pg_enum.c

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/*-------------------------------------------------------------------------
 *
 * pg_enum.c
 *    routines to support manipulation of the pg_enum relation
 *
 * Copyright (c) 2006-2023, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *    src/backend/catalog/pg_enum.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/genam.h"
#include "access/htup_details.h"
#include "access/table.h"
#include "access/xact.h"
#include "catalog/binary_upgrade.h"
#include "catalog/catalog.h"
#include "catalog/indexing.h"
#include "catalog/pg_enum.h"
#include "catalog/pg_type.h"
#include "miscadmin.h"
#include "nodes/value.h"
#include "storage/lmgr.h"
#include "utils/builtins.h"
#include "utils/catcache.h"
#include "utils/fmgroids.h"
#include "utils/hsearch.h"
#include "utils/memutils.h"
#include "utils/syscache.h"
 
/* Potentially set by pg_upgrade_support functions */
Oid         binary_upgrade_next_pg_enum_oid = InvalidOid;
 
/*
 * Hash table of enum value OIDs created during the current transaction by
 * AddEnumLabel.  We disallow using these values until the transaction is
 * committed; otherwise, they might get into indexes where we can't clean
 * them up, and then if the transaction rolls back we have a broken index.
 * (See comments for check_safe_enum_use() in enum.c.)  Values created by
 * EnumValuesCreate are *not* entered into the table; we assume those are
 * created during CREATE TYPE, so they can't go away unless the enum type
 * itself does.
 */
static HTAB *uncommitted_enums = NULL;
 
static void RenumberEnumType(Relation pg_enum, HeapTuple *existing, int nelems);
static int  sort_order_cmp(const void *p1, const void *p2);
 
 
/*
 * EnumValuesCreate
 *      Create an entry in pg_enum for each of the supplied enum values.
 *
 * vals is a list of String values.
 */
void
EnumValuesCreate(Oid enumTypeOid, List *vals)
{
    Relation    pg_enum;
    Oid        *oids;
    int         elemno,
                num_elems;
    ListCell   *lc;
    int         slotCount = 0;
    int         nslots;
    CatalogIndexState indstate;
    TupleTableSlot **slot;
 
    num_elems = list_length(vals);
 
    /*
     * We do not bother to check the list of values for duplicates --- if you
     * have any, you'll get a less-than-friendly unique-index violation. It is
     * probably not worth trying harder.
     */
 
    pg_enum = table_open(EnumRelationId, RowExclusiveLock);
 
    /*
     * Allocate OIDs for the enum's members.
     *
     * While this method does not absolutely guarantee that we generate no
     * duplicate OIDs (since we haven't entered each oid into the table before
     * allocating the next), trouble could only occur if the OID counter wraps
     * all the way around before we finish. Which seems unlikely.
     */
    oids = (Oid *) palloc(num_elems * sizeof(Oid));
 
    for (elemno = 0; elemno < num_elems; elemno++)
    {
        /*
         * We assign even-numbered OIDs to all the new enum labels.  This
         * tells the comparison functions the OIDs are in the correct sort
         * order and can be compared directly.
         */
        Oid         new_oid;
 
        do
        {
            new_oid = GetNewOidWithIndex(pg_enum, EnumOidIndexId,
                                         Anum_pg_enum_oid);
        } while (new_oid & 1);
        oids[elemno] = new_oid;
    }
 
    /* sort them, just in case OID counter wrapped from high to low */
    qsort(oids, num_elems, sizeof(Oid), oid_cmp);
 
    /* and make the entries */
    indstate = CatalogOpenIndexes(pg_enum);
 
    /* allocate the slots to use and initialize them */
    nslots = Min(num_elems,
                 MAX_CATALOG_MULTI_INSERT_BYTES / sizeof(FormData_pg_enum));
    slot = palloc(sizeof(TupleTableSlot *) * nslots);
    for (int i = 0; i < nslots; i++)
        slot[i] = MakeSingleTupleTableSlot(RelationGetDescr(pg_enum),
                                           &TTSOpsHeapTuple);
 
    elemno = 0;
    foreach(lc, vals)
    {
        char       *lab = strVal(lfirst(lc));
        Name        enumlabel = palloc0(NAMEDATALEN);
 
