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Sourcecode: postgresql-8.4 version File versions

transam.c

/*-------------------------------------------------------------------------
 *
 * transam.c
 *      postgres transaction log interface routines
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *      $PostgreSQL$
 *
 * NOTES
 *      This file contains the high level access-method interface to the
 *      transaction system.
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/clog.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "utils/snapmgr.h"


/*
 * Single-item cache for results of TransactionLogFetch.  It's worth having
 * such a cache because we frequently find ourselves repeatedly checking the
 * same XID, for example when scanning a table just after a bulk insert,
 * update, or delete.
 */
static TransactionId cachedFetchXid = InvalidTransactionId;
static XidStatus cachedFetchXidStatus;
static XLogRecPtr cachedCommitLSN;

/* Handy constant for an invalid xlog recptr */
static const XLogRecPtr InvalidXLogRecPtr = {0, 0};

/* Local functions */
static XidStatus TransactionLogFetch(TransactionId transactionId);


/* ----------------------------------------------------------------
 *          Postgres log access method interface
 *
 *          TransactionLogFetch
 * ----------------------------------------------------------------
 */

/*
 * TransactionLogFetch --- fetch commit status of specified transaction id
 */
static XidStatus
TransactionLogFetch(TransactionId transactionId)
{
      XidStatus   xidstatus;
      XLogRecPtr  xidlsn;

      /*
       * Before going to the commit log manager, check our single item cache to
       * see if we didn't just check the transaction status a moment ago.
       */
      if (TransactionIdEquals(transactionId, cachedFetchXid))
            return cachedFetchXidStatus;

      /*
       * Also, check to see if the transaction ID is a permanent one.
       */
      if (!TransactionIdIsNormal(transactionId))
      {
            if (TransactionIdEquals(transactionId, BootstrapTransactionId))
                  return TRANSACTION_STATUS_COMMITTED;
            if (TransactionIdEquals(transactionId, FrozenTransactionId))
                  return TRANSACTION_STATUS_COMMITTED;
            return TRANSACTION_STATUS_ABORTED;
      }

      /*
       * Get the transaction status.
       */
      xidstatus = TransactionIdGetStatus(transactionId, &xidlsn);

      /*
       * Cache it, but DO NOT cache status for unfinished or sub-committed
       * transactions!  We only cache status that is guaranteed not to change.
       */
      if (xidstatus != TRANSACTION_STATUS_IN_PROGRESS &&
            xidstatus != TRANSACTION_STATUS_SUB_COMMITTED)
      {
            cachedFetchXid = transactionId;
            cachedFetchXidStatus = xidstatus;
            cachedCommitLSN = xidlsn;
      }

      return xidstatus;
}

/* ----------------------------------------------------------------
 *                                  Interface functions
 *
 *          TransactionIdDidCommit
 *          TransactionIdDidAbort
 *          ========
 *             these functions test the transaction status of
 *             a specified transaction id.
 *
 *          TransactionIdCommitTree
 *          TransactionIdAsyncCommitTree
 *          TransactionIdAbortTree
 *          ========
 *             these functions set the transaction status of the specified
 *             transaction tree.
 *
 * See also TransactionIdIsInProgress, which once was in this module
 * but now lives in procarray.c.
 * ----------------------------------------------------------------
 */

/*
 * TransactionIdDidCommit
 *          True iff transaction associated with the identifier did commit.
 *
 * Note:
 *          Assumes transaction identifier is valid.
 */
bool                                      /* true if given transaction committed */
TransactionIdDidCommit(TransactionId transactionId)
{
      XidStatus   xidstatus;

      xidstatus = TransactionLogFetch(transactionId);

      /*
       * If it's marked committed, it's committed.
       */
      if (xidstatus == TRANSACTION_STATUS_COMMITTED)
            return true;

