pg_constraint.c

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/*-------------------------------------------------------------------------
 *
 * pg_constraint.c
 *    routines to support manipulation of the pg_constraint relation
 *
 * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/catalog/pg_constraint.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/genam.h"
#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/sysattr.h"
#include "catalog/dependency.h"
#include "catalog/indexing.h"
#include "catalog/objectaccess.h"
#include "catalog/pg_constraint.h"
#include "catalog/pg_constraint_fn.h"
#include "catalog/pg_operator.h"
#include "catalog/pg_type.h"
#include "commands/defrem.h"
#include "utils/array.h"
#include "utils/builtins.h"
#include "utils/fmgroids.h"
#include "utils/lsyscache.h"
#include "utils/rel.h"
#include "utils/syscache.h"
#include "utils/tqual.h"


/*
 * CreateConstraintEntry
 *  Create a constraint table entry.
 *
 * Subsidiary records (such as triggers or indexes to implement the
 * constraint) are *not* created here.  But we do make dependency links
 * from the constraint to the things it depends on.
 *
 * The new constraint's OID is returned.
 */
Oid
CreateConstraintEntry(const char *constraintName,
                      Oid constraintNamespace,
                      char constraintType,
                      bool isDeferrable,
                      bool isDeferred,
                      bool isValidated,
                      Oid relId,
                      const int16 *constraintKey,
                      int constraintNKeys,
                      Oid domainId,
                      Oid indexRelId,
                      Oid foreignRelId,
                      const int16 *foreignKey,
                      const Oid *pfEqOp,
                      const Oid *ppEqOp,
                      const Oid *ffEqOp,
                      int foreignNKeys,
                      char foreignUpdateType,
                      char foreignDeleteType,
                      char foreignMatchType,
                      const Oid *exclOp,
                      Node *conExpr,
                      const char *conBin,
                      const char *conSrc,
                      bool conIsLocal,
                      int conInhCount,
                      bool conNoInherit,
                      bool is_internal)
{
    Relation    conDesc;
    Oid         conOid;
    HeapTuple   tup;
    bool        nulls[Natts_pg_constraint];
    Datum       values[Natts_pg_constraint];
    ArrayType  *conkeyArray;
    ArrayType  *confkeyArray;
    ArrayType  *conpfeqopArray;
    ArrayType  *conppeqopArray;
    ArrayType  *conffeqopArray;
    ArrayType  *conexclopArray;
    NameData    cname;
    int         i;
    ObjectAddress conobject;

    conDesc = heap_open(ConstraintRelationId, RowExclusiveLock);

    Assert(constraintName);
    namestrcpy(&cname, constraintName);

    /*
     * Convert C arrays into Postgres arrays.
     */
    if (constraintNKeys > 0)
    {
        Datum      *conkey;

        conkey = (Datum *) palloc(constraintNKeys * sizeof(Datum));
        for (i = 0; i < constraintNKeys; i++)
            conkey[i] = Int16GetDatum(constraintKey[i]);
        conkeyArray = construct_array(conkey, constraintNKeys,
                                      INT2OID, 2, true, 's');
    }
    else
        conkeyArray = NULL;

    if (foreignNKeys > 0)
    {
        Datum      *fkdatums;

        fkdatums = (Datum *) palloc(foreignNKeys * sizeof(Datum));
        for (i = 0; i < foreignNKeys; i++)
            fkdatums[i] = Int16GetDatum(foreignKey[i]);
        confkeyArray = construct_array(fkdatums, foreignNKeys,
                                       INT2OID, 2, true, 's');
        for (i = 0; i < foreignNKeys; i++)
            fkdatums[i] = ObjectIdGetDatum(pfEqOp[i]);
        conpfeqopArray = construct_array(fkdatums, foreignNKeys,
                                         OIDOID, sizeof(Oid), true, 'i');
        for (i = 0; i < foreignNKeys; i++)
            fkdatums[i] = ObjectIdGetDatum(ppEqOp[i]);
        conppeqopArray = construct_array(fkdatums, foreignNKeys,
                                         OIDOID, sizeof(Oid), true, 'i');
        for (i = 0; i < foreignNKeys; i++)
            fkdatums[i] = ObjectIdGetDatum(ffEqOp[i]);
        conffeqopArray = construct_array(fkdatums, foreignNKeys,
                                         OIDOID, sizeof(Oid), true, 'i');
    }
    else
    {
        confkeyArray = NULL;
        conpfeqopArray = NULL;
        conppeqopArray = NULL;
        conffeqopArray = NULL;
    }