        /*
         * labels are stored in a name field, for easier syscache lookup, so
         * check the length to make sure it's within range.
         */
        if (strlen(lab) > (NAMEDATALEN - 1))
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_NAME),
                     errmsg("invalid enum label \"%s\"", lab),
                     errdetail("Labels must be %d bytes or less.",
                               NAMEDATALEN - 1)));
 
        ExecClearTuple(slot[slotCount]);
 
        memset(slot[slotCount]->tts_isnull, false,
               slot[slotCount]->tts_tupleDescriptor->natts * sizeof(bool));
 
        slot[slotCount]->tts_values[Anum_pg_enum_oid - 1] = ObjectIdGetDatum(oids[elemno]);
        slot[slotCount]->tts_values[Anum_pg_enum_enumtypid - 1] = ObjectIdGetDatum(enumTypeOid);
        slot[slotCount]->tts_values[Anum_pg_enum_enumsortorder - 1] = Float4GetDatum(elemno + 1);
 
        namestrcpy(enumlabel, lab);
        slot[slotCount]->tts_values[Anum_pg_enum_enumlabel - 1] = NameGetDatum(enumlabel);
 
        ExecStoreVirtualTuple(slot[slotCount]);
        slotCount++;
 
        /* if slots are full, insert a batch of tuples */
        if (slotCount == nslots)
        {
            CatalogTuplesMultiInsertWithInfo(pg_enum, slot, slotCount,
                                             indstate);
            slotCount = 0;
        }
 
        elemno++;
    }
 
    /* Insert any tuples left in the buffer */
    if (slotCount > 0)
        CatalogTuplesMultiInsertWithInfo(pg_enum, slot, slotCount,
                                         indstate);
 
    /* clean up */
    pfree(oids);
    for (int i = 0; i < nslots; i++)
        ExecDropSingleTupleTableSlot(slot[i]);
    CatalogCloseIndexes(indstate);
    table_close(pg_enum, RowExclusiveLock);
}
 
 
/*
 * EnumValuesDelete
 *      Remove all the pg_enum entries for the specified enum type.
 */
void
EnumValuesDelete(Oid enumTypeOid)
{
    Relation    pg_enum;
    ScanKeyData key[1];
    SysScanDesc scan;
    HeapTuple   tup;
 
    pg_enum = table_open(EnumRelationId, RowExclusiveLock);
 
    ScanKeyInit(&key[0],
                Anum_pg_enum_enumtypid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(enumTypeOid));
 
    scan = systable_beginscan(pg_enum, EnumTypIdLabelIndexId, true,
                              NULL, 1, key);
 
    while (HeapTupleIsValid(tup = systable_getnext(scan)))
    {
        CatalogTupleDelete(pg_enum, &tup->t_self);
    }
 
    systable_endscan(scan);
 
    table_close(pg_enum, RowExclusiveLock);
}
 
/*
 * Initialize the uncommitted enum table for this transaction.
 */
static void
init_uncommitted_enums(void)
{
    HASHCTL     hash_ctl;
 
    hash_ctl.keysize = sizeof(Oid);
    hash_ctl.entrysize = sizeof(Oid);
    hash_ctl.hcxt = TopTransactionContext;
    uncommitted_enums = hash_create("Uncommitted enums",
                                    32,
                                    &hash_ctl,
                                    HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
}
 
/*
 * AddEnumLabel
 *      Add a new label to the enum set. By default it goes at
 *      the end, but the user can choose to place it before or
 *      after any existing set member.
 */
void
AddEnumLabel(Oid enumTypeOid,
             const char *newVal,
             const char *neighbor,
             bool newValIsAfter,
             bool skipIfExists)
{
    Relation    pg_enum;
    Oid         newOid;
    Datum       values[Natts_pg_enum];
    bool        nulls[Natts_pg_enum];
    NameData    enumlabel;
    HeapTuple   enum_tup;
    float4      newelemorder;
    HeapTuple  *existing;
    CatCList   *list;
    int         nelems;
    int         i;
 
    /* check length of new label is ok */
    if (strlen(newVal) > (NAMEDATALEN - 1))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_NAME),
                 errmsg("invalid enum label \"%s\"", newVal),
                 errdetail("Labels must be %d bytes or less.",
                           NAMEDATALEN - 1)));
 
