Re: Improvement of procArray.xmin for VACUUM - Mailing list pgsql-patches

Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
> > I have been thinking we could improve how quickly VACUUM can expire rows
> > if we update procArray.xmin more frequently for non-SERIALIZABLE
> > transactions.
> > The attached patch updates procArray.xmin in this manner.
>
> This patch is incredibly broken.  Didn't you understand what I said
> about how we don't track which snapshots are still alive?  You can't
> advance procArray.xmin past the xmin of the oldest live snapshot in the
> backend, and you can't assume that there are no live snapshots at the
> places where this patch assumes that.  (Open cursors are one obvious
> counterexample, but I think there are more.)
>
> To make intra-transaction advancing of xmin possible, we'd need to
> explicitly track all of the backend's live snapshots, probably by
> introducing a "snapshot cache" manager that gives out tracked refcounts
> as we do for some other structures like catcache entries.  This might
> have some other advantages (I think most of the current CopySnapshot
> operations could be replaced by refcount increments) but it's a whole
> lot more complicated and invasive than what you've got here.

I updated the patch to save the MyProc->xid at the time the first cursor
is created, and not allow the MyProc->xid to be set lower than that
saved value in the current transaction.  It added only a few more lines
to the patch.

--
  Bruce Momjian  <bruce@momjian.us>          http://momjian.us
  EnterpriseDB                               http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +
Index: src/backend/access/transam/xact.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/access/transam/xact.c,v
retrieving revision 1.238
diff -c -c -r1.238 xact.c
*** src/backend/access/transam/xact.c    22 Mar 2007 19:55:04 -0000    1.238
--- src/backend/access/transam/xact.c    24 Mar 2007 02:18:41 -0000
***************
*** 1563,1569 ****

          LWLockRelease(ProcArrayLock);
      }
!
      PG_TRACE1(transaction__commit, s->transactionId);

      /*
--- 1563,1570 ----

          LWLockRelease(ProcArrayLock);
      }
!     set_portal_min_xid(InvalidTransactionId);
!
      PG_TRACE1(transaction__commit, s->transactionId);

      /*
***************
*** 1802,1807 ****
--- 1803,1810 ----

      LWLockRelease(ProcArrayLock);

+     set_portal_min_xid(InvalidTransactionId);
+
      /*
       * This is all post-transaction cleanup.  Note that if an error is raised
       * here, it's too late to abort the transaction.  This should be just
***************
*** 1969,1974 ****
--- 1972,1979 ----
          LWLockRelease(ProcArrayLock);
      }

+     set_portal_min_xid(InvalidTransactionId);
+
      PG_TRACE1(transaction__abort, s->transactionId);

      /*
Index: src/backend/catalog/index.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/catalog/index.c,v
retrieving revision 1.280
diff -c -c -r1.280 index.c
*** src/backend/catalog/index.c    3 Mar 2007 20:08:41 -0000    1.280
--- src/backend/catalog/index.c    24 Mar 2007 02:18:42 -0000
***************
*** 1394,1400 ****
      }
      else if (indexInfo->ii_Concurrent)
      {
!         snapshot = CopySnapshot(GetTransactionSnapshot());
          OldestXmin = InvalidTransactionId;        /* not used */
      }
      else
--- 1394,1400 ----
      }
      else if (indexInfo->ii_Concurrent)
      {
!         snapshot = CopySnapshot(GetTransactionSnapshot(false));
          OldestXmin = InvalidTransactionId;        /* not used */
      }
      else
Index: src/backend/commands/cluster.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/commands/cluster.c,v
retrieving revision 1.157
diff -c -c -r1.157 cluster.c
*** src/backend/commands/cluster.c    13 Mar 2007 00:33:39 -0000    1.157
--- src/backend/commands/cluster.c    24 Mar 2007 02:18:42 -0000
***************
*** 204,210 ****
              /* Start a new transaction for each relation. */
              StartTransactionCommand();
              /* functions in indexes may want a snapshot set */
!             ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
              cluster_rel(rvtc, true);
              CommitTransactionCommand();
          }
--- 204,210 ----
              /* Start a new transaction for each relation. */
              StartTransactionCommand();
              /* functions in indexes may want a snapshot set */
!             ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(true));
              cluster_rel(rvtc, true);
              CommitTransactionCommand();
          }
Index: src/backend/commands/indexcmds.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/commands/indexcmds.c,v
retrieving revision 1.157
diff -c -c -r1.157 indexcmds.c
*** src/backend/commands/indexcmds.c    13 Mar 2007 00:33:39 -0000    1.157
--- src/backend/commands/indexcmds.c    24 Mar 2007 02:18:42 -0000
***************
*** 493,499 ****
       * We also set ActiveSnapshot to this snap, since functions in indexes may
       * need a snapshot.
       */
!     snapshot = CopySnapshot(GetTransactionSnapshot());
      ActiveSnapshot = snapshot;

