Adding upstream version 16.2.upstream/16.2

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

3 files changed, 836 insertions, 0 deletions

diff --git a/src/backend/foreign/Makefile b/src/backend/foreign/Makefile
new file mode 100644
index 0000000..dfea028
--- /dev/null
+++ b/src/backend/foreign/Makefile
@@ -0,0 +1,18 @@
+#-------------------------------------------------------------------------
+#
+# Makefile--
+#    Makefile for foreign
+#
+# IDENTIFICATION
+#    src/backend/foreign/Makefile
+#
+#-------------------------------------------------------------------------
+
+subdir = src/backend/foreign
+top_builddir = ../../..
+include $(top_builddir)/src/Makefile.global
+
+OBJS = \
+	foreign.o
+
+include $(top_srcdir)/src/backend/common.mk
diff --git a/src/backend/foreign/foreign.c b/src/backend/foreign/foreign.c
new file mode 100644
index 0000000..ca3ad55
--- /dev/null
+++ b/src/backend/foreign/foreign.c
@@ -0,0 +1,813 @@
+/*-------------------------------------------------------------------------
+ *
+ * foreign.c
+ *		  support for foreign-data wrappers, servers and user mappings.
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		  src/backend/foreign/foreign.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/htup_details.h"
+#include "access/reloptions.h"
+#include "catalog/pg_foreign_data_wrapper.h"
+#include "catalog/pg_foreign_server.h"
+#include "catalog/pg_foreign_table.h"
+#include "catalog/pg_user_mapping.h"
+#include "foreign/fdwapi.h"
+#include "foreign/foreign.h"
+#include "funcapi.h"
+#include "lib/stringinfo.h"
+#include "miscadmin.h"
+#include "utils/builtins.h"
+#include "utils/memutils.h"
+#include "utils/rel.h"
+#include "utils/syscache.h"
+#include "utils/varlena.h"
+
+
+/*
+ * GetForeignDataWrapper -	look up the foreign-data wrapper by OID.
+ */
+ForeignDataWrapper *
+GetForeignDataWrapper(Oid fdwid)
+{
+	return GetForeignDataWrapperExtended(fdwid, 0);
+}
+
+
+/*
+ * GetForeignDataWrapperExtended -	look up the foreign-data wrapper
+ * by OID. If flags uses FDW_MISSING_OK, return NULL if the object cannot
+ * be found instead of raising an error.
+ */
+ForeignDataWrapper *
+GetForeignDataWrapperExtended(Oid fdwid, bits16 flags)
+{
+	Form_pg_foreign_data_wrapper fdwform;
+	ForeignDataWrapper *fdw;
+	Datum		datum;
+	HeapTuple	tp;
+	bool		isnull;
+
+	tp = SearchSysCache1(FOREIGNDATAWRAPPEROID, ObjectIdGetDatum(fdwid));
+
+	if (!HeapTupleIsValid(tp))
+	{
+		if ((flags & FDW_MISSING_OK) == 0)
+			elog(ERROR, "cache lookup failed for foreign-data wrapper %u", fdwid);
+		return NULL;
+	}
+
+	fdwform = (Form_pg_foreign_data_wrapper) GETSTRUCT(tp);
+
+	fdw = (ForeignDataWrapper *) palloc(sizeof(ForeignDataWrapper));
+	fdw->fdwid = fdwid;
+	fdw->owner = fdwform->fdwowner;
+	fdw->fdwname = pstrdup(NameStr(fdwform->fdwname));
+	fdw->fdwhandler = fdwform->fdwhandler;
+	fdw->fdwvalidator = fdwform->fdwvalidator;
+
+	/* Extract the fdwoptions */
+	datum = SysCacheGetAttr(FOREIGNDATAWRAPPEROID,
+							tp,
+							Anum_pg_foreign_data_wrapper_fdwoptions,
+							&isnull);
+	if (isnull)
+		fdw->options = NIL;
+	else
+		fdw->options = untransformRelOptions(datum);
+
+	ReleaseSysCache(tp);
+
+	return fdw;
+}
+
+
+/*
+ * GetForeignDataWrapperByName - look up the foreign-data wrapper
+ * definition by name.