      /*
       * If it's marked subcommitted, we have to check the parent recursively.
       * However, if it's older than TransactionXmin, we can't look at
       * pg_subtrans; instead assume that the parent crashed without cleaning up
       * its children.
       *
       * Originally we Assert'ed that the result of SubTransGetParent was not
       * zero. However with the introduction of prepared transactions, there can
       * be a window just after database startup where we do not have complete
       * knowledge in pg_subtrans of the transactions after TransactionXmin.
       * StartupSUBTRANS() has ensured that any missing information will be
       * zeroed.  Since this case should not happen under normal conditions, it
       * seems reasonable to emit a WARNING for it.
       */
      if (xidstatus == TRANSACTION_STATUS_SUB_COMMITTED)
      {
            TransactionId parentXid;

            if (TransactionIdPrecedes(transactionId, TransactionXmin))
                  return false;
            parentXid = SubTransGetParent(transactionId);
            if (!TransactionIdIsValid(parentXid))
            {
                  elog(WARNING, "no pg_subtrans entry for subcommitted XID %u",
                         transactionId);
                  return false;
            }
            return TransactionIdDidCommit(parentXid);
      }

      /*
       * It's not committed.
       */
      return false;
}

/*
 * TransactionIdDidAbort
 *          True iff transaction associated with the identifier did abort.
 *
 * Note:
 *          Assumes transaction identifier is valid.
 */
bool                                      /* true if given transaction aborted */
TransactionIdDidAbort(TransactionId transactionId)
{
      XidStatus   xidstatus;

      xidstatus = TransactionLogFetch(transactionId);

      /*
       * If it's marked aborted, it's aborted.
       */
      if (xidstatus == TRANSACTION_STATUS_ABORTED)
            return true;

      /*
       * If it's marked subcommitted, we have to check the parent recursively.
       * However, if it's older than TransactionXmin, we can't look at
       * pg_subtrans; instead assume that the parent crashed without cleaning up
       * its children.
       */
      if (xidstatus == TRANSACTION_STATUS_SUB_COMMITTED)
      {
            TransactionId parentXid;

            if (TransactionIdPrecedes(transactionId, TransactionXmin))
                  return true;
            parentXid = SubTransGetParent(transactionId);
            if (!TransactionIdIsValid(parentXid))
            {
                  /* see notes in TransactionIdDidCommit */
                  elog(WARNING, "no pg_subtrans entry for subcommitted XID %u",
                         transactionId);
                  return true;
            }
            return TransactionIdDidAbort(parentXid);
      }

      /*
       * It's not aborted.
       */
      return false;
}

/*
 * TransactionIdIsKnownCompleted
 *          True iff transaction associated with the identifier is currently
 *          known to have either committed or aborted.
 *
 * This does NOT look into pg_clog but merely probes our local cache
 * (and so it's not named TransactionIdDidComplete, which would be the
 * appropriate name for a function that worked that way).  The intended
 * use is just to short-circuit TransactionIdIsInProgress calls when doing
 * repeated tqual.c checks for the same XID.  If this isn't extremely fast
 * then it will be counterproductive.
 *
 * Note:
 *          Assumes transaction identifier is valid.
 */
bool
TransactionIdIsKnownCompleted(TransactionId transactionId)
{
      if (TransactionIdEquals(transactionId, cachedFetchXid))
      {
            /* If it's in the cache at all, it must be completed. */
            return true;
      }

      return false;
}

/*
 * TransactionIdCommitTree
 *          Marks the given transaction and children as committed
 *
 * "xid" is a toplevel transaction commit, and the xids array contains its
 * committed subtransactions.
 *
 * This commit operation is not guaranteed to be atomic, but if not, subxids
 * are correctly marked subcommit first.
 */
void
TransactionIdCommitTree(TransactionId xid, int nxids, TransactionId *xids)
{
      TransactionIdSetTreeStatus(xid, nxids, xids,
                                             TRANSACTION_STATUS_COMMITTED,
                                             InvalidXLogRecPtr);
}

/*
 * TransactionIdAsyncCommitTree
 *          Same as above, but for async commits.  The commit record LSN is needed.
 */
void
TransactionIdAsyncCommitTree(TransactionId xid, int nxids, TransactionId *xids,
                                           XLogRecPtr lsn)
{
      TransactionIdSetTreeStatus(xid, nxids, xids,
                                             TRANSACTION_STATUS_COMMITTED, lsn);
}

