foreign.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * foreign.c
 *        support for foreign-data wrappers, servers and user mappings.
 *
 * Portions Copyright (c) 1996-2026, 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 "miscadmin.h"
#include "optimizer/paths.h"
#include "tcop/tcopprot.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 = palloc_object(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 = palloc_object(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))
    {
        ForeignServer *server = GetForeignServer(serverid);
 
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("user mapping not found for user \"%s\", server \"%s\"",
                        MappingUserName(userid), server->servername)));
    }
 
    um = palloc_object(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 = palloc_object(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;
 
    /* Check if the access to foreign tables is restricted */
    if (unlikely((restrict_nonsystem_relation_kind & RESTRICT_RELKIND_FOREIGN_TABLE) != 0))
    {
        /* there must not be built-in FDW handler  */
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("access to non-system foreign table is restricted")));
    }
 
    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 = palloc_object(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;
 
    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 (joinpath->path.pathtype == T_MergeJoin)
                {
                    MergePath  *merge_path = (MergePath *) joinpath;
 
                    /*
                     * If the new outer path is already well enough ordered
                     * for the mergejoin, we can skip doing an explicit sort.
                     */
                    if (merge_path->outersortkeys &&
                        pathkeys_count_contained_in(merge_path->outersortkeys,
                                                    joinpath->outerjoinpath->pathkeys,
                                                    &merge_path->outer_presorted_keys))
                        merge_path->outersortkeys = NIL;
                }
            }
        }
 
        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;
 
                if (joinpath->path.pathtype == T_MergeJoin)
                {
                    MergePath  *merge_path = (MergePath *) joinpath;
 
                    /*
                     * If the new inner path is already well enough ordered
                     * for the mergejoin, we can skip doing an explicit sort.
                     */
                    if (merge_path->innersortkeys &&
                        pathkeys_contained_in(merge_path->innersortkeys,
                                              joinpath->innerjoinpath->pathkeys))
                        merge_path->innersortkeys = NIL;
                }
            }
        }
 
        return (Path *) joinpath;
    }
    return NULL;
}