+ */
+ForeignDataWrapper *
+GetForeignDataWrapperByName(const char *fdwname, bool missing_ok)
+{
+	Oid			fdwId = get_foreign_data_wrapper_oid(fdwname, missing_ok);
+
+	if (!OidIsValid(fdwId))
+		return NULL;
+
+	return GetForeignDataWrapper(fdwId);
+}
+
+
+/*
+ * GetForeignServer - look up the foreign server definition.
+ */
+ForeignServer *
+GetForeignServer(Oid serverid)
+{
+	return GetForeignServerExtended(serverid, 0);
+}
+
+
+/*
+ * GetForeignServerExtended - look up the foreign server definition. If
+ * flags uses FSV_MISSING_OK, return NULL if the object cannot be found
+ * instead of raising an error.
+ */
+ForeignServer *
+GetForeignServerExtended(Oid serverid, bits16 flags)
+{
+	Form_pg_foreign_server serverform;
+	ForeignServer *server;
+	HeapTuple	tp;
+	Datum		datum;
+	bool		isnull;
+
+	tp = SearchSysCache1(FOREIGNSERVEROID, ObjectIdGetDatum(serverid));
+
+	if (!HeapTupleIsValid(tp))
+	{
+		if ((flags & FSV_MISSING_OK) == 0)
+			elog(ERROR, "cache lookup failed for foreign server %u", serverid);
+		return NULL;
+	}
+
+	serverform = (Form_pg_foreign_server) GETSTRUCT(tp);
+
+	server = (ForeignServer *) palloc(sizeof(ForeignServer));
+	server->serverid = serverid;
+	server->servername = pstrdup(NameStr(serverform->srvname));
+	server->owner = serverform->srvowner;
+	server->fdwid = serverform->srvfdw;
+
+	/* Extract server type */
+	datum = SysCacheGetAttr(FOREIGNSERVEROID,
+							tp,
+							Anum_pg_foreign_server_srvtype,
+							&isnull);
+	server->servertype = isnull ? NULL : TextDatumGetCString(datum);
+
+	/* Extract server version */
+	datum = SysCacheGetAttr(FOREIGNSERVEROID,
+							tp,
+							Anum_pg_foreign_server_srvversion,
+							&isnull);
+	server->serverversion = isnull ? NULL : TextDatumGetCString(datum);
+
+	/* Extract the srvoptions */
+	datum = SysCacheGetAttr(FOREIGNSERVEROID,
+							tp,
+							Anum_pg_foreign_server_srvoptions,
+							&isnull);
+	if (isnull)
+		server->options = NIL;
+	else
+		server->options = untransformRelOptions(datum);
+
+	ReleaseSysCache(tp);
+
+	return server;
+}
+
+
+/*
+ * GetForeignServerByName - look up the foreign server definition by name.
+ */
+ForeignServer *
+GetForeignServerByName(const char *srvname, bool missing_ok)
+{
+	Oid			serverid = get_foreign_server_oid(srvname, missing_ok);
+
+	if (!OidIsValid(serverid))
+		return NULL;
+
+	return GetForeignServer(serverid);
+}
+
+
+/*
+ * GetUserMapping - look up the user mapping.
+ *
+ * If no mapping is found for the supplied user, we also look for
+ * PUBLIC mappings (userid == InvalidOid).
