_cxt *context);
static void deparseRelabelType(RelabelType *node, deparse_expr_cxt *context);
static void deparseBoolExpr(BoolExpr *node, deparse_expr_cxt *context);
static void deparseNullTest(NullTest *node, deparse_expr_cxt *context);
static void deparseArrayExpr(ArrayExpr *node, deparse_expr_cxt *context);
static void printRemoteParam(int paramindex, Oid paramtype, int32 paramtypmod,
				 deparse_expr_cxt *context);
static void printRemotePlaceholder(Oid paramtype, int32 paramtypmod,
					   deparse_expr_cxt *context);
static void deparseSelectSql(List *tlist, List **retrieved_attrs,
				 deparse_expr_cxt *context);
static void deparseLockingClause(deparse_expr_cxt *context);
static void appendOrderByClause(List *pathkeys, deparse_expr_cxt *context);
static void appendConditions(List *exprs, deparse_expr_cxt *context);
static void deparseFromExprForRel(StringInfo buf, PlannerInfo *root,
					RelOptInfo *joinrel, bool use_alias, List **params_list);


/*
 * Examine each qual clause in input_conds, and classify them into two groups,
 * which are returned as two lists:
 *	- remote_conds contains expressions that can be evaluated remotely
 *	- local_conds contains expressions that can't be evaluated remotely
 */
void
classifyConditions(PlannerInfo *root,
				   RelOptInfo *baserel,
				   List *input_conds,
				   List **remote_conds,
				   List **local_conds)
{
	ListCell   *lc;

	*remote_conds = NIL;
	*local_conds = NIL;

	foreach(lc, input_conds)
	{
		RestrictInfo *ri = (RestrictInfo *) lfirst(lc);

		if (is_foreign_expr(root, baserel, ri->clause))
			*remote_conds = lappend(*remote_conds, ri);
		else
			*local_conds = lappend(*local_conds, ri);
	}
}

/*
 * Returns true if given expr is safe to evaluate on the foreign server.
 */
bool
is_foreign_expr(PlannerInfo *root,
				RelOptInfo *baserel,
				Expr *expr)
{
	foreign_glob_cxt glob_cxt;
	foreign_loc_cxt loc_cxt;

	/*
	 * Check that the expression consists of nodes that are safe to execute
	 * remotely.
	 */
	glob_cxt.root = root;
	glob_cxt.foreignrel = baserel;
	loc_cxt.collation = InvalidOid;
	loc_cxt.state = FDW_COLLATE_NONE;
	if (!foreign_expr_walker((Node *) expr, &glob_cxt, &loc_cxt))
		return false;

	/*
	 * If the expression has a valid collation that does not arise from a
	 * foreign var, the expression can not be sent over.
	 */
	if (loc_cxt.state == FDW_COLLATE_UNSAFE)
		return false;

	/*
	 * An expression which includes any mutable functions can't be sent over
	 * because its result is not stable.  For example, sending now() remote
	 * side could cause confusion from clock offsets.  Future versions might
	 * be able to make this choice with more granularity.  (We check this last
	 * because it requires a lot of expensive catalog lookups.)
	 */
	if (contain_mutable_functions((Node *) expr))
		return false;

	/* OK to evaluate on the remote server */
	return true;
}

/*
 * Check if expression is safe to execute remotely, and return true if so.
 *
 * In addition, *outer_cxt is updated with collation information.
 *
 * We must check that the expression contains only node types we can deparse,
 * that all types/functions/operators are safe to send (they are "shippable"),
 * and that all collations used in the expression derive from Vars of the
 * foreign table.  Because of the latter, the logic is pretty close to
 * assign_collations_walker() in parse_collate.c, though we can assume here
 * that the given expression is valid.  Note function mutability is not
 * currently considered here.
 */
static bool
foreign_expr_walker(Node *node,
					foreign_glob_cxt *glob_cxt,
					foreign_loc_cxt *outer_cxt)
{
	bool		check_type = true;
	PgFdwRelationInfo *fpinfo;
	foreign_loc_cxt inner_cxt;
	Oid			collation;
	FDWCollateState state;