    /*
     * Acquire a lock on the enum type, which we won't release until commit.
     * This ensures that two backends aren't concurrently modifying the same
     * enum type.  Without that, we couldn't be sure to get a consistent view
     * of the enum members via the syscache.  Note that this does not block
     * other backends from inspecting the type; see comments for
     * RenumberEnumType.
     */
    LockDatabaseObject(TypeRelationId, enumTypeOid, 0, ExclusiveLock);
 
    /*
     * Check if label is already in use.  The unique index on pg_enum would
     * catch this anyway, but we prefer a friendlier error message, and
     * besides we need a check to support IF NOT EXISTS.
     */
    enum_tup = SearchSysCache2(ENUMTYPOIDNAME,
                               ObjectIdGetDatum(enumTypeOid),
                               CStringGetDatum(newVal));
    if (HeapTupleIsValid(enum_tup))
    {
        ReleaseSysCache(enum_tup);
        if (skipIfExists)
        {
            ereport(NOTICE,
                    (errcode(ERRCODE_DUPLICATE_OBJECT),
                     errmsg("enum label \"%s\" already exists, skipping",
                            newVal)));
            return;
        }
        else
            ereport(ERROR,
                    (errcode(ERRCODE_DUPLICATE_OBJECT),
                     errmsg("enum label \"%s\" already exists",
                            newVal)));
    }
 
    pg_enum = table_open(EnumRelationId, RowExclusiveLock);
 
    /* If we have to renumber the existing members, we restart from here */
restart:
 
    /* Get the list of existing members of the enum */
    list = SearchSysCacheList1(ENUMTYPOIDNAME,
                               ObjectIdGetDatum(enumTypeOid));
    nelems = list->n_members;
 
    /* Sort the existing members by enumsortorder */
    existing = (HeapTuple *) palloc(nelems * sizeof(HeapTuple));
    for (i = 0; i < nelems; i++)
        existing[i] = &(list->members[i]->tuple);
 
    qsort(existing, nelems, sizeof(HeapTuple), sort_order_cmp);
 
    if (neighbor == NULL)
    {
        /*
         * Put the new label at the end of the list. No change to existing
         * tuples is required.
         */
        if (nelems > 0)
        {
            Form_pg_enum en = (Form_pg_enum) GETSTRUCT(existing[nelems - 1]);
 
            newelemorder = en->enumsortorder + 1;
        }
        else
            newelemorder = 1;
    }
    else
    {
        /* BEFORE or AFTER was specified */
        int         nbr_index;
        int         other_nbr_index;
        Form_pg_enum nbr_en;
        Form_pg_enum other_nbr_en;
 
        /* Locate the neighbor element */
        for (nbr_index = 0; nbr_index < nelems; nbr_index++)
        {
            Form_pg_enum en = (Form_pg_enum) GETSTRUCT(existing[nbr_index]);
 
            if (strcmp(NameStr(en->enumlabel), neighbor) == 0)
                break;
        }
        if (nbr_index >= nelems)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("\"%s\" is not an existing enum label",
                            neighbor)));
        nbr_en = (Form_pg_enum) GETSTRUCT(existing[nbr_index]);
 
        /*
         * Attempt to assign an appropriate enumsortorder value: one less than
         * the smallest member, one more than the largest member, or halfway
         * between two existing members.
         *
         * In the "halfway" case, because of the finite precision of float4,
         * we might compute a value that's actually equal to one or the other
         * of its neighbors.  In that case we renumber the existing members
         * and try again.
         */
        if (newValIsAfter)
            other_nbr_index = nbr_index + 1;
        else
            other_nbr_index = nbr_index - 1;
 
        if (other_nbr_index < 0)
            newelemorder = nbr_en->enumsortorder - 1;
        else if (other_nbr_index >= nelems)
            newelemorder = nbr_en->enumsortorder + 1;
        else
        {
            /*
             * The midpoint value computed here has to be rounded to float4
             * precision, else our equality comparisons against the adjacent
             * values are meaningless.  The most portable way of forcing that
             * to happen with non-C-standard-compliant compilers is to store
             * it into a volatile variable.
             */
            volatile float4 midpoint;
 
