varsup.c

/*-------------------------------------------------------------------------
 *
 * varsup.c
 *	  postgres OID & XID variables support routines
 *
 * Copyright (c) 2000-2010, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.91 2010/02/26 02:00:34 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/clog.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "access/xact.h"
#include "commands/dbcommands.h"
#include "miscadmin.h"
#include "postmaster/autovacuum.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "utils/builtins.h"
#include "utils/guc.h"
#include "utils/syscache.h"

#include "access/distributedlog.h"
#include "cdb/cdbvars.h"


/* Number of OIDs to prefetch (preallocate) per XLOG write */
#define VAR_OID_PREFETCH		8192

/* pointer to "variable cache" in shared memory (set up by shmem.c) */
VariableCache ShmemVariableCache = NULL;

int xid_stop_limit;
int xid_warn_limit;

/*
 * Allocate the next XID for a new transaction or subtransaction.
 *
 * The new XID is also stored into MyProc before returning.
 *
 * Note: when this is called, we are actually already inside a valid
 * transaction, since XIDs are now not allocated until the transaction
 * does something.	So it is safe to do a database lookup if we want to
 * issue a warning about XID wrap.
 */
TransactionId
GetNewTransactionId(bool isSubXact)
{
	TransactionId xid;

	/*
	 * During bootstrap initialization, we return the special bootstrap
	 * transaction id.
	 */
	if (IsBootstrapProcessingMode())
	{
		Assert(!isSubXact);
		return BootstrapTransactionId;
	}

	/* safety check, we should never get this far in a HS slave */
	if (RecoveryInProgress())
		elog(ERROR, "cannot assign TransactionIds during recovery");

	LWLockAcquire(XidGenLock, LW_EXCLUSIVE);

	xid = ShmemVariableCache->nextXid;

	/*----------
	 * Check to see if it's safe to assign another XID.  This protects against
	 * catastrophic data loss due to XID wraparound.  The basic rules are:
	 *
	 * If we're past xidVacLimit, start trying to force autovacuum cycles.
	 * If we're past xidWarnLimit, start issuing warnings.
	 * If we're past xidStopLimit, refuse to execute transactions, unless
	 * we are running in a standalone backend (which gives an escape hatch
	 * to the DBA who somehow got past the earlier defenses).
	 *----------
	 */
	if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))
	{
		/*
		 * For safety's sake, we release XidGenLock while sending signals,
		 * warnings, etc.  This is not so much because we care about
		 * preserving concurrency in this situation, as to avoid any
		 * possibility of deadlock while doing get_database_name(). First,
		 * copy all the shared values we'll need in this path.
		 */
		TransactionId xidWarnLimit = ShmemVariableCache->xidWarnLimit;
		TransactionId xidStopLimit = ShmemVariableCache->xidStopLimit;
		TransactionId xidWrapLimit = ShmemVariableCache->xidWrapLimit;
		Oid			oldest_datoid = ShmemVariableCache->oldestXidDB;

		LWLockRelease(XidGenLock);

		/*
		 * To avoid swamping the postmaster with signals, we issue the autovac
		 * request only once per 64K transaction starts.  This still gives
		 * plenty of chances before we get into real trouble.
		 */
		if (IsUnderPostmaster && (xid % 65536) == 0)
			SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

		if (IsUnderPostmaster &&
			TransactionIdFollowsOrEquals(xid, xidStopLimit))
		{
			char	   *oldest_datname = get_database_name(oldest_datoid);

			 /*
			  * In GPDB, don't say anything about old prepared transactions, because the system
			  * only uses prepared transactions internally. PREPARE TRANSACTION is not available
			  * to users.
			  */

			/* complain even if that DB has disappeared */
			if (oldest_datname)
				ereport(ERROR,
						(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
						 errmsg("database is not accepting commands to avoid wraparound data loss in database \"%s\"",
								oldest_datname),
						 errhint("Shutdown Greenplum Database. Lower the xid_stop_limit GUC. Execute a database-wide VACUUM in that database. Reset the xid_stop_limit GUC."
								 )));
			else
				ereport(ERROR,
						(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
						 errmsg("database is not accepting commands to avoid wraparound data loss in database with OID %u",
								oldest_datoid),
						 errhint("Shutdown Greenplum Database. Lower the xid_stop_limit GUC. Execute a database-wide VACUUM in that database. Reset the xid_stop_limit GUC."
								 )));
		}
		else if (TransactionIdFollowsOrEquals(xid, xidWarnLimit))
		{
			char	   *oldest_datname = get_database_name(oldest_datoid);