	/* Need do nothing for empty subexpressions */
	if (node == NULL)
		return true;

	/* May need server info from baserel's fdw_private struct */
	fpinfo = (PgFdwRelationInfo *) (glob_cxt->foreignrel->fdw_private);

	/* Set up inner_cxt for possible recursion to child nodes */
	inner_cxt.collation = InvalidOid;
	inner_cxt.state = FDW_COLLATE_NONE;

	switch (nodeTag(node))
	{
		case T_Var:
			{
				Var		   *var = (Var *) node;

				/*
				 * If the Var is from the foreign table, we consider its
				 * collation (if any) safe to use.  If it is from another
				 * table, we treat its collation the same way as we would a
				 * Param's collation, ie it's not safe for it to have a
				 * non-default collation.
				 */
				if (bms_is_member(var->varno, glob_cxt->foreignrel->relids) &&
					var->varlevelsup == 0)
				{
					/* Var belongs to foreign table */

					/*
					 * System columns other than ctid should not be sent to
					 * the remote, since we don't make any effort to ensure
					 * that local and remote values match (tableoid, in
					 * particular, almost certainly doesn't match).
					 */
					if (var->varattno < 0 &&
						var->varattno != SelfItemPointerAttributeNumber)
						return false;

					/* Else check the collation */
					collation = var->varcollid;
					state = OidIsValid(collation) ? FDW_COLLATE_SAFE : FDW_COLLATE_NONE;
				}
				else
				{
					/* Var belongs to some other table */
					collation = var->varcollid;
					if (collation == InvalidOid ||
						collation == DEFAULT_COLLATION_OID)
					{
						/*
						 * It's noncollatable, or it's safe to combine with a
						 * collatable foreign Var, so set state to NONE.
						 */
						state = FDW_COLLATE_NONE;
					}
					else
					{
						/*
						 * Do not fail right away, since the Var might appear
						 * in a collation-insensitive context.
						 */
						state = FDW_COLLATE_UNSAFE;
					}
				}
			}
			break;
		case T_Const:
			{
				Const	   *c = (Const *) node;

				/*
				 * If the constant has nondefault collation, either it's of a
				 * non-builtin type, or it reflects folding of a CollateExpr.
				 * It's unsafe to send to the remote unless it's used in a
				 * non-collation-sensitive context.
				 */
				collation = c->constcollid;
				if (collation == InvalidOid ||
					collation == DEFAULT_COLLATION_OID)
					state = FDW_COLLATE_NONE;
				else
					state = FDW_COLLATE_UNSAFE;
			}
			break;
		case T_Param:
			{
				Param	   *p = (Param *) node;

				/*
				 * Collation rule is same as for Consts and non-foreign Vars.
				 */
				collation = p->paramcollid;
				if (collation == InvalidOid ||
					collation == DEFAULT_COLLATION_OID)
					state = FDW_COLLATE_NONE;
				else
					state = FDW_COLLATE_UNSAFE;
			}
			break;
		case T_ArrayRef:
			{
				ArrayRef   *ar = (ArrayRef *) node;

				/* Assignment should not be in restrictions. */
				if (ar->refassgnexpr != NULL)
					return false;

				/*
				 * Recurse to remaining subexpressions.  Since the array
				 * subscripts must yield (noncollatable) integers, they won't
				 * affect the inner_cxt state.
				 */
				if (!foreign_expr_walker((Node *) ar->refupperindexpr,
										 glob_cxt, &inner_cxt))
					return false;
				if (!foreign_expr_walker((Node *) ar->reflowerindexpr,
										 glob_cxt, &inner_cxt))
					return false;
				if (!foreign_expr_walker((Node *) ar->refexpr,
										 glob_cxt, &inner_cxt))
					return false;