+ */
+UserMapping *
+GetUserMapping(Oid userid, Oid serverid)
+{
+	Datum		datum;
+	HeapTuple	tp;
+	bool		isnull;
+	UserMapping *um;
+
+	tp = SearchSysCache2(USERMAPPINGUSERSERVER,
+						 ObjectIdGetDatum(userid),
+						 ObjectIdGetDatum(serverid));
+
+	if (!HeapTupleIsValid(tp))
+	{
+		/* Not found for the specific user -- try PUBLIC */
+		tp = SearchSysCache2(USERMAPPINGUSERSERVER,
+							 ObjectIdGetDatum(InvalidOid),
+							 ObjectIdGetDatum(serverid));
+	}
+
+	if (!HeapTupleIsValid(tp))
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_OBJECT),
+				 errmsg("user mapping not found for \"%s\"",
+						MappingUserName(userid))));
+
+	um = (UserMapping *) palloc(sizeof(UserMapping));
+	um->umid = ((Form_pg_user_mapping) GETSTRUCT(tp))->oid;
+	um->userid = userid;
+	um->serverid = serverid;
+
+	/* Extract the umoptions */
+	datum = SysCacheGetAttr(USERMAPPINGUSERSERVER,
+							tp,
+							Anum_pg_user_mapping_umoptions,
+							&isnull);
+	if (isnull)
+		um->options = NIL;
+	else
+		um->options = untransformRelOptions(datum);
+
+	ReleaseSysCache(tp);
+
+	return um;
+}
+
+
+/*
+ * GetForeignTable - look up the foreign table definition by relation oid.
+ */
+ForeignTable *
+GetForeignTable(Oid relid)
+{
+	Form_pg_foreign_table tableform;
+	ForeignTable *ft;
+	HeapTuple	tp;
+	Datum		datum;
+	bool		isnull;
+
+	tp = SearchSysCache1(FOREIGNTABLEREL, ObjectIdGetDatum(relid));
+	if (!HeapTupleIsValid(tp))
+		elog(ERROR, "cache lookup failed for foreign table %u", relid);
+	tableform = (Form_pg_foreign_table) GETSTRUCT(tp);
+
+	ft = (ForeignTable *) palloc(sizeof(ForeignTable));
+	ft->relid = relid;
+	ft->serverid = tableform->ftserver;
+
+	/* Extract the ftoptions */
+	datum = SysCacheGetAttr(FOREIGNTABLEREL,
+							tp,
+							Anum_pg_foreign_table_ftoptions,
+							&isnull);
+	if (isnull)
+		ft->options = NIL;
+	else
+		ft->options = untransformRelOptions(datum);
+
+	ReleaseSysCache(tp);
+
+	return ft;
+}
+
+
+/*
+ * GetForeignColumnOptions - Get attfdwoptions of given relation/attnum
+ * as list of DefElem.
+ */
+List *
+GetForeignColumnOptions(Oid relid, AttrNumber attnum)
+{
+	List	   *options;
+	HeapTuple	tp;
+	Datum		datum;
+	bool		isnull;
+
+	tp = SearchSysCache2(ATTNUM,
+						 ObjectIdGetDatum(relid),
+						 Int16GetDatum(attnum));
+	if (!HeapTupleIsValid(tp))
+		elog(ERROR, "cache lookup failed for attribute %d of relation %u",
+			 attnum, relid);
+	datum = SysCacheGetAttr(ATTNUM,
+							tp,
+							Anum_pg_attribute_attfdwoptions,
+							&isnull);
+	if (isnull)
+		options = NIL;
+	else
+		options = untransformRelOptions(datum);
+
+	ReleaseSysCache(tp);
+
+	return options;
+}
+
+
+/*
+ * GetFdwRoutine - call the specified foreign-data wrapper handler routine
+ * to get its FdwRoutine struct.
+ */
+FdwRoutine *
+GetFdwRoutine(Oid fdwhandler)
+{
+	Datum		datum;
+	FdwRoutine *routine;
+
+	datum = OidFunctionCall0(fdwhandler);
+	routine = (FdwRoutine *) DatumGetPointer(datum);
+
+	if (routine == NULL || !IsA(routine, FdwRoutine))
+		elog(ERROR, "foreign-data wrapper handler function %u did not return an FdwRoutine struct",
+			 fdwhandler);
+
+	return routine;
+}
+
+
+/*
+ * GetForeignServerIdByRelId - look up the foreign server
+ * for the given foreign table, and return its OID.