			 /*
			  * In GPDB, don't say anything about old prepared transactions, because the system
			  * only uses prepared transactions internally. PREPARE TRANSACTION is not available
			  * to users.
			  */

			/* complain even if that DB has disappeared */
			if (oldest_datname)
				ereport(WARNING,
						(errmsg("database \"%s\" must be vacuumed within %u transactions",
								oldest_datname,
								xidWrapLimit - xid),
						 errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database."
								 )));
			else
				ereport(WARNING,
						(errmsg("database with OID %u must be vacuumed within %u transactions",
								oldest_datoid,
								xidWrapLimit - xid),
						 errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database."
								 )));
		}

		/* Re-acquire lock and start over */
		LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
		xid = ShmemVariableCache->nextXid;
	}

	/*
	 * If we are allocating the first XID of a new page of the commit log,
	 * zero out that commit-log page before returning. We must do this while
	 * holding XidGenLock, else another xact could acquire and commit a later
	 * XID before we zero the page.  Fortunately, a page of the commit log
	 * holds 32K or more transactions, so we don't have to do this very often.
	 *
	 * Extend pg_subtrans too.
	 */
	ExtendCLOG(xid);
	ExtendSUBTRANS(xid);
	DistributedLog_Extend(xid);

	/*
	 * Now advance the nextXid counter.  This must not happen until after we
	 * have successfully completed ExtendCLOG() --- if that routine fails, we
	 * want the next incoming transaction to try it again.	We cannot assign
	 * more XIDs until there is CLOG space for them.
	 */
	TransactionIdAdvance(ShmemVariableCache->nextXid);

	/*
	 * To aid testing, you can set the debug_burn_xids GUC, to consume XIDs
	 * faster. If set, we bump the XID counter to the next value divisible by
	 * 4096, minus one. The idea is to skip over "boring" XID ranges, but
	 * still step through XID wraparound, CLOG page boundaries etc. one XID
	 * at a time.
	 */
	if (Debug_burn_xids)
	{
		TransactionId xx;
		uint32		r;

		/*
		 * Based on the minimum of ENTRIES_PER_PAGE (DistributedLog),
		 * SUBTRANS_XACTS_PER_PAGE, CLOG_XACTS_PER_PAGE.
		 */
		const uint32      page_extend_limit = 4 * 1024;

		xx = ShmemVariableCache->nextXid;

		r = xx % page_extend_limit;
		if (r > 1 && r < (page_extend_limit - 1))
		{
			xx += page_extend_limit - r - 1;
			ShmemVariableCache->nextXid = xx;
		}
	}

	/*
	 * We must store the new XID into the shared ProcArray before releasing
	 * XidGenLock.	This ensures that every active XID older than
	 * latestCompletedXid is present in the ProcArray, which is essential for
	 * correct OldestXmin tracking; see src/backend/access/transam/README.
	 *
	 * XXX by storing xid into MyProc without acquiring ProcArrayLock, we are
	 * relying on fetch/store of an xid to be atomic, else other backends
	 * might see a partially-set xid here.	But holding both locks at once
	 * would be a nasty concurrency hit.  So for now, assume atomicity.
	 *
	 * Note that readers of PGPROC xid fields should be careful to fetch the
	 * value only once, rather than assume they can read a value multiple
	 * times and get the same answer each time.
	 *
	 * The same comments apply to the subxact xid count and overflow fields.
	 *
	 * A solution to the atomic-store problem would be to give each PGPROC its
	 * own spinlock used only for fetching/storing that PGPROC's xid and
	 * related fields.
	 *
	 * If there's no room to fit a subtransaction XID into PGPROC, set the
	 * cache-overflowed flag instead.  This forces readers to look in
	 * pg_subtrans to map subtransaction XIDs up to top-level XIDs. There is a
	 * race-condition window, in that the new XID will not appear as running
	 * until its parent link has been placed into pg_subtrans. However, that
	 * will happen before anyone could possibly have a reason to inquire about
	 * the status of the XID, so it seems OK.  (Snapshots taken during this
	 * window *will* include the parent XID, so they will deliver the correct
	 * answer later on when someone does have a reason to inquire.)
	 */
	{
		/*
		 * Use volatile pointer to prevent code rearrangement; other backends
		 * could be examining my subxids info concurrently, and we don't want
		 * them to see an invalid intermediate state, such as incrementing
		 * nxids before filling the array entry.  Note we are assuming that
		 * TransactionId and int fetch/store are atomic.
		 */
		volatile PGPROC *myproc = MyProc;