      /*
--- 493,499 ----
       * We also set ActiveSnapshot to this snap, since functions in indexes may
       * need a snapshot.
       */
!     snapshot = CopySnapshot(GetTransactionSnapshot(false));
      ActiveSnapshot = snapshot;

      /*
***************
*** 1304,1310 ****

          StartTransactionCommand();
          /* functions in indexes may want a snapshot set */
!         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
          if (reindex_relation(relid, true))
              ereport(NOTICE,
                      (errmsg("table \"%s\" was reindexed",
--- 1304,1310 ----

          StartTransactionCommand();
          /* functions in indexes may want a snapshot set */
!         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(true));
          if (reindex_relation(relid, true))
              ereport(NOTICE,
                      (errmsg("table \"%s\" was reindexed",
Index: src/backend/commands/trigger.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/commands/trigger.c,v
retrieving revision 1.214
diff -c -c -r1.214 trigger.c
*** src/backend/commands/trigger.c    19 Mar 2007 23:38:29 -0000    1.214
--- src/backend/commands/trigger.c    24 Mar 2007 02:18:43 -0000
***************
*** 2645,2651 ****
       */
      events = &afterTriggers->events;
      if (events->head != NULL)
!         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());

      /*
       * Run all the remaining triggers.    Loop until they are all gone, just in
--- 2645,2651 ----
       */
      events = &afterTriggers->events;
      if (events->head != NULL)
!         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(false));

      /*
       * Run all the remaining triggers.    Loop until they are all gone, just in
Index: src/backend/commands/vacuum.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/commands/vacuum.c,v
retrieving revision 1.349
diff -c -c -r1.349 vacuum.c
*** src/backend/commands/vacuum.c    14 Mar 2007 18:48:55 -0000    1.349
--- src/backend/commands/vacuum.c    24 Mar 2007 02:18:45 -0000
***************
*** 412,418 ****
                  {
                      StartTransactionCommand();
                      /* functions in indexes may want a snapshot set */
!                     ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
                  }
                  else
                      old_context = MemoryContextSwitchTo(anl_context);
--- 412,418 ----
                  {
                      StartTransactionCommand();
                      /* functions in indexes may want a snapshot set */
!                     ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(false));
                  }
                  else
                      old_context = MemoryContextSwitchTo(anl_context);
***************
*** 470,476 ****
           * necessary if we are called from autovacuum because autovacuum might
           * continue on to do an ANALYZE-only call.
           */
!         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
      }

      if (vacstmt->vacuum && !IsAutoVacuumWorkerProcess())
--- 470,476 ----
           * necessary if we are called from autovacuum because autovacuum might
           * continue on to do an ANALYZE-only call.
           */
!         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(false));
      }