+ */
+Oid
+GetForeignServerIdByRelId(Oid relid)
+{
+	HeapTuple	tp;
+	Form_pg_foreign_table tableform;
+	Oid			serverid;
+
+	tp = SearchSysCache1(FOREIGNTABLEREL, ObjectIdGetDatum(relid));
+	if (!HeapTupleIsValid(tp))
+		elog(ERROR, "cache lookup failed for foreign table %u", relid);
+	tableform = (Form_pg_foreign_table) GETSTRUCT(tp);
+	serverid = tableform->ftserver;
+	ReleaseSysCache(tp);
+
+	return serverid;
+}
+
+
+/*
+ * GetFdwRoutineByServerId - look up the handler of the foreign-data wrapper
+ * for the given foreign server, and retrieve its FdwRoutine struct.
+ */
+FdwRoutine *
+GetFdwRoutineByServerId(Oid serverid)
+{
+	HeapTuple	tp;
+	Form_pg_foreign_data_wrapper fdwform;
+	Form_pg_foreign_server serverform;
+	Oid			fdwid;
+	Oid			fdwhandler;
+
+	/* Get foreign-data wrapper OID for the server. */
+	tp = SearchSysCache1(FOREIGNSERVEROID, ObjectIdGetDatum(serverid));
+	if (!HeapTupleIsValid(tp))
+		elog(ERROR, "cache lookup failed for foreign server %u", serverid);
+	serverform = (Form_pg_foreign_server) GETSTRUCT(tp);
+	fdwid = serverform->srvfdw;
+	ReleaseSysCache(tp);
+
+	/* Get handler function OID for the FDW. */
+	tp = SearchSysCache1(FOREIGNDATAWRAPPEROID, ObjectIdGetDatum(fdwid));
+	if (!HeapTupleIsValid(tp))
+		elog(ERROR, "cache lookup failed for foreign-data wrapper %u", fdwid);
+	fdwform = (Form_pg_foreign_data_wrapper) GETSTRUCT(tp);
+	fdwhandler = fdwform->fdwhandler;
+
+	/* Complain if FDW has been set to NO HANDLER. */
+	if (!OidIsValid(fdwhandler))
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("foreign-data wrapper \"%s\" has no handler",
+						NameStr(fdwform->fdwname))));
+
+	ReleaseSysCache(tp);
+
+	/* And finally, call the handler function. */
+	return GetFdwRoutine(fdwhandler);
+}
+
+
+/*
+ * GetFdwRoutineByRelId - look up the handler of the foreign-data wrapper
+ * for the given foreign table, and retrieve its FdwRoutine struct.
+ */
+FdwRoutine *
+GetFdwRoutineByRelId(Oid relid)
+{
+	Oid			serverid;
+
+	/* Get server OID for the foreign table. */
+	serverid = GetForeignServerIdByRelId(relid);
+
+	/* Now retrieve server's FdwRoutine struct. */
+	return GetFdwRoutineByServerId(serverid);
+}
+
+/*
+ * GetFdwRoutineForRelation - look up the handler of the foreign-data wrapper
+ * for the given foreign table, and retrieve its FdwRoutine struct.
+ *
+ * This function is preferred over GetFdwRoutineByRelId because it caches
+ * the data in the relcache entry, saving a number of catalog lookups.
+ *
+ * If makecopy is true then the returned data is freshly palloc'd in the
+ * caller's memory context.  Otherwise, it's a pointer to the relcache data,
+ * which will be lost in any relcache reset --- so don't rely on it long.
+ */
+FdwRoutine *
+GetFdwRoutineForRelation(Relation relation, bool makecopy)
+{
+	FdwRoutine *fdwroutine;
+	FdwRoutine *cfdwroutine;
+
+	if (relation->rd_fdwroutine == NULL)
+	{
+		/* Get the info by consulting the catalogs and the FDW code */
+		fdwroutine = GetFdwRoutineByRelId(RelationGetRelid(relation));
+
+		/* Save the data for later reuse in CacheMemoryContext */
+		cfdwroutine = (FdwRoutine *) MemoryContextAlloc(CacheMemoryContext,
+														sizeof(FdwRoutine));
+		memcpy(cfdwroutine, fdwroutine, sizeof(FdwRoutine));
+		relation->rd_fdwroutine = cfdwroutine;
+
+		/* Give back the locally palloc'd copy regardless of makecopy */
+		return fdwroutine;
+	}
+
+	/* We have valid cached data --- does the caller want a copy? */
+	if (makecopy)
+	{
+		fdwroutine = (FdwRoutine *) palloc(sizeof(FdwRoutine));
+		memcpy(fdwroutine, relation->rd_fdwroutine, sizeof(FdwRoutine));
+		return fdwroutine;
+	}
+
+	/* Only a short-lived reference is needed, so just hand back cached copy */
+	return relation->rd_fdwroutine;
+}
+
+
+/*
+ * IsImportableForeignTable - filter table names for IMPORT FOREIGN SCHEMA
+ *
+ * Returns true if given table name should be imported according to the
+ * statement's import filter options.