				/*
				 * Array subscripting should yield same collation as input,
				 * but for safety use same logic as for function nodes.
				 */
				collation = ar->refcollid;
				if (collation == InvalidOid)
					state = FDW_COLLATE_NONE;
				else if (inner_cxt.state == FDW_COLLATE_SAFE &&
						 collation == inner_cxt.collation)
					state = FDW_COLLATE_SAFE;
				else if (collation == DEFAULT_COLLATION_OID)
					state = FDW_COLLATE_NONE;
				else
					state = FDW_COLLATE_UNSAFE;
			}
			break;
		case T_FuncExpr:
			{
				FuncExpr   *fe = (FuncExpr *) node;

				/*
				 * If function used by the expression is not shippable, it
				 * can't be sent to remote because it might have incompatible
				 * semantics on remote side.
				 */
				if (!is_shippable(fe->funcid, ProcedureRelationId, fpinfo))
					return false;

				/*
				 * Recurse to input subexpressions.
				 */
				if (!foreign_expr_walker((Node *) fe->args,
										 glob_cxt, &inner_cxt))
					return false;

				/*
				 * If function's input collation is not derived from a foreign
				 * Var, it can't be sent to remote.
				 */
				if (fe->inputcollid == InvalidOid)
					 /* OK, inputs are all noncollatable */ ;
				else if (inner_cxt.state != FDW_COLLATE_SAFE ||
						 fe->inputcollid != inner_cxt.collation)
					return false;

				/*
				 * Detect whether node is introducing a collation not derived
				 * from a foreign Var.  (If so, we just mark it unsafe for now
				 * rather than immediately returning false, since the parent
				 * node might not care.)
				 */
				collation = fe->funccollid;
				if (collation == InvalidOid)
					state = FDW_COLLATE_NONE;
				else if (inner_cxt.state == FDW_COLLATE_SAFE &&
						 collation == inner_cxt.collation)
					state = FDW_COLLATE_SAFE;
				else if (collation == DEFAULT_COLLATION_OID)
					state = FDW_COLLATE_NONE;
				else
					state = FDW_COLLATE_UNSAFE;
			}
			break;
		case T_OpExpr:
		case T_DistinctExpr:	/* struct-equivalent to OpExpr */
			{
				OpExpr	   *oe = (OpExpr *) node;

				/*
				 * Similarly, only shippable operators can be sent to remote.
				 * (If the operator is shippable, we assume its underlying
				 * function is too.)
				 */
				if (!is_shippable(oe->opno, OperatorRelationId, fpinfo))
					return false;

				/*
				 * Recurse to input subexpressions.
				 */
				if (!foreign_expr_walker((Node *) oe->args,
										 glob_cxt, &inner_cxt))
					return false;

				/*
				 * If operator's input collation is not derived from a foreign
				 * Var, it can't be sent to remote.
				 */
				if (oe->inputcollid == InvalidOid)
					 /* OK, inputs are all noncollatable */ ;
				else if (inner_cxt.state != FDW_COLLATE_SAFE ||
						 oe->inputcollid != inner_cxt.collation)
					return false;

				/* Result-collation handling is same as for functions */
				collation = oe->opcollid;
				if (collation == InvalidOid)
					state = FDW_COLLATE_NONE;
				else if (inner_cxt.state == FDW_COLLATE_SAFE &&
						 collation == inner_cxt.collation)
					state = FDW_COLLATE_SAFE;
				else if (collation == DEFAULT_COLLATION_OID)
					state = FDW_COLLATE_NONE;
				else
					state = FDW_COLLATE_UNSAFE;
			}
			break;
		case T_ScalarArrayOpExpr:
			{
				ScalarArrayOpExpr *oe = (ScalarArrayOpExpr *) node;

				/*
				 * Again, only shippable operators can be sent to remote.
				 */
				if (!is_shippable(oe->opno, OperatorRelationId, fpinfo))
					return false;