/*
 * TransactionIdAbortTree
 *          Marks the given transaction and children as aborted.
 *
 * "xid" is a toplevel transaction commit, and the xids array contains its
 * committed subtransactions.
 *
 * We don't need to worry about the non-atomic behavior, since any onlookers
 * will consider all the xacts as not-yet-committed anyway.
 */
void
TransactionIdAbortTree(TransactionId xid, int nxids, TransactionId *xids)
{
      TransactionIdSetTreeStatus(xid, nxids, xids,
                                             TRANSACTION_STATUS_ABORTED, InvalidXLogRecPtr);
}

/*
 * TransactionIdPrecedes --- is id1 logically < id2?
 */
bool
TransactionIdPrecedes(TransactionId id1, TransactionId id2)
{
      /*
       * If either ID is a permanent XID then we can just do unsigned
       * comparison.    If both are normal, do a modulo-2^31 comparison.
       */
      int32       diff;

      if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
            return (id1 < id2);

      diff = (int32) (id1 - id2);
      return (diff < 0);
}

/*
 * TransactionIdPrecedesOrEquals --- is id1 logically <= id2?
 */
bool
TransactionIdPrecedesOrEquals(TransactionId id1, TransactionId id2)
{
      int32       diff;

      if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
            return (id1 <= id2);

      diff = (int32) (id1 - id2);
      return (diff <= 0);
}

/*
 * TransactionIdFollows --- is id1 logically > id2?
 */
bool
TransactionIdFollows(TransactionId id1, TransactionId id2)
{
      int32       diff;

      if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
            return (id1 > id2);

      diff = (int32) (id1 - id2);
      return (diff > 0);
}

/*
 * TransactionIdFollowsOrEquals --- is id1 logically >= id2?
 */
bool
TransactionIdFollowsOrEquals(TransactionId id1, TransactionId id2)
{
      int32       diff;

      if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
            return (id1 >= id2);

      diff = (int32) (id1 - id2);
      return (diff >= 0);
}


/*
 * TransactionIdLatest --- get latest XID among a main xact and its children
 */
TransactionId
TransactionIdLatest(TransactionId mainxid,
                              int nxids, const TransactionId *xids)
{
      TransactionId result;

      /*
       * In practice it is highly likely that the xids[] array is sorted, and so
       * we could save some cycles by just taking the last child XID, but this
       * probably isn't so performance-critical that it's worth depending on
       * that assumption.  But just to show we're not totally stupid, scan the
       * array back-to-front to avoid useless assignments.
       */
      result = mainxid;
      while (--nxids >= 0)
      {
            if (TransactionIdPrecedes(result, xids[nxids]))
                  result = xids[nxids];
      }
      return result;
}


/*
 * TransactionIdGetCommitLSN
 *
 * This function returns an LSN that is late enough to be able
 * to guarantee that if we flush up to the LSN returned then we
 * will have flushed the transaction's commit record to disk.
 *
 * The result is not necessarily the exact LSN of the transaction's
 * commit record!  For example, for long-past transactions (those whose
 * clog pages already migrated to disk), we'll return InvalidXLogRecPtr.
 * Also, because we group transactions on the same clog page to conserve
 * storage, we might return the LSN of a later transaction that falls into
 * the same group.
 */
XLogRecPtr
TransactionIdGetCommitLSN(TransactionId xid)
{
      XLogRecPtr  result;

      /*
       * Currently, all uses of this function are for xids that were just
       * reported to be committed by TransactionLogFetch, so we expect that
       * checking TransactionLogFetch's cache will usually succeed and avoid an
       * extra trip to shared memory.
       */
      if (TransactionIdEquals(xid, cachedFetchXid))
            return cachedCommitLSN;

      /* Special XIDs are always known committed */
      if (!TransactionIdIsNormal(xid))
            return InvalidXLogRecPtr;

      /*
       * Get the transaction status.
       */
      (void) TransactionIdGetStatus(xid, &result);

      return result;
}

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