      if (vacstmt->vacuum && !IsAutoVacuumWorkerProcess())
***************
*** 964,970 ****
      if (vacstmt->full)
      {
          /* functions in indexes may want a snapshot set */
!         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
      }
      else
      {
--- 964,970 ----
      if (vacstmt->full)
      {
          /* functions in indexes may want a snapshot set */
!         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(false));
      }
      else
      {
Index: src/backend/executor/functions.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/executor/functions.c,v
retrieving revision 1.112
diff -c -c -r1.112 functions.c
*** src/backend/executor/functions.c    13 Mar 2007 00:33:40 -0000    1.112
--- src/backend/executor/functions.c    24 Mar 2007 02:18:45 -0000
***************
*** 300,306 ****
      else
      {
          CommandCounterIncrement();
!         snapshot = CopySnapshot(GetTransactionSnapshot());
      }

      if (IsA(es->stmt, PlannedStmt))
--- 300,306 ----
      else
      {
          CommandCounterIncrement();
!         snapshot = CopySnapshot(GetTransactionSnapshot(false));
      }

      if (IsA(es->stmt, PlannedStmt))
Index: src/backend/executor/spi.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/executor/spi.c,v
retrieving revision 1.173
diff -c -c -r1.173 spi.c
*** src/backend/executor/spi.c    17 Mar 2007 03:15:38 -0000    1.173
--- src/backend/executor/spi.c    24 Mar 2007 02:18:46 -0000
***************
*** 15,23 ****
--- 15,25 ----
  #include "postgres.h"

  #include "access/printtup.h"
+ #include "access/transam.h"
  #include "catalog/heap.h"
  #include "commands/trigger.h"
  #include "executor/spi_priv.h"
+ #include "storage/procarray.h"
  #include "utils/lsyscache.h"
  #include "utils/memutils.h"
  #include "utils/typcache.h"
***************
*** 57,63 ****
  static MemoryContext _SPI_procmem(void);
  static bool _SPI_checktuples(void);

-
  /* =================== interface functions =================== */

  int
--- 59,64 ----
***************
*** 997,1004 ****
      else
      {
          CommandCounterIncrement();
!         snapshot = GetTransactionSnapshot();
      }

      /*
       * Start portal execution.
--- 998,1006 ----
      else
      {
          CommandCounterIncrement();
!         snapshot = GetTransactionSnapshot(false);
      }
+     set_portal_min_xid(TransactionXmin);

      /*
       * Start portal execution.
***************
*** 1530,1536 ****
                      if (read_only)
                          ActiveSnapshot = CopySnapshot(saveActiveSnapshot);
                      else
!                         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
                  }
                  else
                  {
--- 1532,1538 ----
                      if (read_only)
                          ActiveSnapshot = CopySnapshot(saveActiveSnapshot);
                      else
!                         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(false));
                  }
                  else
                  {
Index: src/backend/postmaster/autovacuum.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/postmaster/autovacuum.c,v
retrieving revision 1.34
diff -c -c -r1.34 autovacuum.c
*** src/backend/postmaster/autovacuum.c    13 Mar 2007 00:33:41 -0000    1.34
--- src/backend/postmaster/autovacuum.c    24 Mar 2007 02:18:46 -0000
***************
*** 868,874 ****
      StartTransactionCommand();

      /* functions in indexes may want a snapshot set */
!     ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());

      /*
       * Clean up any dead statistics collector entries for this DB. We always
--- 868,874 ----
      StartTransactionCommand();

      /* functions in indexes may want a snapshot set */
!     ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(false));

      /*
       * Clean up any dead statistics collector entries for this DB. We always
Index: src/backend/storage/ipc/procarray.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/storage/ipc/procarray.c,v
retrieving revision 1.22
diff -c -c -r1.22 procarray.c
*** src/backend/storage/ipc/procarray.c    23 Mar 2007 03:16:39 -0000    1.22
--- src/backend/storage/ipc/procarray.c    24 Mar 2007 02:18:47 -0000
***************
*** 54,60 ****
  } ProcArrayStruct;

  static ProcArrayStruct *procArray;
!