            other_nbr_en = (Form_pg_enum) GETSTRUCT(existing[other_nbr_index]);
            midpoint = (nbr_en->enumsortorder +
                        other_nbr_en->enumsortorder) / 2;
 
            if (midpoint == nbr_en->enumsortorder ||
                midpoint == other_nbr_en->enumsortorder)
            {
                RenumberEnumType(pg_enum, existing, nelems);
                /* Clean up and start over */
                pfree(existing);
                ReleaseCatCacheList(list);
                goto restart;
            }
 
            newelemorder = midpoint;
        }
    }
 
    /* Get a new OID for the new label */
    if (IsBinaryUpgrade)
    {
        if (!OidIsValid(binary_upgrade_next_pg_enum_oid))
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("pg_enum OID value not set when in binary upgrade mode")));
 
        /*
         * Use binary-upgrade override for pg_enum.oid, if supplied. During
         * binary upgrade, all pg_enum.oid's are set this way so they are
         * guaranteed to be consistent.
         */
        if (neighbor != NULL)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("ALTER TYPE ADD BEFORE/AFTER is incompatible with binary upgrade")));
 
        newOid = binary_upgrade_next_pg_enum_oid;
        binary_upgrade_next_pg_enum_oid = InvalidOid;
    }
    else
    {
        /*
         * Normal case: we need to allocate a new Oid for the value.
         *
         * We want to give the new element an even-numbered Oid if it's safe,
         * which is to say it compares correctly to all pre-existing even
         * numbered Oids in the enum.  Otherwise, we must give it an odd Oid.
         */
        for (;;)
        {
            bool        sorts_ok;
 
            /* Get a new OID (different from all existing pg_enum tuples) */
            newOid = GetNewOidWithIndex(pg_enum, EnumOidIndexId,
                                        Anum_pg_enum_oid);
 
            /*
             * Detect whether it sorts correctly relative to existing
             * even-numbered labels of the enum.  We can ignore existing
             * labels with odd Oids, since a comparison involving one of those
             * will not take the fast path anyway.
             */
            sorts_ok = true;
            for (i = 0; i < nelems; i++)
            {
                HeapTuple   exists_tup = existing[i];
                Form_pg_enum exists_en = (Form_pg_enum) GETSTRUCT(exists_tup);
                Oid         exists_oid = exists_en->oid;
 
                if (exists_oid & 1)
                    continue;   /* ignore odd Oids */
 
                if (exists_en->enumsortorder < newelemorder)
                {
                    /* should sort before */
                    if (exists_oid >= newOid)
                    {
                        sorts_ok = false;
                        break;
                    }
                }
                else
                {
                    /* should sort after */
                    if (exists_oid <= newOid)
                    {
                        sorts_ok = false;
                        break;
                    }
                }
            }
 
            if (sorts_ok)
            {
                /* If it's even and sorts OK, we're done. */
                if ((newOid & 1) == 0)
                    break;
 
                /*
                 * If it's odd, and sorts OK, loop back to get another OID and
                 * try again.  Probably, the next available even OID will sort
                 * correctly too, so it's worth trying.
                 */
            }
            else
            {
                /*
                 * If it's odd, and does not sort correctly, we're done.
                 * (Probably, the next available even OID would sort
                 * incorrectly too, so no point in trying again.)
                 */
                if (newOid & 1)
                    break;
 
                /*
                 * If it's even, and does not sort correctly, loop back to get
                 * another OID and try again.  (We *must* reject this case.)
                 */
            }
        }
    }
 
    /* Done with info about existing members */
    pfree(existing);
    ReleaseCatCacheList(list);
 
    /* Create the new pg_enum entry */
    memset(nulls, false, sizeof(nulls));
    values[Anum_pg_enum_oid - 1] = ObjectIdGetDatum(newOid);
    values[Anum_pg_enum_enumtypid - 1] = ObjectIdGetDatum(enumTypeOid);
    values[Anum_pg_enum_enumsortorder - 1] = Float4GetDatum(newelemorder);
    namestrcpy(&enumlabel, newVal);
    values[Anum_pg_enum_enumlabel - 1] = NameGetDatum(&enumlabel);
    enum_tup = heap_form_tuple(RelationGetDescr(pg_enum), values, nulls);
    CatalogTupleInsert(pg_enum, enum_tup);
    heap_freetuple(enum_tup);
 
    table_close(pg_enum, RowExclusiveLock);
 