		if (!isSubXact)
			myproc->xid = xid;
		else
		{
			int			nxids = myproc->subxids.nxids;

			if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
			{
				myproc->subxids.xids[nxids] = xid;
				myproc->subxids.nxids = nxids + 1;
			}
			else
				myproc->subxids.overflowed = true;
		}
	}

	LWLockRelease(XidGenLock);

	return xid;
}

/*
 * Read nextXid but don't allocate it.
 */
TransactionId
ReadNewTransactionId(void)
{
	TransactionId xid;

	LWLockAcquire(XidGenLock, LW_SHARED);
	xid = ShmemVariableCache->nextXid;
	LWLockRelease(XidGenLock);

	return xid;
}

/*
 * Get the last safe XID, i.e. the oldest XID that might exist in any
 * database of our cluster.
 */
TransactionId
GetTransactionIdLimit(void)
{
	TransactionId xid;

	LWLockAcquire(XidGenLock, LW_SHARED);
	xid = ShmemVariableCache->oldestXid;
	LWLockRelease(XidGenLock);

	if (!TransactionIdIsNormal(xid))
	{
		/*
		 * shouldn't happen, but since this value is used in the computation
		 * of oldest xmin, which determines which tuples be safely vacuumed
		 * away, let's be paranoid.
		 */
		elog(ERROR, "invalid oldestXid limit: %u", xid);
	}

	return xid;
}

/*
 * Determine the last safe XID to allocate given the currently oldest
 * datfrozenxid (ie, the oldest XID that might exist in any database
 * of our cluster), and the OID of the (or a) database with that value.
 */
void
SetTransactionIdLimit(TransactionId oldest_datfrozenxid, Oid oldest_datoid)
{
	TransactionId xidVacLimit;
	TransactionId xidWarnLimit;
	TransactionId xidStopLimit;
	TransactionId xidWrapLimit;
	TransactionId curXid;

	Assert(TransactionIdIsNormal(oldest_datfrozenxid));

	/*
	 * The place where we actually get into deep trouble is halfway around
	 * from the oldest potentially-existing XID.  (This calculation is
	 * probably off by one or two counts, because the special XIDs reduce the
	 * size of the loop a little bit.  But we throw in plenty of slop below,
	 * so it doesn't matter.)
	 */
	xidWrapLimit = oldest_datfrozenxid + (MaxTransactionId >> 1);
	if (xidWrapLimit < FirstNormalTransactionId)
		xidWrapLimit += FirstNormalTransactionId;

	/*
	 * We'll refuse to continue assigning XIDs in interactive mode once we get
	 * within xid_stop_limit transactions of data loss.  This leaves lots of
	 * room for the DBA to fool around fixing things in a standalone backend,
	 * while not being significant compared to total XID space. (Note that since
	 * vacuuming requires one transaction per table cleaned, we had better be
	 * sure there's lots of XIDs left...)
	 */
	xidStopLimit = xidWrapLimit - (TransactionId)xid_stop_limit;
	if (xidStopLimit < FirstNormalTransactionId)
		xidStopLimit -= FirstNormalTransactionId;