				/*
				 * Recurse to input subexpressions.
				 */
				if (!foreign_expr_walker((Node *) oe->args,
										 glob_cxt, &inner_cxt))
					return false;

				/*
				 * If operator's input collation is not derived from a foreign
				 * Var, it can't be sent to remote.
				 */
				if (oe->inputcollid == InvalidOid)
					 /* OK, inputs are all noncollatable */ ;
				else if (inner_cxt.state != FDW_COLLATE_SAFE ||
						 oe->inputcollid != inner_cxt.collation)
					return false;

				/* Output is always boolean and so noncollatable. */
				collation = InvalidOid;
				state = FDW_COLLATE_NONE;
			}
			break;
		case T_RelabelType:
			{
				RelabelType *r = (RelabelType *) node;

				/*
				 * Recurse to input subexpression.
				 */
				if (!foreign_expr_walker((Node *) r->arg,
										 glob_cxt, &inner_cxt))
					return false;

				/*
				 * RelabelType must not introduce a collation not derived from
				 * an input foreign Var (same logic as for a real function).
				 */
				collation = r->resultcollid;
				if (collation == InvalidOid)
					state = FDW_COLLATE_NONE;
				else if (inner_cxt.state == FDW_COLLATE_SAFE &&
						 collation == inner_cxt.collation)
					state = FDW_COLLATE_SAFE;
				else if (collation == DEFAULT_COLLATION_OID)
					state = FDW_COLLATE_NONE;
				else
					state = FDW_COLLATE_UNSAFE;
			}
			break;
		case T_BoolExpr:
			{
				BoolExpr   *b = (BoolExpr *) node;

				/*
				 * Recurse to input subexpressions.
				 */
				if (!foreign_expr_walker((Node *) b->args,
										 glob_cxt, &inner_cxt))
					return false;

				/* Output is always boolean and so noncollatable. */
				collation = InvalidOid;
				state = FDW_COLLATE_NONE;
			}
			break;
		case T_NullTest:
			{
				NullTest   *nt = (NullTest *) node;

				/*
				 * Recurse to input subexpressions.
				 */
				if (!foreign_expr_walker((Node *) nt->arg,
										 glob_cxt, &inner_cxt))
					return false;

				/* Output is always boolean and so noncollatable. */
				collation = InvalidOid;
				state = FDW_COLLATE_NONE;
			}
			break;
		case T_ArrayExpr:
			{
				ArrayExpr  *a = (ArrayExpr *) node;

				/*
				 * Recurse to input subexpressions.
				 */
				if (!foreign_expr_walker((Node *) a->elements,
										 glob_cxt, &inner_cxt))
					return false;

				/*
				 * ArrayExpr must not introduce a collation not derived from
				 * an input foreign Var (same logic as for a function).
				 */
				collation = a->array_collid;
				if (collation == InvalidOid)
					state = FDW_COLLATE_NONE;
				else if (inner_cxt.state == FDW_COLLATE_SAFE &&
						 collation == inner_cxt.collation)
					state = FDW_COLLATE_SAFE;
				else if (collation == DEFAULT_COLLATION_OID)
					state = FDW_COLLATE_NONE;
				else
					state = FDW_COLLATE_UNSAFE;
			}
			break;
		case T_List:
			{
				List	   *l = (List *) node;
				ListCell   *lc;

				/*
				 * Recurse to component subexpressions.
				 */
				foreach(lc, l)
				{
					if (!foreign_expr_walker((Node *) lfirst(lc),
											 glob_cxt, &inner_cxt))
						return false;
				}

				/*
				 * When processing a list, collation state just bubbles up
				 * from the list elements.
				 */
				collation = inner_cxt.collation;
				state = inner_cxt.state;

				/* Don't apply exprType() to the list. */
				check_type = false;
			}
			break;
		default:

			/*
			 * If it's anything else, assume it's unsafe.  This list can be
			 * expanded later, but don't forget to add deparse support below.
			 */
			return false;
	}