    /* Set up the uncommitted enum table if not already done in this tx */
    if (uncommitted_enums == NULL)
        init_uncommitted_enums();
 
    /* Add the new value to the table */
    (void) hash_search(uncommitted_enums, &newOid, HASH_ENTER, NULL);
}
 
 
/*
 * RenameEnumLabel
 *      Rename a label in an enum set.
 */
void
RenameEnumLabel(Oid enumTypeOid,
                const char *oldVal,
                const char *newVal)
{
    Relation    pg_enum;
    HeapTuple   enum_tup;
    Form_pg_enum en;
    CatCList   *list;
    int         nelems;
    HeapTuple   old_tup;
    bool        found_new;
    int         i;
 
    /* check length of new label is ok */
    if (strlen(newVal) > (NAMEDATALEN - 1))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_NAME),
                 errmsg("invalid enum label \"%s\"", newVal),
                 errdetail("Labels must be %d bytes or less.",
                           NAMEDATALEN - 1)));
 
    /*
     * Acquire a lock on the enum type, which we won't release until commit.
     * This ensures that two backends aren't concurrently modifying the same
     * enum type.  Since we are not changing the type's sort order, this is
     * probably not really necessary, but there seems no reason not to take
     * the lock to be sure.
     */
    LockDatabaseObject(TypeRelationId, enumTypeOid, 0, ExclusiveLock);
 
    pg_enum = table_open(EnumRelationId, RowExclusiveLock);
 
    /* Get the list of existing members of the enum */
    list = SearchSysCacheList1(ENUMTYPOIDNAME,
                               ObjectIdGetDatum(enumTypeOid));
    nelems = list->n_members;
 
    /*
     * Locate the element to rename and check if the new label is already in
     * use.  (The unique index on pg_enum would catch that anyway, but we
     * prefer a friendlier error message.)
     */
    old_tup = NULL;
    found_new = false;
    for (i = 0; i < nelems; i++)
    {
        enum_tup = &(list->members[i]->tuple);
        en = (Form_pg_enum) GETSTRUCT(enum_tup);
        if (strcmp(NameStr(en->enumlabel), oldVal) == 0)
            old_tup = enum_tup;
        if (strcmp(NameStr(en->enumlabel), newVal) == 0)
            found_new = true;
    }
    if (!old_tup)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("\"%s\" is not an existing enum label",
                        oldVal)));
    if (found_new)
        ereport(ERROR,
                (errcode(ERRCODE_DUPLICATE_OBJECT),
                 errmsg("enum label \"%s\" already exists",
                        newVal)));
 
    /* OK, make a writable copy of old tuple */
    enum_tup = heap_copytuple(old_tup);
    en = (Form_pg_enum) GETSTRUCT(enum_tup);
 
    ReleaseCatCacheList(list);
 
    /* Update the pg_enum entry */
    namestrcpy(&en->enumlabel, newVal);
    CatalogTupleUpdate(pg_enum, &enum_tup->t_self, enum_tup);
    heap_freetuple(enum_tup);
 
    table_close(pg_enum, RowExclusiveLock);
}
 
 
/*
 * Test if the given enum value is in the table of uncommitted enums.
 */
bool
EnumUncommitted(Oid enum_id)
{
    bool        found;
 
    /* If we've made no uncommitted table, all values are safe */
    if (uncommitted_enums == NULL)
        return false;
 
    /* Else, is it in the table? */
    (void) hash_search(uncommitted_enums, &enum_id, HASH_FIND, &found);
    return found;
}
 
 
/*
 * Clean up enum stuff after end of top-level transaction.
 */
void
AtEOXact_Enum(void)
{
    /*
     * Reset the uncommitted table, as all our enum values are now committed.
     * The memory will go away automatically when TopTransactionContext is
     * freed; it's sufficient to clear our pointer.
     */
    uncommitted_enums = NULL;
}
 