	/*
	 * We'll start complaining loudly when we get within xid_warn_limit
	 * transactions of the stop point.	This is kind of arbitrary, but if
	 * you let your gas gauge get down to 1% of full, would you be looking for
	 * the next gas station?  We need to be fairly liberal about this number
	 * because there are lots of scenarios where most transactions are done by
	 * automatic clients that won't pay attention to warnings. (No, we're not
	 * gonna make this configurable.  If you know enough to configure it, you
	 * know enough to not get in this kind of trouble in the first place.)
	 */
	xidWarnLimit = xidStopLimit  - (TransactionId)xid_warn_limit;
	if (xidWarnLimit < FirstNormalTransactionId)
		xidWarnLimit -= FirstNormalTransactionId;

	/*
	 * We'll start trying to force autovacuums when oldest_datfrozenxid gets
	 * to be more than autovacuum_freeze_max_age transactions old.
	 *
	 * Note: guc.c ensures that autovacuum_freeze_max_age is in a sane range,
	 * so that xidVacLimit will be well before xidWarnLimit.
	 *
	 * Note: autovacuum_freeze_max_age is a PGC_POSTMASTER parameter so that
	 * we don't have to worry about dealing with on-the-fly changes in its
	 * value.  It doesn't look practical to update shared state from a GUC
	 * assign hook (too many processes would try to execute the hook,
	 * resulting in race conditions as well as crashes of those not connected
	 * to shared memory).  Perhaps this can be improved someday.
	 */
	xidVacLimit = oldest_datfrozenxid + autovacuum_freeze_max_age;
	if (xidVacLimit < FirstNormalTransactionId)
		xidVacLimit += FirstNormalTransactionId;

	/* Grab lock for just long enough to set the new limit values */
	LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
	ShmemVariableCache->oldestXid = oldest_datfrozenxid;
	ShmemVariableCache->xidVacLimit = xidVacLimit;
	ShmemVariableCache->xidWarnLimit = xidWarnLimit;
	ShmemVariableCache->xidStopLimit = xidStopLimit;
	ShmemVariableCache->xidWrapLimit = xidWrapLimit;
	ShmemVariableCache->oldestXidDB = oldest_datoid;
	curXid = ShmemVariableCache->nextXid;
	LWLockRelease(XidGenLock);

	/* Log the info */
	ereport(DEBUG1,
			(errmsg("transaction ID wrap limit is %u, limited by database with OID %u",
					xidWrapLimit, oldest_datoid)));

	/*
	 * If past the autovacuum force point, immediately signal an autovac
	 * request.  The reason for this is that autovac only processes one
	 * database per invocation.  Once it's finished cleaning up the oldest
	 * database, it'll call here, and we'll signal the postmaster to start
	 * another iteration immediately if there are still any old databases.
	 */
	if (TransactionIdFollowsOrEquals(curXid, xidVacLimit) &&
		IsUnderPostmaster && !InRecovery)
		SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

	/* Give an immediate warning if past the wrap warn point */
	if (TransactionIdFollowsOrEquals(curXid, xidWarnLimit) && !InRecovery)
	{
		char	   *oldest_datname;

		/*
		 * We can be called when not inside a transaction, for example
		 * during StartupXLOG().  In such a case we cannot do database
		 * access, so we must just report the oldest DB's OID.
		 *
		 * Note: it's also possible that get_database_name fails and returns
		 * NULL, for example because the database just got dropped.  We'll
		 * still warn, even though the warning might now be unnecessary.
		 */
		if (IsTransactionState())
			oldest_datname = get_database_name(oldest_datoid);
		else
			oldest_datname = NULL;

		if (oldest_datname)
			ereport(WARNING,
			(errmsg("database \"%s\" must be vacuumed within %u transactions",
					oldest_datname,
					xidWrapLimit - curXid),
			 errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
					 "You might also need to commit or roll back old prepared transactions.")));
		else
			ereport(WARNING,
					(errmsg("database with OID %u must be vacuumed within %u transactions",
							oldest_datoid,
							xidWrapLimit - curXid),
					 errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
							 "You might also need to commit or roll back old prepared transactions.")));
	}
}