    if (exclOp != NULL)
    {
        Datum      *opdatums;

        opdatums = (Datum *) palloc(constraintNKeys * sizeof(Datum));
        for (i = 0; i < constraintNKeys; i++)
            opdatums[i] = ObjectIdGetDatum(exclOp[i]);
        conexclopArray = construct_array(opdatums, constraintNKeys,
                                         OIDOID, sizeof(Oid), true, 'i');
    }
    else
        conexclopArray = NULL;

    /* initialize nulls and values */
    for (i = 0; i < Natts_pg_constraint; i++)
    {
        nulls[i] = false;
        values[i] = (Datum) NULL;
    }

    values[Anum_pg_constraint_conname - 1] = NameGetDatum(&cname);
    values[Anum_pg_constraint_connamespace - 1] = ObjectIdGetDatum(constraintNamespace);
    values[Anum_pg_constraint_contype - 1] = CharGetDatum(constraintType);
    values[Anum_pg_constraint_condeferrable - 1] = BoolGetDatum(isDeferrable);
    values[Anum_pg_constraint_condeferred - 1] = BoolGetDatum(isDeferred);
    values[Anum_pg_constraint_convalidated - 1] = BoolGetDatum(isValidated);
    values[Anum_pg_constraint_conrelid - 1] = ObjectIdGetDatum(relId);
    values[Anum_pg_constraint_contypid - 1] = ObjectIdGetDatum(domainId);
    values[Anum_pg_constraint_conindid - 1] = ObjectIdGetDatum(indexRelId);
    values[Anum_pg_constraint_confrelid - 1] = ObjectIdGetDatum(foreignRelId);
    values[Anum_pg_constraint_confupdtype - 1] = CharGetDatum(foreignUpdateType);
    values[Anum_pg_constraint_confdeltype - 1] = CharGetDatum(foreignDeleteType);
    values[Anum_pg_constraint_confmatchtype - 1] = CharGetDatum(foreignMatchType);
    values[Anum_pg_constraint_conislocal - 1] = BoolGetDatum(conIsLocal);
    values[Anum_pg_constraint_coninhcount - 1] = Int32GetDatum(conInhCount);
    values[Anum_pg_constraint_connoinherit - 1] = BoolGetDatum(conNoInherit);

    if (conkeyArray)
        values[Anum_pg_constraint_conkey - 1] = PointerGetDatum(conkeyArray);
    else
        nulls[Anum_pg_constraint_conkey - 1] = true;

    if (confkeyArray)
        values[Anum_pg_constraint_confkey - 1] = PointerGetDatum(confkeyArray);
    else
        nulls[Anum_pg_constraint_confkey - 1] = true;

    if (conpfeqopArray)
        values[Anum_pg_constraint_conpfeqop - 1] = PointerGetDatum(conpfeqopArray);
    else
        nulls[Anum_pg_constraint_conpfeqop - 1] = true;

    if (conppeqopArray)
        values[Anum_pg_constraint_conppeqop - 1] = PointerGetDatum(conppeqopArray);
    else
        nulls[Anum_pg_constraint_conppeqop - 1] = true;

    if (conffeqopArray)
        values[Anum_pg_constraint_conffeqop - 1] = PointerGetDatum(conffeqopArray);
    else
        nulls[Anum_pg_constraint_conffeqop - 1] = true;

    if (conexclopArray)
        values[Anum_pg_constraint_conexclop - 1] = PointerGetDatum(conexclopArray);
    else
        nulls[Anum_pg_constraint_conexclop - 1] = true;

    /*
     * initialize the binary form of the check constraint.
     */
    if (conBin)
        values[Anum_pg_constraint_conbin - 1] = CStringGetTextDatum(conBin);
    else
        nulls[Anum_pg_constraint_conbin - 1] = true;