+ */
+bool
+IsImportableForeignTable(const char *tablename,
+						 ImportForeignSchemaStmt *stmt)
+{
+	ListCell   *lc;
+
+	switch (stmt->list_type)
+	{
+		case FDW_IMPORT_SCHEMA_ALL:
+			return true;
+
+		case FDW_IMPORT_SCHEMA_LIMIT_TO:
+			foreach(lc, stmt->table_list)
+			{
+				RangeVar   *rv = (RangeVar *) lfirst(lc);
+
+				if (strcmp(tablename, rv->relname) == 0)
+					return true;
+			}
+			return false;
+
+		case FDW_IMPORT_SCHEMA_EXCEPT:
+			foreach(lc, stmt->table_list)
+			{
+				RangeVar   *rv = (RangeVar *) lfirst(lc);
+
+				if (strcmp(tablename, rv->relname) == 0)
+					return false;
+			}
+			return true;
+	}
+	return false;				/* shouldn't get here */
+}
+
+
+/*
+ * pg_options_to_table - Convert options array to name/value table
+ *
+ * This is useful to provide details for information_schema and pg_dump.
+ */
+Datum
+pg_options_to_table(PG_FUNCTION_ARGS)
+{
+	Datum		array = PG_GETARG_DATUM(0);
+	ListCell   *cell;
+	List	   *options;
+	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+
+	options = untransformRelOptions(array);
+	rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+
+	/* prepare the result set */
+	InitMaterializedSRF(fcinfo, MAT_SRF_USE_EXPECTED_DESC);
+
+	foreach(cell, options)
+	{
+		DefElem    *def = lfirst(cell);
+		Datum		values[2];
+		bool		nulls[2];
+
+		values[0] = CStringGetTextDatum(def->defname);
+		nulls[0] = false;
+		if (def->arg)
+		{
+			values[1] = CStringGetTextDatum(strVal(def->arg));
+			nulls[1] = false;
+		}
+		else
+		{
+			values[1] = (Datum) 0;
+			nulls[1] = true;
+		}
+		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
+							 values, nulls);
+	}
+
+	return (Datum) 0;
+}
+
+
+/*
+ * Describes the valid options for postgresql FDW, server, and user mapping.
+ */
+struct ConnectionOption
+{
+	const char *optname;
+	Oid			optcontext;		/* Oid of catalog in which option may appear */
+};
+
+/*
+ * Copied from fe-connect.c PQconninfoOptions.
+ *
+ * The list is small - don't bother with bsearch if it stays so.
+ */
+static const struct ConnectionOption libpq_conninfo_options[] = {
+	{"authtype", ForeignServerRelationId},
+	{"service", ForeignServerRelationId},
+	{"user", UserMappingRelationId},
+	{"password", UserMappingRelationId},
+	{"connect_timeout", ForeignServerRelationId},
+	{"dbname", ForeignServerRelationId},
+	{"host", ForeignServerRelationId},
+	{"hostaddr", ForeignServerRelationId},
+	{"port", ForeignServerRelationId},
+	{"tty", ForeignServerRelationId},
+	{"options", ForeignServerRelationId},
+	{"requiressl", ForeignServerRelationId},
+	{"sslmode", ForeignServerRelationId},
+	{"gsslib", ForeignServerRelationId},
+	{"gssdelegation", ForeignServerRelationId},
+	{NULL, InvalidOid}
+};
+
+
+/*
+ * Check if the provided option is one of libpq conninfo options.