	/*
	 * If result type of given expression is not shippable, it can't be sent
	 * to remote because it might have incompatible semantics on remote side.
	 */
	if (check_type && !is_shippable(exprType(node), TypeRelationId, fpinfo))
		return false;

	/*
	 * Now, merge my collation information into my parent's state.
	 */
	if (state > outer_cxt->state)
	{
		/* Override previous parent state */
		outer_cxt->collation = collation;
		outer_cxt->state = state;
	}
	else if (state == outer_cxt->state)
	{
		/* Merge, or detect error if there's a collation conflict */
		switch (state)
		{
			case FDW_COLLATE_NONE:
				/* Nothing + nothing is still nothing */
				break;
			case FDW_COLLATE_SAFE:
				if (collation != outer_cxt->collation)
				{
					/*
					 * Non-default collation always beats default.
					 */
					if (outer_cxt->collation == DEFAULT_COLLATION_OID)
					{
						/* Override previous parent state */
						outer_cxt->collation = collation;
					}
					else if (collation != DEFAULT_COLLATION_OID)
					{
						/*
						 * Conflict; show state as indeterminate.  We don't
						 * want to "return false" right away, since parent
						 * node might not care about collation.
						 */
						outer_cxt->state = FDW_COLLATE_UNSAFE;
					}
				}
				break;
			case FDW_COLLATE_UNSAFE:
				/* We're still conflicted ... */
				break;
		}
	}

	/* It looks OK */
	return true;
}

/*
 * Convert type OID + typmod info into a type name we can ship to the remote
 * server.  Someplace else had better have verified that this type name is
 * expected to be known on the remote end.
 *
 * This is almost just format_type_with_typemod(), except that if left to its
 * own devices, that function will make schema-qualification decisions based
 * on the local search_path, which is wrong.  We must schema-qualify all
 * type names that are not in pg_catalog.  We assume here that built-in types
 * are all in pg_catalog and need not be qualified; otherwise, qualify.
 */
static char *
deparse_type_name(Oid type_oid, int32 typemod)
{
	if (is_builtin(type_oid))
		return format_type_with_typemod(type_oid, typemod);
	else
		return format_type_with_typemod_qualified(type_oid, typemod);
}

/*
 * Build the targetlist for given relation to be deparsed as SELECT clause.
 *
 * The output targetlist contains the columns that need to be fetched from the
 * foreign server for the given relation.
 */
List *
build_tlist_to_deparse(RelOptInfo *foreignrel)
{
	List	   *tlist = NIL;
	PgFdwRelationInfo *fpinfo = (PgFdwRelationInfo *) foreignrel->fdw_private;

	/*
	 * We require columns specified in foreignrel->reltarget->exprs and those
	 * required for evaluating the local conditions.
	 */
	tlist = add_to_flat_tlist(tlist,
					   pull_var_clause((Node *) foreignrel->reltarget->exprs,
									   PVC_RECURSE_PLACEHOLDERS));
	tlist = add_to_flat_tlist(tlist,
							  pull_var_clause((Node *) fpinfo->local_conds,
											  PVC_RECURSE_PLACEHOLDERS));

	return tlist;
}

/*
 * Deparse SELECT statement for given relation into buf.
 *
 * tlist contains the list of desired columns to be fetched from foreign server.
 * For a base relation fpinfo->attrs_used is used to construct SELECT clause,
 * hence the tlist is ignored for a base relation.
 *
 * remote_conds is the list of conditions to be deparsed as WHERE clause.
 *
 * If params_list is not NULL, it receives a list of Params and other-relation
 * Vars used in the clauses; these values must be transmitted to the remote
 * server as parameter values.
 *
 * If params_list is NULL, we're generating the query for EXPLAIN purposes,
 * so Params and other-relation Vars should be replaced by dummy values.
 *
 * pathkeys is the list of pathkeys to order the result by.
 *
 * List of columns selected is returned in retrieved_attrs.
 */
extern void
deparseSelectStmtForRel(StringInfo buf, PlannerInfo *root, RelOptInfo *rel,
						List *tlist, List *remote_conds, List *pathkeys,
						List **retrieved_attrs, List **params_list)
{
	deparse_expr_cxt context;