  #ifdef XIDCACHE_DEBUG

--- 54,60 ----
  } ProcArrayStruct;

  static ProcArrayStruct *procArray;
! static TransactionId portal_min_xid = InvalidTransactionId;

  #ifdef XIDCACHE_DEBUG

***************
*** 498,504 ****
   *----------
   */
  Snapshot
! GetSnapshotData(Snapshot snapshot, bool serializable)
  {
      ProcArrayStruct *arrayP = procArray;
      TransactionId xmin;
--- 498,504 ----
   *----------
   */
  Snapshot
! GetSnapshotData(Snapshot snapshot, bool set_procArray_xmin)
  {
      ProcArrayStruct *arrayP = procArray;
      TransactionId xmin;
***************
*** 510,519 ****

      Assert(snapshot != NULL);

!     /* Serializable snapshot must be computed before any other... */
!     Assert(serializable ?
!            !TransactionIdIsValid(MyProc->xmin) :
!            TransactionIdIsValid(MyProc->xmin));

      /*
       * Allocating space for maxProcs xids is usually overkill; numProcs would
--- 510,517 ----

      Assert(snapshot != NULL);

!     /* Either we set an xmin, or we already have one.  */
!     Assert(set_procArray_xmin || TransactionIdIsValid(MyProc->xmin));

      /*
       * Allocating space for maxProcs xids is usually overkill; numProcs would
***************
*** 656,664 ****
          }
      }

!     if (serializable)
!         MyProc->xmin = TransactionXmin = xmin;
!
      LWLockRelease(ProcArrayLock);

      /*
--- 654,684 ----
          }
      }

!     if (set_procArray_xmin)
!     {
!         if (xmin != MyProc->xmin)
!             fprintf(stderr, "%x:%x: set new xmin old=%x new=%x\n", MyProcPid, MyProc->xid, MyProc->xmin, xmin);
!         else
!             fprintf(stderr, "%x:%x: not set\n", MyProcPid, MyProc->xid);
!         /*
!          * ---
!          * We need to set MyProc->xmin in a few cases:
!          *    o  set at transaction start for SERIALIZABLE visibility
!          *    o  set for new command in READ COMMITTED mode so
!          *       VACUUM knows we don't care about completed transactions
!          *
!          *    The problem is that we might have old cursors/portals
!          *    that need to have old transaction visibility, so we use
!          *    portal_min_xid to record MyProc->xmin during the first
!          *    cursors/portals creation, and then don't allow MyProc->xmin
!          *    to be set lower.
!          */
!         if (!TransactionIdIsValid(MyProc->xmin) ||
!             !TransactionIdIsValid(portal_min_xid) ||
!             TransactionIdPrecedesOrEquals(xmin, portal_min_xid))
!             MyProc->xmin = TransactionXmin = xmin;
!     }
!
      LWLockRelease(ProcArrayLock);

      /*
***************
*** 987,992 ****
--- 1007,1023 ----
      LWLockRelease(ProcArrayLock);
  }

+ void set_portal_min_xid(TransactionId xid)
+ {
+     /*
+      *    We only want portal_min_xid to be the first
+      *    portal's MyProc->xmin, or set to invalid.
+      */
+     if (portal_min_xid == InvalidTransactionId ||
+         xid == InvalidTransactionId)
+         portal_min_xid = xid;
+ }
+
  #ifdef XIDCACHE_DEBUG

  /*
Index: src/backend/tcop/fastpath.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/tcop/fastpath.c,v
retrieving revision 1.96
diff -c -c -r1.96 fastpath.c
*** src/backend/tcop/fastpath.c    3 Mar 2007 19:32:54 -0000    1.96
--- src/backend/tcop/fastpath.c    24 Mar 2007 02:18:47 -0000
***************
*** 308,314 ****
       * Now that we know we are in a valid transaction, set snapshot in case
       * needed by function itself or one of the datatype I/O routines.
       */
!     ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());

      /*
       * Begin parsing the buffer contents.
--- 308,314 ----
       * Now that we know we are in a valid transaction, set snapshot in case
       * needed by function itself or one of the datatype I/O routines.
       */
!     ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(false));