+ * context is the Oid of the catalog the option came from, or 0 if we
+ * don't care.
+ */
+static bool
+is_conninfo_option(const char *option, Oid context)
+{
+	const struct ConnectionOption *opt;
+
+	for (opt = libpq_conninfo_options; opt->optname; opt++)
+		if (context == opt->optcontext && strcmp(opt->optname, option) == 0)
+			return true;
+	return false;
+}
+
+
+/*
+ * Validate the generic option given to SERVER or USER MAPPING.
+ * Raise an ERROR if the option or its value is considered invalid.
+ *
+ * Valid server options are all libpq conninfo options except
+ * user and password -- these may only appear in USER MAPPING options.
+ *
+ * Caution: this function is deprecated, and is now meant only for testing
+ * purposes, because the list of options it knows about doesn't necessarily
+ * square with those known to whichever libpq instance you might be using.
+ * Inquire of libpq itself, instead.
+ */
+Datum
+postgresql_fdw_validator(PG_FUNCTION_ARGS)
+{
+	List	   *options_list = untransformRelOptions(PG_GETARG_DATUM(0));
+	Oid			catalog = PG_GETARG_OID(1);
+
+	ListCell   *cell;
+
+	foreach(cell, options_list)
+	{
+		DefElem    *def = lfirst(cell);
+
+		if (!is_conninfo_option(def->defname, catalog))
+		{
+			const struct ConnectionOption *opt;
+			const char *closest_match;
+			ClosestMatchState match_state;
+			bool		has_valid_options = false;
+
+			/*
+			 * Unknown option specified, complain about it. Provide a hint
+			 * with a valid option that looks similar, if there is one.
+			 */
+			initClosestMatch(&match_state, def->defname, 4);
+			for (opt = libpq_conninfo_options; opt->optname; opt++)
+			{
+				if (catalog == opt->optcontext)
+				{
+					has_valid_options = true;
+					updateClosestMatch(&match_state, opt->optname);
+				}
+			}
+
+			closest_match = getClosestMatch(&match_state);
+			ereport(ERROR,
+					(errcode(ERRCODE_SYNTAX_ERROR),
+					 errmsg("invalid option \"%s\"", def->defname),
+					 has_valid_options ? closest_match ?
+					 errhint("Perhaps you meant the option \"%s\".",
+							 closest_match) : 0 :
+					 errhint("There are no valid options in this context.")));
+
+			PG_RETURN_BOOL(false);
+		}
+	}
+
+	PG_RETURN_BOOL(true);
+}
+
+
+/*
+ * get_foreign_data_wrapper_oid - given a FDW name, look up the OID
+ *
+ * If missing_ok is false, throw an error if name not found.  If true, just
+ * return InvalidOid.
+ */
+Oid
+get_foreign_data_wrapper_oid(const char *fdwname, bool missing_ok)
+{
+	Oid			oid;
+
+	oid = GetSysCacheOid1(FOREIGNDATAWRAPPERNAME,
+						  Anum_pg_foreign_data_wrapper_oid,
+						  CStringGetDatum(fdwname));
+	if (!OidIsValid(oid) && !missing_ok)
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_OBJECT),
+				 errmsg("foreign-data wrapper \"%s\" does not exist",
+						fdwname)));
+	return oid;
+}
+
+
+/*
+ * get_foreign_server_oid - given a server name, look up the OID
+ *
+ * If missing_ok is false, throw an error if name not found.  If true, just
+ * return InvalidOid.
+ */
+Oid
+get_foreign_server_oid(const char *servername, bool missing_ok)
+{
+	Oid			oid;
+
+	oid = GetSysCacheOid1(FOREIGNSERVERNAME, Anum_pg_foreign_server_oid,
+						  CStringGetDatum(servername));
+	if (!OidIsValid(oid) && !missing_ok)
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_OBJECT),
+				 errmsg("server \"%s\" does not exist", servername)));
+	return oid;
+}
+
+/*
+ * Get a copy of an existing local path for a given join relation.