/*
 * ForceTransactionIdLimitUpdate -- does the XID wrap-limit data need updating?
 *
 * We primarily check whether oldestXidDB is valid.  The cases we have in
 * mind are that that database was dropped, or the field was reset to zero
 * by pg_resetxlog.  In either case we should force recalculation of the
 * wrap limit.	Also do it if oldestXid is old enough to be forcing
 * autovacuums or other actions; this ensures we update our state as soon
 * as possible once extra overhead is being incurred.
 */
bool
ForceTransactionIdLimitUpdate(void)
{
	TransactionId nextXid;
	TransactionId xidVacLimit;
	TransactionId oldestXid;
	Oid			oldestXidDB;

	/* Locking is probably not really necessary, but let's be careful */
	LWLockAcquire(XidGenLock, LW_SHARED);
	nextXid = ShmemVariableCache->nextXid;
	xidVacLimit = ShmemVariableCache->xidVacLimit;
	oldestXid = ShmemVariableCache->oldestXid;
	oldestXidDB = ShmemVariableCache->oldestXidDB;
	LWLockRelease(XidGenLock);

	if (!TransactionIdIsNormal(oldestXid))
		return true;			/* shouldn't happen, but just in case */
	if (!TransactionIdIsValid(xidVacLimit))
		return true;			/* this shouldn't happen anymore either */
	if (TransactionIdFollowsOrEquals(nextXid, xidVacLimit))
		return true;			/* past VacLimit, don't delay updating */
	if (!SearchSysCacheExists1(DATABASEOID, ObjectIdGetDatum(oldestXidDB)))
		return true;			/* could happen, per comments above */
	return false;
}

/*
 * Requires OidGenLock to be held by caller.
 */
static Oid
GetNewObjectIdUnderLock(void)
{
	Oid result;

	/* safety check, we should never get this far in a HS slave */
	if (RecoveryInProgress())
		elog(ERROR, "cannot assign OIDs during recovery");

	Assert(LWLockHeldByMe(OidGenLock));

	/*
	 * Check for wraparound of the OID counter.  We *must* not return 0
	 * (InvalidOid); and as long as we have to check that, it seems a good
	 * idea to skip over everything below FirstNormalObjectId too. (This
	 * basically just avoids lots of collisions with bootstrap-assigned OIDs
	 * right after a wrap occurs, so as to avoid a possibly large number of
	 * iterations in GetNewOid.)  Note we are relying on unsigned comparison.
	 *
	 * During initdb, we start the OID generator at FirstBootstrapObjectId, so
	 * we only enforce wrapping to that point when in bootstrap or standalone
	 * mode.  The first time through this routine after normal postmaster
	 * start, the counter will be forced up to FirstNormalObjectId. This
	 * mechanism leaves the OIDs between FirstBootstrapObjectId and
	 * FirstNormalObjectId available for automatic assignment during initdb,
	 * while ensuring they will never conflict with user-assigned OIDs.
	 */
	if (ShmemVariableCache->nextOid < ((Oid) FirstNormalObjectId))
	{
		if (IsPostmasterEnvironment)
		{
			/* wraparound in normal environment */
			ShmemVariableCache->nextOid = FirstNormalObjectId;
			ShmemVariableCache->oidCount = 0;
		}
		else
		{
			/* we may be bootstrapping, so don't enforce the full range */
			if (ShmemVariableCache->nextOid < ((Oid) FirstBootstrapObjectId))
			{
				/* wraparound in standalone environment? */
				ShmemVariableCache->nextOid = FirstBootstrapObjectId;
				ShmemVariableCache->oidCount = 0;
			}
		}
	}

	/* If we run out of logged for use oids then we must log more */
	if (ShmemVariableCache->oidCount == 0)
	{
		XLogPutNextOid(ShmemVariableCache->nextOid + VAR_OID_PREFETCH);
		ShmemVariableCache->oidCount = VAR_OID_PREFETCH;
	}

	result = ShmemVariableCache->nextOid;

	(ShmemVariableCache->nextOid)++;
	(ShmemVariableCache->oidCount)--;

	return result;
}

/*
 * GetNewObjectId -- allocate a new OID
 *
 * OIDs are generated by a cluster-wide counter. Since they are only
 * 32 bits wide, counter wraparound will occur eventually, and
 * therefore it is unwise to assume they are unique unless precautions
 * are taken to make them so. Hence, this routine should generally not
 * be used directly. The only direct callers should be GetNewOid() and
 * GetNewOidWithIndex() in catalog/catalog.c. It's also called from
 * cdb_sync_oid_to_segments() in cdb/cdboidsync.c to synchronize the
 * OID counter on the QD with its QEs.
 */
Oid
GetNewObjectId(void)
{
	Oid			result;