      /*
       * Begin parsing the buffer contents.
Index: src/backend/tcop/postgres.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/tcop/postgres.c,v
retrieving revision 1.529
diff -c -c -r1.529 postgres.c
*** src/backend/tcop/postgres.c    22 Mar 2007 19:55:04 -0000    1.529
--- src/backend/tcop/postgres.c    24 Mar 2007 02:18:48 -0000
***************
*** 738,744 ****
          {
              if (needSnapshot)
              {
!                 ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
                  needSnapshot = false;
              }
              stmt = (Node *) pg_plan_query(query, boundParams);
--- 738,744 ----
          {
              if (needSnapshot)
              {
!                 ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(true));
                  needSnapshot = false;
              }
              stmt = (Node *) pg_plan_query(query, boundParams);
Index: src/backend/tcop/pquery.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/tcop/pquery.c,v
retrieving revision 1.115
diff -c -c -r1.115 pquery.c
*** src/backend/tcop/pquery.c    13 Mar 2007 00:33:42 -0000    1.115
--- src/backend/tcop/pquery.c    24 Mar 2007 02:18:48 -0000
***************
*** 154,160 ****
       * Must always set snapshot for plannable queries.    Note we assume that
       * caller will take care of restoring ActiveSnapshot on exit/error.
       */
!     ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());

      /*
       * Create the QueryDesc object
--- 154,160 ----
       * Must always set snapshot for plannable queries.    Note we assume that
       * caller will take care of restoring ActiveSnapshot on exit/error.
       */
!     ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(false));

      /*
       * Create the QueryDesc object
***************
*** 489,495 ****
                  if (snapshot)
                      ActiveSnapshot = CopySnapshot(snapshot);
                  else
!                     ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());

                  /*
                   * Create QueryDesc in portal's context; for the moment, set
--- 489,495 ----
                  if (snapshot)
                      ActiveSnapshot = CopySnapshot(snapshot);
                  else
!                     ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(false));

                  /*
                   * Create QueryDesc in portal's context; for the moment, set
***************
*** 1163,1169 ****
            IsA(utilityStmt, NotifyStmt) ||
            IsA(utilityStmt, UnlistenStmt) ||
            IsA(utilityStmt, CheckPointStmt)))
!         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
      else
          ActiveSnapshot = NULL;

--- 1163,1169 ----
            IsA(utilityStmt, NotifyStmt) ||
            IsA(utilityStmt, UnlistenStmt) ||
            IsA(utilityStmt, CheckPointStmt)))
!         ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(false));
      else
          ActiveSnapshot = NULL;

Index: src/backend/utils/adt/ri_triggers.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/utils/adt/ri_triggers.c,v
retrieving revision 1.92
diff -c -c -r1.92 ri_triggers.c
*** src/backend/utils/adt/ri_triggers.c    15 Mar 2007 23:12:06 -0000    1.92
--- src/backend/utils/adt/ri_triggers.c    24 Mar 2007 02:18:50 -0000
***************
*** 3283,3289 ****
      {
          CommandCounterIncrement();        /* be sure all my own work is visible */
          test_snapshot = CopySnapshot(GetLatestSnapshot());
!         crosscheck_snapshot = CopySnapshot(GetTransactionSnapshot());
      }
      else
      {
--- 3283,3289 ----
      {
          CommandCounterIncrement();        /* be sure all my own work is visible */
          test_snapshot = CopySnapshot(GetLatestSnapshot());
!         crosscheck_snapshot = CopySnapshot(GetTransactionSnapshot(false));
      }
      else
      {
Index: src/backend/utils/time/tqual.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/utils/time/tqual.c,v
retrieving revision 1.101
diff -c -c -r1.101 tqual.c
*** src/backend/utils/time/tqual.c    5 Jan 2007 22:19:47 -0000    1.101
--- src/backend/utils/time/tqual.c    24 Mar 2007 02:18:50 -0000
***************
*** 1202,1208 ****
   * the result with CopySnapshot() if it is to be used very long.
   */
  Snapshot
! GetTransactionSnapshot(void)
  {
      /* First call in transaction? */
      if (SerializableSnapshot == NULL)
--- 1202,1208 ----
   * the result with CopySnapshot() if it is to be used very long.
   */
  Snapshot
! GetTransactionSnapshot(bool set_procArray_xmin)
  {
      /* First call in transaction? */
      if (SerializableSnapshot == NULL)
***************
*** 1214,1220 ****
      if (IsXactIsoLevelSerializable)
          return SerializableSnapshot;