+ *
+ * This function is usually helpful to obtain an alternate local path for EPQ
+ * checks.
+ *
+ * Right now, this function only supports unparameterized foreign joins, so we
+ * only search for unparameterized path in the given list of paths. Since we
+ * are searching for a path which can be used to construct an alternative local
+ * plan for a foreign join, we look for only MergeJoin, HashJoin or NestLoop
+ * paths.
+ *
+ * If the inner or outer subpath of the chosen path is a ForeignScan, we
+ * replace it with its outer subpath.  For this reason, and also because the
+ * planner might free the original path later, the path returned by this
+ * function is a shallow copy of the original.  There's no need to copy
+ * the substructure, so we don't.
+ *
+ * Since the plan created using this path will presumably only be used to
+ * execute EPQ checks, efficiency of the path is not a concern. But since the
+ * path list in RelOptInfo is anyway sorted by total cost we are likely to
+ * choose the most efficient path, which is all for the best.
+ */
+Path *
+GetExistingLocalJoinPath(RelOptInfo *joinrel)
+{
+	ListCell   *lc;
+
+	Assert(IS_JOIN_REL(joinrel));
+
+	foreach(lc, joinrel->pathlist)
+	{
+		Path	   *path = (Path *) lfirst(lc);
+		JoinPath   *joinpath = NULL;
+
+		/* Skip parameterized paths. */
+		if (path->param_info != NULL)
+			continue;
+
+		switch (path->pathtype)
+		{
+			case T_HashJoin:
+				{
+					HashPath   *hash_path = makeNode(HashPath);
+
+					memcpy(hash_path, path, sizeof(HashPath));
+					joinpath = (JoinPath *) hash_path;
+				}
+				break;
+
+			case T_NestLoop:
+				{
+					NestPath   *nest_path = makeNode(NestPath);
+
+					memcpy(nest_path, path, sizeof(NestPath));
+					joinpath = (JoinPath *) nest_path;
+				}
+				break;
+
+			case T_MergeJoin:
+				{
+					MergePath  *merge_path = makeNode(MergePath);
+
+					memcpy(merge_path, path, sizeof(MergePath));
+					joinpath = (JoinPath *) merge_path;
+				}
+				break;
+
+			default:
+
+				/*
+				 * Just skip anything else. We don't know if corresponding
+				 * plan would build the output row from whole-row references
+				 * of base relations and execute the EPQ checks.
+				 */
+				break;
+		}
+
+		/* This path isn't good for us, check next. */
+		if (!joinpath)
+			continue;
+
+		/*
+		 * If either inner or outer path is a ForeignPath corresponding to a
+		 * pushed down join, replace it with the fdw_outerpath, so that we
+		 * maintain path for EPQ checks built entirely of local join
+		 * strategies.
+		 */
+		if (IsA(joinpath->outerjoinpath, ForeignPath))
+		{
+			ForeignPath *foreign_path;
+
+			foreign_path = (ForeignPath *) joinpath->outerjoinpath;
+			if (IS_JOIN_REL(foreign_path->path.parent))
+				joinpath->outerjoinpath = foreign_path->fdw_outerpath;
+		}
+
+		if (IsA(joinpath->innerjoinpath, ForeignPath))
+		{
+			ForeignPath *foreign_path;
+
+			foreign_path = (ForeignPath *) joinpath->innerjoinpath;
+			if (IS_JOIN_REL(foreign_path->path.parent))
+				joinpath->innerjoinpath = foreign_path->fdw_outerpath;
+		}
+
+		return (Path *) joinpath;
+	}
+	return NULL;
+}
diff --git a/src/backend/foreign/meson.build b/src/backend/foreign/meson.build
new file mode 100644
index 0000000..6638a6a
--- /dev/null
+++ b/src/backend/foreign/meson.build
@@ -0,0 +1,5 @@
+# Copyright (c) 2022-2023, PostgreSQL Global Development Group
+
+backend_sources += files(
+  'foreign.c'
+)