	/* We handle relations for foreign tables and joins between those */
	Assert(rel->reloptkind == RELOPT_JOINREL ||
		   rel->reloptkind == RELOPT_BASEREL ||
		   rel->reloptkind == RELOPT_OTHER_MEMBER_REL);

	/* Fill portions of context common to join and base relation */
	context.buf = buf;
	context.root = root;
	context.foreignrel = rel;
	context.params_list = params_list;

	/* Construct SELECT clause and FROM clause */
	deparseSelectSql(tlist, retrieved_attrs, &context);

	/*
	 * Construct WHERE clause
	 */
	if (remote_conds)
	{
		appendStringInfo(buf, " WHERE ");
		appendConditions(remote_conds, &context);
	}

	/* Add ORDER BY clause if we found any useful pathkeys */
	if (pathkeys)
		appendOrderByClause(pathkeys, &context);

	/* Add any necessary FOR UPDATE/SHARE. */
	deparseLockingClause(&context);
}

/*
 * Construct a simple SELECT statement that retrieves desired columns
 * of the specified foreign table, and append it to "buf".  The output
 * contains just "SELECT ... FROM ....".
 *
 * We also create an integer List of the columns being retrieved, which is
 * returned to *retrieved_attrs.
 *
 * tlist is the list of desired columns. Read prologue of
 * deparseSelectStmtForRel() for details.
 */
static void
deparseSelectSql(List *tlist, List **retrieved_attrs, deparse_expr_cxt *context)
{
	StringInfo	buf = context->buf;
	RelOptInfo *foreignrel = context->foreignrel;
	PlannerInfo *root = context->root;
	PgFdwRelationInfo *fpinfo = (PgFdwRelationInfo *) foreignrel->fdw_private;

	/*
	 * Construct SELECT list
	 */
	appendStringInfoString(buf, "SELECT ");

	if (foreignrel->reloptkind == RELOPT_JOINREL)
	{
		/* For a join relation use the input tlist */
		deparseExplicitTargetList(tlist, retrieved_attrs, context);
	}
	else
	{
		/*
		 * For a base relation fpinfo->attrs_used gives the list of columns
		 * required to be fetched from the foreign server.
		 */
		RangeTblEntry *rte = planner_rt_fetch(foreignrel->relid, root);

		/*
		 * Core code already has some lock on each rel being planned, so we
		 * can use NoLock here.
		 */
		Relation	rel = heap_open(rte->relid, NoLock);

		deparseTargetList(buf, root, foreignrel->relid, rel, false,
						  fpinfo->attrs_used, false, retrieved_attrs);
		heap_close(rel, NoLock);
	}

	/*
	 * Construct FROM clause
	 */
	appendStringInfoString(buf, " FROM ");
	deparseFromExprForRel(buf, root, foreignrel,
						  (foreignrel->reloptkind == RELOPT_JOINREL),
						  context->params_list);
}

/*
 * Emit a target list that retrieves the columns specified in attrs_used.
 * This is used for both SELECT and RETURNING targetlists; the is_returning
 * parameter is true only for a RETURNING targetlist.
 *
 * The tlist text is appended to buf, and we also create an integer List
 * of the columns being retrieved, which is returned to *retrieved_attrs.
 *
 * If qualify_col is true, add relation alias before the column name.
 */
static void
deparseTargetList(StringInfo buf,
				  PlannerInfo *root,
				  Index rtindex,
				  Relation rel,
				  bool is_returning,
				  Bitmapset *attrs_used,
				  bool qualify_col,
				  List **retrieved_attrs)
{
	TupleDesc	tupdesc = RelationGetDescr(rel);
	bool		have_wholerow;
	bool		first;
	int			i;

	*retrieved_attrs = NIL;