	LWLockAcquire(OidGenLock, LW_EXCLUSIVE);

	result = GetNewObjectIdUnderLock();

	LWLockRelease(OidGenLock);

	return result;
}

/*
 * AdvanceObjectId -- advance object id counter for QE nodes
 *
 * The QD provides the preassigned OID to the QE nodes which will be
 * used as the relation's OID. QE nodes do not use this OID as the
 * relfilenode value anymore so the OID counter is not
 * incremented. This function forcefully increments the QE node's OID
 * counter to be about the same as the OID provided by the QD node.
 */
void
AdvanceObjectId(Oid newOid)
{
	LWLockAcquire(OidGenLock, LW_EXCLUSIVE);

	while(GetNewObjectIdUnderLock() <= newOid);

	LWLockRelease(OidGenLock);
}

/*
 * Requires RelfilenodeGenLock to be held by caller.
 */
static Oid
GetNewSegRelfilenodeUnderLock(void)
{
	Oid result;

	Assert(LWLockHeldByMe(RelfilenodeGenLock));

	if (ShmemVariableCache->nextRelfilenode < ((Oid) FirstNormalObjectId) &&
		IsPostmasterEnvironment)
	{
		/* wraparound in normal environment */
		ShmemVariableCache->nextRelfilenode = FirstNormalObjectId;
		ShmemVariableCache->relfilenodeCount = 0;
	}

	if (ShmemVariableCache->relfilenodeCount == 0)
	{
		XLogPutNextRelfilenode(ShmemVariableCache->nextRelfilenode + VAR_OID_PREFETCH);
		ShmemVariableCache->relfilenodeCount = VAR_OID_PREFETCH;
	}

	result = ShmemVariableCache->nextRelfilenode;

	(ShmemVariableCache->nextRelfilenode)++;
	(ShmemVariableCache->relfilenodeCount)--;

	return result;
}

/*
 * GetNewSegRelfilenode -- allocate a new relfilenode value
 *
 * Similar to GetNewObjectId but for relfilenodes. This function has its own
 * separate counter and is used to allocate relfilenode values instead of
 * trying to use the newly generated OIDs (QD) or preassigned OIDs (QE) as the
 * relfilenode.
 */
Oid
GetNewSegRelfilenode(void)
{
	Oid result;

	LWLockAcquire(RelfilenodeGenLock, LW_EXCLUSIVE);

	result = GetNewSegRelfilenodeUnderLock();

	LWLockRelease(RelfilenodeGenLock);

	return result;
}

/*
 * Only used for GP_ROLE_DISPATCH and GP_ROLE_UTILITY to make sure
 * sequence relation has OID same as relfilenode. This is required due
 * to sequence server doing direct opens to filesystem assuming relfilenode is
 * the same as OID.
 *
 * Note: Delete this function and its calls if sequence relations
 * change to where we no longer have to assume relfilenode is the same
 * as OID.
 */
Oid
GetNewSequenceRelationObjectId(void)
{
	Oid currentOidCount;
	Oid currentRelfilenodeCount;
	Oid result = InvalidOid;

	Assert(Gp_role == GP_ROLE_DISPATCH || Gp_role == GP_ROLE_UTILITY);

	LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
	LWLockAcquire(RelfilenodeGenLock, LW_EXCLUSIVE);

	currentOidCount = ShmemVariableCache->nextOid;
	currentRelfilenodeCount = ShmemVariableCache->nextRelfilenode;

	if (currentOidCount >= currentRelfilenodeCount)
	{
		result = GetNewObjectIdUnderLock();
		while(GetNewSegRelfilenodeUnderLock() <= result);
	}
	else if (currentOidCount < currentRelfilenodeCount)
	{
		result = GetNewSegRelfilenodeUnderLock();
		while(GetNewObjectIdUnderLock() <= result);
	}

	LWLockRelease(OidGenLock);
	LWLockRelease(RelfilenodeGenLock);

	return result;
}