!     LatestSnapshot = GetSnapshotData(&LatestSnapshotData, false);

      return LatestSnapshot;
  }
--- 1214,1221 ----
      if (IsXactIsoLevelSerializable)
          return SerializableSnapshot;

!     LatestSnapshot = GetSnapshotData(&LatestSnapshotData,
!     /* we know we are not Serializable */ set_procArray_xmin);

      return LatestSnapshot;
  }
Index: src/include/storage/procarray.h
===================================================================
RCS file: /cvsroot/pgsql/src/include/storage/procarray.h,v
retrieving revision 1.12
diff -c -c -r1.12 procarray.h
*** src/include/storage/procarray.h    16 Jan 2007 13:28:57 -0000    1.12
--- src/include/storage/procarray.h    24 Mar 2007 02:18:51 -0000
***************
*** 37,41 ****
--- 37,42 ----

  extern void XidCacheRemoveRunningXids(TransactionId xid,
                            int nxids, TransactionId *xids);
+ extern void set_portal_min_xid(TransactionId xid);

  #endif   /* PROCARRAY_H */
Index: src/include/utils/tqual.h
===================================================================
RCS file: /cvsroot/pgsql/src/include/utils/tqual.h,v
retrieving revision 1.65
diff -c -c -r1.65 tqual.h
*** src/include/utils/tqual.h    5 Jan 2007 22:19:59 -0000    1.65
--- src/include/utils/tqual.h    24 Mar 2007 02:18:51 -0000
***************
*** 142,154 ****
  extern HTSV_Result HeapTupleSatisfiesVacuum(HeapTupleHeader tuple,
                           TransactionId OldestXmin, Buffer buffer);

! extern Snapshot GetTransactionSnapshot(void);
  extern Snapshot GetLatestSnapshot(void);
  extern Snapshot CopySnapshot(Snapshot snapshot);
  extern void FreeSnapshot(Snapshot snapshot);
  extern void FreeXactSnapshot(void);

  /* in procarray.c; declared here to avoid including tqual.h in procarray.h: */
! extern Snapshot GetSnapshotData(Snapshot snapshot, bool serializable);

  #endif   /* TQUAL_H */
--- 142,154 ----
  extern HTSV_Result HeapTupleSatisfiesVacuum(HeapTupleHeader tuple,
                           TransactionId OldestXmin, Buffer buffer);

! extern Snapshot GetTransactionSnapshot(bool set_procArray_xmin);
  extern Snapshot GetLatestSnapshot(void);
  extern Snapshot CopySnapshot(Snapshot snapshot);
  extern void FreeSnapshot(Snapshot snapshot);
  extern void FreeXactSnapshot(void);

  /* in procarray.c; declared here to avoid including tqual.h in procarray.h: */
! extern Snapshot GetSnapshotData(Snapshot snapshot, bool set_procArray_xmin);

  #endif   /* TQUAL_H */
Index: src/pl/plpgsql/src/pl_exec.c
===================================================================
RCS file: /cvsroot/pgsql/src/pl/plpgsql/src/pl_exec.c,v
retrieving revision 1.190
diff -c -c -r1.190 pl_exec.c
*** src/pl/plpgsql/src/pl_exec.c    15 Mar 2007 23:12:07 -0000    1.190
--- src/pl/plpgsql/src/pl_exec.c    24 Mar 2007 02:18:53 -0000
***************
*** 4124,4130 ****
          if (!estate->readonly_func)
          {
              CommandCounterIncrement();
!             ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
          }

          /*
--- 4124,4130 ----
          if (!estate->readonly_func)
          {
              CommandCounterIncrement();
!             ActiveSnapshot = CopySnapshot(GetTransactionSnapshot(false));
          }

          /*