    /*
     * initialize the text form of the check constraint
     */
    if (conSrc)
        values[Anum_pg_constraint_consrc - 1] = CStringGetTextDatum(conSrc);
    else
        nulls[Anum_pg_constraint_consrc - 1] = true;

    tup = heap_form_tuple(RelationGetDescr(conDesc), values, nulls);

    conOid = simple_heap_insert(conDesc, tup);

    /* update catalog indexes */
    CatalogUpdateIndexes(conDesc, tup);

    conobject.classId = ConstraintRelationId;
    conobject.objectId = conOid;
    conobject.objectSubId = 0;

    heap_close(conDesc, RowExclusiveLock);

    if (OidIsValid(relId))
    {
        /*
         * Register auto dependency from constraint to owning relation, or to
         * specific column(s) if any are mentioned.
         */
        ObjectAddress relobject;

        relobject.classId = RelationRelationId;
        relobject.objectId = relId;
        if (constraintNKeys > 0)
        {
            for (i = 0; i < constraintNKeys; i++)
            {
                relobject.objectSubId = constraintKey[i];

                recordDependencyOn(&conobject, &relobject, DEPENDENCY_AUTO);
            }
        }
        else
        {
            relobject.objectSubId = 0;

            recordDependencyOn(&conobject, &relobject, DEPENDENCY_AUTO);
        }
    }

    if (OidIsValid(domainId))
    {
        /*
         * Register auto dependency from constraint to owning domain
         */
        ObjectAddress domobject;

        domobject.classId = TypeRelationId;
        domobject.objectId = domainId;
        domobject.objectSubId = 0;

        recordDependencyOn(&conobject, &domobject, DEPENDENCY_AUTO);
    }

    if (OidIsValid(foreignRelId))
    {
        /*
         * Register normal dependency from constraint to foreign relation, or
         * to specific column(s) if any are mentioned.
         */
        ObjectAddress relobject;

        relobject.classId = RelationRelationId;
        relobject.objectId = foreignRelId;
        if (foreignNKeys > 0)
        {
            for (i = 0; i < foreignNKeys; i++)
            {
                relobject.objectSubId = foreignKey[i];

                recordDependencyOn(&conobject, &relobject, DEPENDENCY_NORMAL);
            }
        }
        else
        {
            relobject.objectSubId = 0;

            recordDependencyOn(&conobject, &relobject, DEPENDENCY_NORMAL);
        }
    }

    if (OidIsValid(indexRelId) && constraintType == CONSTRAINT_FOREIGN)
    {
        /*
         * Register normal dependency on the unique index that supports a
         * foreign-key constraint.  (Note: for indexes associated with unique
         * or primary-key constraints, the dependency runs the other way, and
         * is not made here.)
         */
        ObjectAddress relobject;

        relobject.classId = RelationRelationId;
        relobject.objectId = indexRelId;
        relobject.objectSubId = 0;

        recordDependencyOn(&conobject, &relobject, DEPENDENCY_NORMAL);
    }

    if (foreignNKeys > 0)
    {
        /*
         * Register normal dependencies on the equality operators that support
         * a foreign-key constraint.  If the PK and FK types are the same then
         * all three operators for a column are the same; otherwise they are
         * different.
         */
        ObjectAddress oprobject;

        oprobject.classId = OperatorRelationId;
        oprobject.objectSubId = 0;

        for (i = 0; i < foreignNKeys; i++)
        {
            oprobject.objectId = pfEqOp[i];
            recordDependencyOn(&conobject, &oprobject, DEPENDENCY_NORMAL);
            if (ppEqOp[i] != pfEqOp[i])
            {
                oprobject.objectId = ppEqOp[i];
                recordDependencyOn(&conobject, &oprobject, DEPENDENCY_NORMAL);
            }
            if (ffEqOp[i] != pfEqOp[i])
            {
                oprobject.objectId = ffEqOp[i];
                recordDependencyOn(&conobject, &oprobject, DEPENDENCY_NORMAL);
            }
        }
    }

    /*
     * We don't bother to register dependencies on the exclusion operators of
     * an exclusion constraint.  We assume they are members of the opclass
     * supporting the index, so there's an indirect dependency via that. (This
     * would be pretty dicey for cross-type operators, but exclusion operators
     * can never be cross-type.)
     */

    if (conExpr != NULL)
    {
        /*
         * Register dependencies from constraint to objects mentioned in CHECK
         * expression.
         */
        recordDependencyOnSingleRelExpr(&conobject, conExpr, relId,
                                        DEPENDENCY_NORMAL,
                                        DEPENDENCY_NORMAL);
    }

    /* Post creation hook for new constraint */
    InvokeObjectPostCreateHookArg(ConstraintRelationId, conOid, 0,
                                  is_internal);

    return conOid;
}


/*
 * Test whether given name is currently used as a constraint name
 * for the given object (relation or domain).
 *
 * This is used to decide whether to accept a user-specified constraint name.
 * It is deliberately not the same test as ChooseConstraintName uses to decide
 * whether an auto-generated name is OK: here, we will allow it unless there
 * is an identical constraint name in use *on the same object*.
 *
 * NB: Caller should hold exclusive lock on the given object, else
 * this test can be fooled by concurrent additions.
 */
bool
ConstraintNameIsUsed(ConstraintCategory conCat, Oid objId,
                     Oid objNamespace, const char *conname)
{
    bool        found;
    Relation    conDesc;
    SysScanDesc conscan;
    ScanKeyData skey[2];
    HeapTuple   tup;

    conDesc = heap_open(ConstraintRelationId, AccessShareLock);

    found = false;

    ScanKeyInit(&skey[0],
                Anum_pg_constraint_conname,
                BTEqualStrategyNumber, F_NAMEEQ,
                CStringGetDatum(conname));

    ScanKeyInit(&skey[1],
                Anum_pg_constraint_connamespace,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(objNamespace));

    conscan = systable_beginscan(conDesc, ConstraintNameNspIndexId, true,
                                 NULL, 2, skey);

    while (HeapTupleIsValid(tup = systable_getnext(conscan)))
    {
        Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(tup);

        if (conCat == CONSTRAINT_RELATION && con->conrelid == objId)
        {
            found = true;
            break;
        }
        else if (conCat == CONSTRAINT_DOMAIN && con->contypid == objId)
        {
            found = true;
            break;
        }
    }

    systable_endscan(conscan);
    heap_close(conDesc, AccessShareLock);

    return found;
}

/*
 * Select a nonconflicting name for a new constraint.
 *
 * The objective here is to choose a name that is unique within the
 * specified namespace.  Postgres does not require this, but the SQL
 * spec does, and some apps depend on it.  Therefore we avoid choosing
 * default names that so conflict.
 *
 * name1, name2, and label are used the same way as for makeObjectName(),
 * except that the label can't be NULL; digits will be appended to the label
 * if needed to create a name that is unique within the specified namespace.
 *
 * 'others' can be a list of string names already chosen within the current
 * command (but not yet reflected into the catalogs); we will not choose
 * a duplicate of one of these either.
 *
 * Note: it is theoretically possible to get a collision anyway, if someone
 * else chooses the same name concurrently.  This is fairly unlikely to be
 * a problem in practice, especially if one is holding an exclusive lock on
 * the relation identified by name1.
 *
 * Returns a palloc'd string.
 */
char *
ChooseConstraintName(const char *name1, const char *name2,
                     const char *label, Oid namespaceid,
                     List *others)
{
    int         pass = 0;
    char       *conname = NULL;
    char        modlabel[NAMEDATALEN];
    Relation    conDesc;
    SysScanDesc conscan;
    ScanKeyData skey[2];
    bool        found;
    ListCell   *l;

    conDesc = heap_open(ConstraintRelationId, AccessShareLock);

    /* try the unmodified label first */
    StrNCpy(modlabel, label, sizeof(modlabel));

    for (;;)
    {
        conname = makeObjectName(name1, name2, modlabel);

        found = false;

        foreach(l, others)
        {
            if (strcmp((char *) lfirst(l), conname) == 0)
            {
                found = true;
                break;
            }
        }

        if (!found)
        {
            ScanKeyInit(&skey[0],
                        Anum_pg_constraint_conname,
                        BTEqualStrategyNumber, F_NAMEEQ,
                        CStringGetDatum(conname));

            ScanKeyInit(&skey[1],
                        Anum_pg_constraint_connamespace,
                        BTEqualStrategyNumber, F_OIDEQ,
                        ObjectIdGetDatum(namespaceid));

            conscan = systable_beginscan(conDesc, ConstraintNameNspIndexId, true,
                                         NULL, 2, skey);

            found = (HeapTupleIsValid(systable_getnext(conscan)));

            systable_endscan(conscan);
        }

        if (!found)
            break;

        /* found a conflict, so try a new name component */
        pfree(conname);
        snprintf(modlabel, sizeof(modlabel), "%s%d", label, ++pass);
    }

    heap_close(conDesc, AccessShareLock);

    return conname;
}

/*
 * Delete a single constraint record.
 */
void
RemoveConstraintById(Oid conId)
{
    Relation    conDesc;
    HeapTuple   tup;
    Form_pg_constraint con;

    conDesc = heap_open(ConstraintRelationId, RowExclusiveLock);

    tup = SearchSysCache1(CONSTROID, ObjectIdGetDatum(conId));
    if (!HeapTupleIsValid(tup)) /* should not happen */
        elog(ERROR, "cache lookup failed for constraint %u", conId);
    con = (Form_pg_constraint) GETSTRUCT(tup);

    /*
     * Special processing depending on what the constraint is for.
     */
    if (OidIsValid(con->conrelid))
    {
        Relation    rel;

        /*
         * If the constraint is for a relation, open and exclusive-lock the
         * relation it's for.
         */
        rel = heap_open(con->conrelid, AccessExclusiveLock);

        /*
         * We need to update the relcheck count if it is a check constraint
         * being dropped.  This update will force backends to rebuild relcache
         * entries when we commit.
         */
        if (con->contype == CONSTRAINT_CHECK)
        {
            Relation    pgrel;
            HeapTuple   relTup;
            Form_pg_class classForm;

            pgrel = heap_open(RelationRelationId, RowExclusiveLock);
            relTup = SearchSysCacheCopy1(RELOID,
                                         ObjectIdGetDatum(con->conrelid));
            if (!HeapTupleIsValid(relTup))
                elog(ERROR, "cache lookup failed for relation %u",
                     con->conrelid);
            classForm = (Form_pg_class) GETSTRUCT(relTup);

            if (classForm->relchecks == 0)      /* should not happen */
                elog(ERROR, "relation \"%s\" has relchecks = 0",
                     RelationGetRelationName(rel));
            classForm->relchecks--;

            simple_heap_update(pgrel, &relTup->t_self, relTup);

            CatalogUpdateIndexes(pgrel, relTup);

            heap_freetuple(relTup);

            heap_close(pgrel, RowExclusiveLock);
        }

        /* Keep lock on constraint's rel until end of xact */
        heap_close(rel, NoLock);
    }
    else if (OidIsValid(con->contypid))
    {
        /*
         * XXX for now, do nothing special when dropping a domain constraint
         *
         * Probably there should be some form of locking on the domain type,
         * but we have no such concept at the moment.
         */
    }
    else
        elog(ERROR, "constraint %u is not of a known type", conId);

    /* Fry the constraint itself */
    simple_heap_delete(conDesc, &tup->t_self);

    /* Clean up */
    ReleaseSysCache(tup);
    heap_close(conDesc, RowExclusiveLock);
}

/*
 * RenameConstraintById
 *      Rename a constraint.
 *
 * Note: this isn't intended to be a user-exposed function; it doesn't check
 * permissions etc.  Currently this is only invoked when renaming an index
 * that is associated with a constraint, but it's made a little more general
 * than that with the expectation of someday having ALTER TABLE RENAME
 * CONSTRAINT.
 */
void
RenameConstraintById(Oid conId, const char *newname)
{
    Relation    conDesc;
    HeapTuple   tuple;
    Form_pg_constraint con;

    conDesc = heap_open(ConstraintRelationId, RowExclusiveLock);

    tuple = SearchSysCacheCopy1(CONSTROID, ObjectIdGetDatum(conId));
    if (!HeapTupleIsValid(tuple))
        elog(ERROR, "cache lookup failed for constraint %u", conId);
    con = (Form_pg_constraint) GETSTRUCT(tuple);

    /*
     * We need to check whether the name is already in use --- note that there
     * currently is not a unique index that would catch this.
     */
    if (OidIsValid(con->conrelid) &&
        ConstraintNameIsUsed(CONSTRAINT_RELATION,
                             con->conrelid,
                             con->connamespace,
                             newname))
        ereport(ERROR,
                (errcode(ERRCODE_DUPLICATE_OBJECT),
               errmsg("constraint \"%s\" for relation \"%s\" already exists",
                      newname, get_rel_name(con->conrelid))));
    if (OidIsValid(con->contypid) &&
        ConstraintNameIsUsed(CONSTRAINT_DOMAIN,
                             con->contypid,
                             con->connamespace,
                             newname))
        ereport(ERROR,
                (errcode(ERRCODE_DUPLICATE_OBJECT),
                 errmsg("constraint \"%s\" for domain %s already exists",
                        newname, format_type_be(con->contypid))));

    /* OK, do the rename --- tuple is a copy, so OK to scribble on it */
    namestrcpy(&(con->conname), newname);

    simple_heap_update(conDesc, &tuple->t_self, tuple);

    /* update the system catalog indexes */
    CatalogUpdateIndexes(conDesc, tuple);

    InvokeObjectPostAlterHook(ConstraintRelationId, conId, 0);

    heap_freetuple(tuple);
    heap_close(conDesc, RowExclusiveLock);
}

/*
 * AlterConstraintNamespaces
 *      Find any constraints belonging to the specified object,
 *      and move them to the specified new namespace.
 *
 * isType indicates whether the owning object is a type or a relation.
 */
void
AlterConstraintNamespaces(Oid ownerId, Oid oldNspId,
                       Oid newNspId, bool isType, ObjectAddresses *objsMoved)
{
    Relation    conRel;
    ScanKeyData key[1];
    SysScanDesc scan;
    HeapTuple   tup;

    conRel = heap_open(ConstraintRelationId, RowExclusiveLock);

    if (isType)
    {
        ScanKeyInit(&key[0],
                    Anum_pg_constraint_contypid,
                    BTEqualStrategyNumber, F_OIDEQ,
                    ObjectIdGetDatum(ownerId));

        scan = systable_beginscan(conRel, ConstraintTypidIndexId, true,
                                  NULL, 1, key);
    }
    else
    {
        ScanKeyInit(&key[0],
                    Anum_pg_constraint_conrelid,
                    BTEqualStrategyNumber, F_OIDEQ,
                    ObjectIdGetDatum(ownerId));

        scan = systable_beginscan(conRel, ConstraintRelidIndexId, true,
                                  NULL, 1, key);
    }

    while (HeapTupleIsValid((tup = systable_getnext(scan))))
    {
        Form_pg_constraint conform = (Form_pg_constraint) GETSTRUCT(tup);
        ObjectAddress thisobj;

        thisobj.classId = ConstraintRelationId;
        thisobj.objectId = HeapTupleGetOid(tup);
        thisobj.objectSubId = 0;

        if (object_address_present(&thisobj, objsMoved))
            continue;

        /* Don't update if the object is already part of the namespace */
        if (conform->connamespace == oldNspId && oldNspId != newNspId)
        {
            tup = heap_copytuple(tup);
            conform = (Form_pg_constraint) GETSTRUCT(tup);

            conform->connamespace = newNspId;

            simple_heap_update(conRel, &tup->t_self, tup);
            CatalogUpdateIndexes(conRel, tup);

            /*
             * Note: currently, the constraint will not have its own
             * dependency on the namespace, so we don't need to do
             * changeDependencyFor().
             */
        }

        InvokeObjectPostAlterHook(ConstraintRelationId, thisobj.objectId, 0);

        add_exact_object_address(&thisobj, objsMoved);
    }

    systable_endscan(scan);

    heap_close(conRel, RowExclusiveLock);
}

/*
 * get_relation_constraint_oid
 *      Find a constraint on the specified relation with the specified name.
 *      Returns constraint's OID.
 */
Oid
get_relation_constraint_oid(Oid relid, const char *conname, bool missing_ok)
{
    Relation    pg_constraint;
    HeapTuple   tuple;
    SysScanDesc scan;
    ScanKeyData skey[1];
    Oid         conOid = InvalidOid;

    /*
     * Fetch the constraint tuple from pg_constraint.  There may be more than
     * one match, because constraints are not required to have unique names;
     * if so, error out.
     */
    pg_constraint = heap_open(ConstraintRelationId, AccessShareLock);

    ScanKeyInit(&skey[0],
                Anum_pg_constraint_conrelid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(relid));

    scan = systable_beginscan(pg_constraint, ConstraintRelidIndexId, true,
                              NULL, 1, skey);

    while (HeapTupleIsValid(tuple = systable_getnext(scan)))
    {
        Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(tuple);

        if (strcmp(NameStr(con->conname), conname) == 0)
        {
            if (OidIsValid(conOid))
                ereport(ERROR,
                        (errcode(ERRCODE_DUPLICATE_OBJECT),
                 errmsg("table \"%s\" has multiple constraints named \"%s\"",
                        get_rel_name(relid), conname)));
            conOid = HeapTupleGetOid(tuple);
        }
    }

    systable_endscan(scan);

    /* If no such constraint exists, complain */
    if (!OidIsValid(conOid) && !missing_ok)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("constraint \"%s\" for table \"%s\" does not exist",
                        conname, get_rel_name(relid))));

    heap_close(pg_constraint, AccessShareLock);

    return conOid;
}

/*
 * get_domain_constraint_oid
 *      Find a constraint on the specified domain with the specified name.
 *      Returns constraint's OID.
 */
Oid
get_domain_constraint_oid(Oid typid, const char *conname, bool missing_ok)
{
    Relation    pg_constraint;
    HeapTuple   tuple;
    SysScanDesc scan;
    ScanKeyData skey[1];
    Oid         conOid = InvalidOid;

    /*
     * Fetch the constraint tuple from pg_constraint.  There may be more than
     * one match, because constraints are not required to have unique names;
     * if so, error out.
     */
    pg_constraint = heap_open(ConstraintRelationId, AccessShareLock);

    ScanKeyInit(&skey[0],
                Anum_pg_constraint_contypid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(typid));

    scan = systable_beginscan(pg_constraint, ConstraintTypidIndexId, true,
                              NULL, 1, skey);

    while (HeapTupleIsValid(tuple = systable_getnext(scan)))
    {
        Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(tuple);

        if (strcmp(NameStr(con->conname), conname) == 0)
        {
            if (OidIsValid(conOid))
                ereport(ERROR,
                        (errcode(ERRCODE_DUPLICATE_OBJECT),
                errmsg("domain \"%s\" has multiple constraints named \"%s\"",
                       format_type_be(typid), conname)));
            conOid = HeapTupleGetOid(tuple);
        }
    }

    systable_endscan(scan);

    /* If no such constraint exists, complain */
    if (!OidIsValid(conOid) && !missing_ok)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("constraint \"%s\" for domain \"%s\" does not exist",
                        conname, format_type_be(typid))));

    heap_close(pg_constraint, AccessShareLock);

    return conOid;
}

/*
 * get_primary_key_attnos
 *      Identify the columns in a relation's primary key, if any.
 *
 * Returns a Bitmapset of the column attnos of the primary key's columns,
 * with attnos being offset by FirstLowInvalidHeapAttributeNumber so that
 * system columns can be represented.
 *
 * If there is no primary key, return NULL.  We also return NULL if the pkey
 * constraint is deferrable and deferrableOk is false.
 *
 * *constraintOid is set to the OID of the pkey constraint, or InvalidOid
 * on failure.
 */
Bitmapset *
get_primary_key_attnos(Oid relid, bool deferrableOk, Oid *constraintOid)
{
    Bitmapset  *pkattnos = NULL;
    Relation    pg_constraint;
    HeapTuple   tuple;
    SysScanDesc scan;
    ScanKeyData skey[1];

    /* Set *constraintOid, to avoid complaints about uninitialized vars */
    *constraintOid = InvalidOid;

    /* Scan pg_constraint for constraints of the target rel */
    pg_constraint = heap_open(ConstraintRelationId, AccessShareLock);

    ScanKeyInit(&skey[0],
                Anum_pg_constraint_conrelid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(relid));

    scan = systable_beginscan(pg_constraint, ConstraintRelidIndexId, true,
                              NULL, 1, skey);

    while (HeapTupleIsValid(tuple = systable_getnext(scan)))
    {
        Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(tuple);
        Datum       adatum;
        bool        isNull;
        ArrayType  *arr;
        int16      *attnums;
        int         numkeys;
        int         i;

        /* Skip constraints that are not PRIMARY KEYs */
        if (con->contype != CONSTRAINT_PRIMARY)
            continue;

        /*
         * If the primary key is deferrable, but we've been instructed to
         * ignore deferrable constraints, then we might as well give up
         * searching, since there can only be a single primary key on a table.
         */
        if (con->condeferrable && !deferrableOk)
            break;

        /* Extract the conkey array, ie, attnums of PK's columns */
        adatum = heap_getattr(tuple, Anum_pg_constraint_conkey,
                              RelationGetDescr(pg_constraint), &isNull);
        if (isNull)
            elog(ERROR, "null conkey for constraint %u",
                 HeapTupleGetOid(tuple));
        arr = DatumGetArrayTypeP(adatum);       /* ensure not toasted */
        numkeys = ARR_DIMS(arr)[0];
        if (ARR_NDIM(arr) != 1 ||
            numkeys < 0 ||
            ARR_HASNULL(arr) ||
            ARR_ELEMTYPE(arr) != INT2OID)
            elog(ERROR, "conkey is not a 1-D smallint array");
        attnums = (int16 *) ARR_DATA_PTR(arr);

        /* Construct the result value */
        for (i = 0; i < numkeys; i++)
        {
            pkattnos = bms_add_member(pkattnos,
                            attnums[i] - FirstLowInvalidHeapAttributeNumber);
        }
        *constraintOid = HeapTupleGetOid(tuple);

        /* No need to search further */
        break;
    }

    systable_endscan(scan);

    heap_close(pg_constraint, AccessShareLock);

    return pkattnos;
}

/*
 * Determine whether a relation can be proven functionally dependent on
 * a set of grouping columns.  If so, return TRUE and add the pg_constraint
 * OIDs of the constraints needed for the proof to the *constraintDeps list.
 *
 * grouping_columns is a list of grouping expressions, in which columns of
 * the rel of interest are Vars with the indicated varno/varlevelsup.
 *
 * Currently we only check to see if the rel has a primary key that is a
 * subset of the grouping_columns.  We could also use plain unique constraints
 * if all their columns are known not null, but there's a problem: we need
 * to be able to represent the not-null-ness as part of the constraints added
 * to *constraintDeps.  FIXME whenever not-null constraints get represented
 * in pg_constraint.
 */
bool
check_functional_grouping(Oid relid,
                          Index varno, Index varlevelsup,
                          List *grouping_columns,
                          List **constraintDeps)
{
    Bitmapset  *pkattnos;
    Bitmapset  *groupbyattnos;
    Oid         constraintOid;
    ListCell   *gl;

    /* If the rel has no PK, then we can't prove functional dependency */
    pkattnos = get_primary_key_attnos(relid, false, &constraintOid);
    if (pkattnos == NULL)
        return false;

    /* Identify all the rel's columns that appear in grouping_columns */
    groupbyattnos = NULL;
    foreach(gl, grouping_columns)
    {
        Var        *gvar = (Var *) lfirst(gl);

        if (IsA(gvar, Var) &&
            gvar->varno == varno &&
            gvar->varlevelsup == varlevelsup)
            groupbyattnos = bms_add_member(groupbyattnos,
                        gvar->varattno - FirstLowInvalidHeapAttributeNumber);
    }

    if (bms_is_subset(pkattnos, groupbyattnos))
    {
        /* The PK is a subset of grouping_columns, so we win */
        *constraintDeps = lappend_oid(*constraintDeps, constraintOid);
        return true;
    }

    return false;
}