 
/*
 * RenumberEnumType
 *      Renumber existing enum elements to have sort positions 1..n.
 *
 * We avoid doing this unless absolutely necessary; in most installations
 * it will never happen.  The reason is that updating existing pg_enum
 * entries creates hazards for other backends that are concurrently reading
 * pg_enum.  Although system catalog scans now use MVCC semantics, the
 * syscache machinery might read different pg_enum entries under different
 * snapshots, so some other backend might get confused about the proper
 * ordering if a concurrent renumbering occurs.
 *
 * We therefore make the following choices:
 *
 * 1. Any code that is interested in the enumsortorder values MUST read
 * all the relevant pg_enum entries with a single MVCC snapshot, or else
 * acquire lock on the enum type to prevent concurrent execution of
 * AddEnumLabel().
 *
 * 2. Code that is not examining enumsortorder can use a syscache
 * (for example, enum_in and enum_out do so).
 */
static void
RenumberEnumType(Relation pg_enum, HeapTuple *existing, int nelems)
{
    int         i;
 
    /*
     * We should only need to increase existing elements' enumsortorders,
     * never decrease them.  Therefore, work from the end backwards, to avoid
     * unwanted uniqueness violations.
     */
    for (i = nelems - 1; i >= 0; i--)
    {
        HeapTuple   newtup;
        Form_pg_enum en;
        float4      newsortorder;
 
        newtup = heap_copytuple(existing[i]);
        en = (Form_pg_enum) GETSTRUCT(newtup);
 
        newsortorder = i + 1;
        if (en->enumsortorder != newsortorder)
        {
            en->enumsortorder = newsortorder;
 
            CatalogTupleUpdate(pg_enum, &newtup->t_self, newtup);
        }
 
        heap_freetuple(newtup);
    }
 
    /* Make the updates visible */
    CommandCounterIncrement();
}
 
 
/* qsort comparison function for tuples by sort order */
static int
sort_order_cmp(const void *p1, const void *p2)
{
    HeapTuple   v1 = *((const HeapTuple *) p1);
    HeapTuple   v2 = *((const HeapTuple *) p2);
    Form_pg_enum en1 = (Form_pg_enum) GETSTRUCT(v1);
    Form_pg_enum en2 = (Form_pg_enum) GETSTRUCT(v2);
 
    if (en1->enumsortorder < en2->enumsortorder)
        return -1;
    else if (en1->enumsortorder > en2->enumsortorder)
        return 1;
    else
        return 0;
}
 
Size
EstimateUncommittedEnumsSpace(void)
{
    size_t      entries;
 
    if (uncommitted_enums)
        entries = hash_get_num_entries(uncommitted_enums);
    else
        entries = 0;
 
    /* Add one for the terminator. */
    return sizeof(Oid) * (entries + 1);
}
 
void
SerializeUncommittedEnums(void *space, Size size)
{
    Oid        *serialized = (Oid *) space;
 
    /*
     * Make sure the hash table hasn't changed in size since the caller
     * reserved the space.
     */
    Assert(size == EstimateUncommittedEnumsSpace());
 
    /* Write out all the values from the hash table, if there is one. */
    if (uncommitted_enums)
    {
        HASH_SEQ_STATUS status;
        Oid        *value;
 
        hash_seq_init(&status, uncommitted_enums);
        while ((value = (Oid *) hash_seq_search(&status)))
            *serialized++ = *value;
    }
 
    /* Write out the terminator. */
    *serialized = InvalidOid;
 
    /*
     * Make sure the amount of space we actually used matches what was
     * estimated.
     */
    Assert((char *) (serialized + 1) == ((char *) space) + size);
}
 
void
RestoreUncommittedEnums(void *space)
{
    Oid        *serialized = (Oid *) space;
 
    Assert(!uncommitted_enums);
 
    /*
     * As a special case, if the list is empty then don't even bother to
     * create the hash table.  This is the usual case, since enum alteration
     * is expected to be rare.
     */
    if (!OidIsValid(*serialized))
        return;
 
    /* Read all the values into a new hash table. */
    init_uncommitted_enums();
    do
    {
        hash_search(uncommitted_enums, serialized++, HASH_ENTER, NULL);
    } while (OidIsValid(*serialized));
}