	/* If there's a whole-row reference, we'll need all the columns. */
	have_wholerow = bms_is_member(0 - FirstLowInvalidHeapAttributeNumber,
								  attrs_used);

	first = true;
	for (i = 1; i <= tupdesc->natts; i++)
	{
		Form_pg_attribute attr = tupdesc->attrs[i - 1];

		/* Ignore dropped attributes. */
		if (attr->attisdropped)
			continue;

		if (have_wholerow ||
			bms_is_member(i - FirstLowInvalidHeapAttributeNumber,
						  attrs_used))
		{
			if (!first)
				appendStringInfoString(buf, ", ");
			else if (is_returning)
				appendStringInfoString(buf, " RETURNING ");
			first = false;

			deparseColumnRef(buf, rtindex, i, root, qualify_col);

			*retrieved_attrs = lappend_int(*retrieved_attrs, i);
		}
	}

	/*
	 * Add ctid if needed.  We currently don't support retrieving any other
	 * system columns.
	 */
	if (bms_is_member(SelfItemPointerAttributeNumber - FirstLowInvalidHeapAttributeNumber,
					  attrs_used))
	{
		if (!first)
			appendStringInfoString(buf, ", ");
		else if (is_returning)
			appendStringInfoString(buf, " RETURNING ");
		first = false;

		if (qualify_col)
			ADD_REL_QUALIFIER(buf, rtindex);
		appendStringInfoString(buf, "ctid");

		*retrieved_attrs = lappend_int(*retrieved_attrs,
									   SelfItemPointerAttributeNumber);
	}

	/* Don't generate bad syntax if no undropped columns */
	if (first && !is_returning)
		appendStringInfoString(buf, "NULL");
}

/*
 * Deparse the appropriate locking clause (FOR SELECT or FOR SHARE) for a
 * given relation (context->foreignrel).
 */
static void
deparseLockingClause(deparse_expr_cxt *context)
{
	StringInfo	buf = context->buf;
	PlannerInfo *root = context->root;
	RelOptInfo *rel = context->foreignrel;
	int			relid = -1;

	while ((relid = bms_next_member(rel->relids, relid)) >= 0)
	{
		/*
		 * Add FOR UPDATE/SHARE if appropriate.  We apply locking during the
		 * initial row fetch, rather than later on as is done for local
		 * tables. The extra roundtrips involved in trying to duplicate the
		 * local semantics exactly don't seem worthwhile (see also comments
		 * for RowMarkType).
		 *
		 * Note: because we actually run the query as a cursor, this assumes
		 * that DECLARE CURSOR ... FOR UPDATE is supported, which it isn't
		 * before 8.3.
		 */
		if (relid == root->parse->resultRelation &&
			(root->parse->commandType == CMD_UPDATE ||
			 root->parse->commandType == CMD_DELETE))
		{
			/* Relation is UPDATE/DELETE target, so use FOR UPDATE */
			appendStringInfoString(buf, " FOR UPDATE");

			/* Add the relation alias if we are here for a join relation */
			if (rel->reloptkind == RELOPT_JOINREL)
				appendStringInfo(buf, " OF %s%d", REL_ALIAS_PREFIX, relid);
		}
		else
		{
			PlanRowMark *rc = get_plan_rowmark(root->rowMarks, relid);

			if (rc)
			{
				/*
				 * Relation is specified as a FOR UPDATE/SHARE target, so
				 * handle that.  (But we could also see LCS_NONE, meaning this
				 * isn't a target relation after all.)
				 *
				 * For now, just ignore any [NO] KEY specification, since (a)
				 * it's not clear what that means for a remote table that we
				 * don't have complete information about, and (b) it wouldn't
				 * work anyway on older remote servers.  Likewise, we don't
				 * worry about NOWAIT.
				 */
				switch (rc->strength)
				{
					case LCS_NONE:
						/* No locking needed */
						break;
					case LCS_FORKEYSHARE:
					case LCS_FORSHARE: