17 files changed, 1030 insertions, 34 deletions

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 45b99566973..8dd69337f11 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -13803,6 +13803,14 @@ SELECT typlen FROM pg_type WHERE oid = pg_typeof(33);
    <title>System Administration Functions</title>
 
    <para>
+    The functions described in this section are used to control and
+    monitor a <productname>PostgreSQL</> installation.
+   </para>
+
+  <sect2 id="functions-admin-set">
+   <title>Configuration Settings Functions</title>
+
+   <para>
     <xref linkend="functions-admin-set-table"> shows the functions
     available to query and alter run-time configuration parameters.
    </para>
@@ -13889,6 +13897,11 @@ SELECT set_config('log_statement_stats', 'off', false);
 </programlisting>
    </para>
 
+  </sect2>
+
+  <sect2 id="functions-admin-signal">
+   <title>Server Signalling Functions</title>
+
    <indexterm>
     <primary>pg_cancel_backend</primary>
    </indexterm>
@@ -13985,6 +13998,11 @@ SELECT set_config('log_statement_stats', 'off', false);
     subprocess.
    </para>
 
+  </sect2>
+
+  <sect2 id="functions-admin-backup">
+   <title>Backup Control Functions</title>
+
    <indexterm>
     <primary>backup</primary>
    </indexterm>
@@ -14181,6 +14199,11 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
     <xref linkend="continuous-archiving">.
    </para>
 
+  </sect2>
+
+  <sect2 id="functions-recovery-control">
+   <title>Recovery Control Functions</title>
+
    <indexterm>
     <primary>pg_is_in_recovery</primary>
    </indexterm>
@@ -14198,7 +14221,7 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
     The functions shown in <xref
     linkend="functions-recovery-info-table"> provide information
     about the current status of the standby.
-    These functions may be executed during both recovery and in normal running.
+    These functions may be executed both during recovery and in normal running.
    </para>
 
    <table id="functions-recovery-info-table">
@@ -14333,6 +14356,87 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
     the pause, the rate of WAL generation and available disk space.
    </para>
 
+  </sect2>
+
+  <sect2 id="functions-snapshot-synchronization">
+   <title>Snapshot Synchronization Functions</title>
+
+   <indexterm>
+     <primary>pg_export_snapshot</primary>
+   </indexterm>
+
+   <para>
+    <productname>PostgreSQL</> allows database sessions to synchronize their
+    snapshots. A <firstterm>snapshot</> determines which data is visible to the
+    transaction that is using the snapshot. Synchronized snapshots are
+    necessary when two or more sessions need to see identical content in the
+    database. If two sessions just start their transactions independently,
+    there is always a possibility that some third transaction commits
+    between the executions of the two <command>START TRANSACTION</> commands,
+    so that one session sees the effects of that transaction and the other
+    does not.
+   </para>
+
+   <para>
+    To solve this problem, <productname>PostgreSQL</> allows a transaction to
+    <firstterm>export</> the snapshot it is using.  As long as the exporting
+    transaction remains open, other transactions can <firstterm>import</> its
+    snapshot, and thereby be guaranteed that they see exactly the same view
+    of the database that the first transaction sees.  But note that any
+    database changes made by any one of these transactions remain invisible
+    to the other transactions, as is usual for changes made by uncommitted
+    transactions.  So the transactions are synchronized with respect to
+    pre-existing data, but act normally for changes they make themselves.
+   </para>
+
+   <para>
+    Snapshots are exported with the <function>pg_export_snapshot</> function,
+    shown in <xref linkend="functions-snapshot-synchronization-table">, and
+    imported with the <xref linkend="sql-set-transaction"> command.
+   </para>
+
+   <table id="functions-snapshot-synchronization-table">
+    <title>Snapshot Synchronization Functions</title>
+    <tgroup cols="3">
+     <thead>
+      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
+      </row>
+     </thead>
+
+     <tbody>
+      <row>
+       <entry>
+        <literal><function>pg_export_snapshot()</function></literal>
+       </entry>
+       <entry><type>text</type></entry>
+       <entry>Save the current snapshot and return its identifier</entry>
+      </row>
+     </tbody>
+    </tgroup>
+   </table>
+
+   <para>
+    The function <function>pg_export_snapshot</> saves the current snapshot
+    and returns a <type>text</> string identifying the snapshot.  This string
+    must be passed (outside the database) to clients that want to import the
+    snapshot.  The snapshot is available for import only until the end of the
+    transaction that exported it.  A transaction can export more than one
+    snapshot, if needed.  Note that doing so is only useful in <literal>READ
+    COMMITTED</> transactions, since in <literal>REPEATABLE READ</> and
+    higher isolation levels, transactions use the same snapshot throughout
+    their lifetime.  Once a transaction has exported any snapshots, it cannot
+    be prepared with <xref linkend="sql-prepare-transaction">.
+   </para>
+
+   <para>
+    See  <xref linkend="sql-set-transaction"> for details of how to use an
+    exported snapshot.
+   </para>
+  </sect2>
+
+  <sect2 id="functions-admin-dbobject">
+   <title>Database Object Management Functions</title>
+
    <para>
     The functions shown in <xref linkend="functions-admin-dbsize"> calculate
     the disk space usage of database objects.
@@ -14591,9 +14695,14 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
     the relation.
    </para>
 
+  </sect2>
+
+  <sect2 id="functions-admin-genfile">
+   <title>Generic File Access Functions</title>
+
    <para>
     The functions shown in <xref
-    linkend="functions-admin-genfile"> provide native access to
+    linkend="functions-admin-genfile-table"> provide native access to
     files on the machine hosting the server. Only files within the
     database cluster directory and the <varname>log_directory</> can be
     accessed.  Use a relative path for files in the cluster directory,
@@ -14601,7 +14710,7 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
     for log files.  Use of these functions is restricted to superusers.
    </para>
 
-   <table id="functions-admin-genfile">
+   <table id="functions-admin-genfile-table">
     <title>Generic File Access Functions</title>
     <tgroup cols="3">
      <thead>
@@ -14694,13 +14803,18 @@ SELECT (pg_stat_file('filename')).modification;
 </programlisting>
    </para>
 
+  </sect2>
+
+  <sect2 id="functions-advisory-locks">
+   <title>Advisory Lock Functions</title>
+
    <para>
-    The functions shown in <xref linkend="functions-advisory-locks"> manage
-    advisory locks.  For details about proper use of these functions, see
-    <xref linkend="advisory-locks">.
+    The functions shown in <xref linkend="functions-advisory-locks-table">
+    manage advisory locks.  For details about proper use of these functions,
+    see <xref linkend="advisory-locks">.
    </para>
 
-   <table id="functions-advisory-locks">
+   <table id="functions-advisory-locks-table">
     <title>Advisory Lock Functions</title>
     <tgroup cols="3">
      <thead>
@@ -14972,6 +15086,8 @@ SELECT (pg_stat_file('filename')).modification;
     at session end, even if the client disconnects ungracefully.)
    </para>
 
+  </sect2>
+
   </sect1>
 
   <sect1 id="functions-trigger">
diff --git a/doc/src/sgml/ref/set_transaction.sgml b/doc/src/sgml/ref/set_transaction.sgml
index e28a7e1cde2..4327ca51a69 100644
--- a/doc/src/sgml/ref/set_transaction.sgml
+++ b/doc/src/sgml/ref/set_transaction.sgml
@@ -33,6 +33,7 @@
  <refsynopsisdiv>
 <synopsis>
 SET TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]
+SET TRANSACTION SNAPSHOT <replaceable class="parameter">snapshot_id</replaceable>
 SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]
 
 <phrase>where <replaceable class="parameter">transaction_mode</replaceable> is one of:</phrase>
@@ -60,6 +61,8 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
    The available transaction characteristics are the transaction
    isolation level, the transaction access mode (read/write or
    read-only), and the deferrable mode.
+   In addition, a snapshot can be selected, though only for the current
+   transaction, not as a session default.
   </para>
 
   <para>
@@ -98,7 +101,7 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
        serializable transactions would create a situation which could not
        have occurred for any serial (one-at-a-time) execution of those
        transactions, one of them will be rolled back with a
-       <literal>serialization_failure</literal> <literal>SQLSTATE</literal>.
+       <literal>serialization_failure</literal> error.
       </para>
      </listitem>
     </varlistentry>
@@ -139,13 +142,41 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
   <para>
    The <literal>DEFERRABLE</literal> transaction property has no effect
    unless the transaction is also <literal>SERIALIZABLE</literal> and
-   <literal>READ ONLY</literal>.  When all of these properties are set on a
+   <literal>READ ONLY</literal>.  When all three of these properties are
+   selected for a
    transaction, the transaction may block when first acquiring its snapshot,
    after which it is able to run without the normal overhead of a
    <literal>SERIALIZABLE</literal> transaction and without any risk of
    contributing to or being canceled by a serialization failure.  This mode
    is well suited for long-running reports or backups.
   </para>
+
+  <para>
+   The <literal>SET TRANSACTION SNAPSHOT</literal> command allows a new
+   transaction to run with the same <firstterm>snapshot</> as an existing
+   transaction.  The pre-existing transaction must have exported its snapshot
+   with the <literal>pg_export_snapshot</literal> function (see <xref
+   linkend="functions-snapshot-synchronization">).  That function returns a
+   snapshot identifier, which must be given to <literal>SET TRANSACTION
+   SNAPSHOT</literal> to specify which snapshot is to be imported.  The
+   identifier must be written as a string literal in this command, for example
+   <literal>'000003A1-1'</>.
+   <literal>SET TRANSACTION SNAPSHOT</literal> can only be executed at the
+   start of a transaction, before the first query or
+   data-modification statement (<command>SELECT</command>,
+   <command>INSERT</command>, <command>DELETE</command>,
+   <command>UPDATE</command>, <command>FETCH</command>, or
+   <command>COPY</command>) of the transaction.  Furthermore, the transaction
+   must already be set to <literal>SERIALIZABLE</literal> or
+   <literal>REPEATABLE READ</literal> isolation level (otherwise, the snapshot
+   would be discarded immediately, since <literal>READ COMMITTED</> mode takes
+   a new snapshot for each command).  If the importing transaction uses
+   <literal>SERIALIZABLE</literal> isolation level, then the transaction that
+   exported the snapshot must also use that isolation level.  Also, a
+   non-read-only serializable transaction cannot import a snapshot from a
+   read-only transaction.
+  </para>
+
  </refsect1>
 
  <refsect1>
@@ -163,6 +194,8 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
    by instead specifying the desired <replaceable
    class="parameter">transaction_modes</replaceable> in
    <command>BEGIN</command> or <command>START TRANSACTION</command>.
+   But that option is not available for <command>SET TRANSACTION
+   SNAPSHOT</command>.
   </para>
 
   <para>
@@ -178,11 +211,45 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
   </para>
  </refsect1>
 
+ <refsect1>
+  <title>Examples</title>
+
+  <para>
+   To begin a new transaction with the same snapshot as an already
+   existing transaction, first export the snapshot from the existing
+   transaction. That will return the snapshot identifier, for example:
+
+<programlisting>
+BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+SELECT pg_export_snapshot();
+ pg_export_snapshot
+--------------------
+ 000003A1-1
+(1 row)
+</programlisting>
+
+   Then give the snapshot identifier in a <command>SET TRANSACTION
+   SNAPSHOT</command> command at the beginning of the newly opened
+   transaction:
+
+<programlisting>
+BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+SET TRANSACTION SNAPSHOT '000003A1-1';
+</programlisting>
+  </para>
+ </refsect1>
+
  <refsect1 id="R1-SQL-SET-TRANSACTION-3">
   <title>Compatibility</title>
 
   <para>
-   Both commands are defined in the <acronym>SQL</acronym> standard.
+   These commands are defined in the <acronym>SQL</acronym> standard,
+   except for the <literal>DEFERRABLE</literal> transaction mode
+   and the <command>SET TRANSACTION SNAPSHOT</> form, which are
+   <productname>PostgreSQL</productname> extensions.
+  </para>
+
+  <para>
    <literal>SERIALIZABLE</literal> is the default transaction
    isolation level in the standard.  In
    <productname>PostgreSQL</productname> the default is ordinarily
@@ -198,12 +265,6 @@ SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transa
   </para>
 
   <para>
-   The <literal>DEFERRABLE</literal>
-   <replaceable class="parameter">transaction_mode</replaceable>
-   is a <productname>PostgreSQL</productname> language extension.
-  </para>
-
-  <para>
    The SQL standard requires commas between successive <replaceable
    class="parameter">transaction_modes</replaceable>, but for historical
    reasons <productname>PostgreSQL</productname> allows the commas to be
diff --git a/doc/src/sgml/storage.sgml b/doc/src/sgml/storage.sgml
index 0a133bb7c7e..cb2f60e1eee 100644
--- a/doc/src/sgml/storage.sgml
+++ b/doc/src/sgml/storage.sgml
@@ -88,6 +88,11 @@ Item
 </row>
 
 <row>
+ <entry><filename>pg_snapshots</></entry>
+ <entry>Subdirectory containing exported snapshots</entry>
+</row>
+
+<row>
  <entry><filename>pg_stat_tmp</></entry>
  <entry>Subdirectory containing temporary files for the statistics
   subsystem</entry>
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 3dab45c2da6..c151d3be191 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -2067,6 +2067,16 @@ PrepareTransaction(void)
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("cannot PREPARE a transaction that has operated on temporary tables")));
 
+	/*
+	 * Likewise, don't allow PREPARE after pg_export_snapshot.  This could be
+	 * supported if we added cleanup logic to twophase.c, but for now it
+	 * doesn't seem worth the trouble.
+	 */
+	if (XactHasExportedSnapshots())
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot PREPARE a transaction that has exported snapshots")));
+
 	/* Prevent cancel/die interrupt while cleaning up */
 	HOLD_INTERRUPTS();
 
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 1c17348472e..5fec88691a3 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -58,6 +58,7 @@
 #include "utils/guc.h"
 #include "utils/ps_status.h"
 #include "utils/relmapper.h"
+#include "utils/snapmgr.h"
 #include "utils/timestamp.h"
 #include "pg_trace.h"
 
@@ -6382,6 +6383,12 @@ StartupXLOG(void)
 		ResetUnloggedRelations(UNLOGGED_RELATION_CLEANUP);
 
 		/*
+		 * Likewise, delete any saved transaction snapshot files that got
+		 * left behind by crashed backends.
+		 */
+		DeleteAllExportedSnapshotFiles();
+
+		/*
 		 * Initialize for Hot Standby, if enabled. We won't let backends in
 		 * yet, not until we've reached the min recovery point specified in
 		 * control file and we've established a recovery snapshot from a
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index e9f3896badb..e2edcde024e 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -553,8 +553,8 @@ static void processCASbits(int cas_bits, int location, const char *constrType,
 
 	SAVEPOINT SCHEMA SCROLL SEARCH SECOND_P SECURITY SELECT SEQUENCE SEQUENCES
 	SERIALIZABLE SERVER SESSION SESSION_USER SET SETOF SHARE
-	SHOW SIMILAR SIMPLE SMALLINT SOME STABLE STANDALONE_P START STATEMENT
-	STATISTICS STDIN STDOUT STORAGE STRICT_P STRIP_P SUBSTRING
+	SHOW SIMILAR SIMPLE SMALLINT SNAPSHOT SOME STABLE STANDALONE_P START
+	STATEMENT STATISTICS STDIN STDOUT STORAGE STRICT_P STRIP_P SUBSTRING
 	SYMMETRIC SYSID SYSTEM_P
 
 	TABLE TABLES TABLESPACE TEMP TEMPLATE TEMPORARY TEXT_P THEN TIME TIMESTAMP
@@ -1352,6 +1352,15 @@ set_rest:	/* Generic SET syntaxes: */
 					n->args = list_make1(makeStringConst($3 == XMLOPTION_DOCUMENT ? "DOCUMENT" : "CONTENT", @3));
 					$$ = n;
 				}
+			/* Special syntaxes invented by PostgreSQL: */
+			| TRANSACTION SNAPSHOT Sconst
+				{
+					VariableSetStmt *n = makeNode(VariableSetStmt);
+					n->kind = VAR_SET_MULTI;
+					n->name = "TRANSACTION SNAPSHOT";
+					n->args = list_make1(makeStringConst($3, @3));
+					$$ = n;
+				}
 		;
 
 var_name:	ColId								{ $$ = $1; }
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 7d44a34d025..a8ff54037c4 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -1123,6 +1123,28 @@ GetOldestXmin(bool allDbs, bool ignoreVacuum)
 }
 
 /*
+ * GetMaxSnapshotXidCount -- get max size for snapshot XID array
+ *
+ * We have to export this for use by snapmgr.c.
+ */
+int
+GetMaxSnapshotXidCount(void)
+{
+	return procArray->maxProcs;
+}
+
+/*
+ * GetMaxSnapshotSubxidCount -- get max size for snapshot sub-XID array
+ *
+ * We have to export this for use by snapmgr.c.
+ */
+int
+GetMaxSnapshotSubxidCount(void)
+{
+	return TOTAL_MAX_CACHED_SUBXIDS;
+}
+
+/*
  * GetSnapshotData -- returns information about running transactions.
  *
  * The returned snapshot includes xmin (lowest still-running xact ID),
@@ -1187,14 +1209,14 @@ GetSnapshotData(Snapshot snapshot)
 		 * we are in recovery, see later comments.
 		 */
 		snapshot->xip = (TransactionId *)
-			malloc(arrayP->maxProcs * sizeof(TransactionId));
+			malloc(GetMaxSnapshotXidCount() * sizeof(TransactionId));
 		if (snapshot->xip == NULL)
 			ereport(ERROR,
 					(errcode(ERRCODE_OUT_OF_MEMORY),
 					 errmsg("out of memory")));
 		Assert(snapshot->subxip == NULL);
 		snapshot->subxip = (TransactionId *)
-			malloc(TOTAL_MAX_CACHED_SUBXIDS * sizeof(TransactionId));
+			malloc(GetMaxSnapshotSubxidCount() * sizeof(TransactionId));
 		if (snapshot->subxip == NULL)
 			ereport(ERROR,
 					(errcode(ERRCODE_OUT_OF_MEMORY),
@@ -1377,6 +1399,77 @@ GetSnapshotData(Snapshot snapshot)
 }
 
 /*
+ * ProcArrayInstallImportedXmin -- install imported xmin into MyProc->xmin
+ *
+ * This is called when installing a snapshot imported from another
+ * transaction.  To ensure that OldestXmin doesn't go backwards, we must
+ * check that the source transaction is still running, and we'd better do
+ * that atomically with installing the new xmin.
+ *
+ * Returns TRUE if successful, FALSE if source xact is no longer running.
+ */
+bool
+ProcArrayInstallImportedXmin(TransactionId xmin, TransactionId sourcexid)
+{
+	bool		result = false;
+	ProcArrayStruct *arrayP = procArray;
+	int			index;
+
+	Assert(TransactionIdIsNormal(xmin));
+	if (!TransactionIdIsNormal(sourcexid))
+		return false;
+
+	/* Get lock so source xact can't end while we're doing this */
+	LWLockAcquire(ProcArrayLock, LW_SHARED);
+
+	for (index = 0; index < arrayP->numProcs; index++)
+	{
+		volatile PGPROC *proc = arrayP->procs[index];
+		TransactionId xid;
+
+		/* Ignore procs running LAZY VACUUM */
+		if (proc->vacuumFlags & PROC_IN_VACUUM)
+			continue;
+
+		xid = proc->xid;	/* fetch just once */
+		if (xid != sourcexid)
+			continue;
+
+		/*
+		 * We check the transaction's database ID for paranoia's sake: if
+		 * it's in another DB then its xmin does not cover us.  Caller should
+		 * have detected this already, so we just treat any funny cases as
+		 * "transaction not found".
+		 */
+		if (proc->databaseId != MyDatabaseId)
+			continue;
+
+		/*
+		 * Likewise, let's just make real sure its xmin does cover us.
+		 */
+		xid = proc->xmin;	/* fetch just once */
+		if (!TransactionIdIsNormal(xid) ||
+			!TransactionIdPrecedesOrEquals(xid, xmin))
+			continue;
+
+		/*
+		 * We're good.  Install the new xmin.  As in GetSnapshotData, set
+		 * TransactionXmin too.  (Note that because snapmgr.c called
+		 * GetSnapshotData first, we'll be overwriting a valid xmin here,
+		 * so we don't check that.)
+		 */
+		MyProc->xmin = TransactionXmin = xmin;
+
+		result = true;
+		break;
+	}
+
+	LWLockRelease(ProcArrayLock);
+
+	return result;
+}
+
+/*
  * GetRunningTransactionData -- returns information about running transactions.
  *
  * Similar to GetSnapshotData but returns more information. We include
diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index d39f8975f88..345f6f56a69 100644
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -147,6 +147,8 @@
  *
  * predicate lock maintenance
  *		GetSerializableTransactionSnapshot(Snapshot snapshot)
+ *		SetSerializableTransactionSnapshot(Snapshot snapshot,
+ *										   TransactionId sourcexid)
  *		RegisterPredicateLockingXid(void)
  *		PredicateLockRelation(Relation relation, Snapshot snapshot)
  *		PredicateLockPage(Relation relation, BlockNumber blkno,
@@ -417,7 +419,8 @@ static void OldSerXidSetActiveSerXmin(TransactionId xid);
 static uint32 predicatelock_hash(const void *key, Size keysize);
 static void SummarizeOldestCommittedSxact(void);
 static Snapshot GetSafeSnapshot(Snapshot snapshot);
-static Snapshot GetSerializableTransactionSnapshotInt(Snapshot snapshot);
+static Snapshot GetSerializableTransactionSnapshotInt(Snapshot snapshot,
+									  TransactionId sourcexid);
 static bool PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag);
 static bool GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
 						  PREDICATELOCKTARGETTAG *parent);
@@ -1505,7 +1508,8 @@ GetSafeSnapshot(Snapshot origSnapshot)
 		 * our caller passed to us.  The pointer returned is actually the same
 		 * one passed to it, but we avoid assuming that here.
 		 */
-		snapshot = GetSerializableTransactionSnapshotInt(origSnapshot);
+		snapshot = GetSerializableTransactionSnapshotInt(origSnapshot,
+														 InvalidTransactionId);
 
 		if (MySerializableXact == InvalidSerializableXact)
 			return snapshot;	/* no concurrent r/w xacts; it's safe */
@@ -1574,11 +1578,52 @@ GetSerializableTransactionSnapshot(Snapshot snapshot)
 	if (XactReadOnly && XactDeferrable)
 		return GetSafeSnapshot(snapshot);
 
-	return GetSerializableTransactionSnapshotInt(snapshot);
+	return GetSerializableTransactionSnapshotInt(snapshot,
+												 InvalidTransactionId);
 }
 
+/*
+ * Import a snapshot to be used for the current transaction.
+ *
+ * This is nearly the same as GetSerializableTransactionSnapshot, except that
+ * we don't take a new snapshot, but rather use the data we're handed.
+ *
+ * The caller must have verified that the snapshot came from a serializable
+ * transaction; and if we're read-write, the source transaction must not be
+ * read-only.
+ */
+void
+SetSerializableTransactionSnapshot(Snapshot snapshot,
+								   TransactionId sourcexid)
+{
+	Assert(IsolationIsSerializable());
+
+	/*
+	 * We do not allow SERIALIZABLE READ ONLY DEFERRABLE transactions to
+	 * import snapshots, since there's no way to wait for a safe snapshot
+	 * when we're using the snap we're told to.  (XXX instead of throwing
+	 * an error, we could just ignore the XactDeferrable flag?)
+	 */
+	if (XactReadOnly && XactDeferrable)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("a snapshot-importing transaction must not be READ ONLY DEFERRABLE")));
+
+	(void) GetSerializableTransactionSnapshotInt(snapshot, sourcexid);
+}
+
+/*
+ * Guts of GetSerializableTransactionSnapshot
+ *
+ * If sourcexid is valid, this is actually an import operation and we should
+ * skip calling GetSnapshotData, because the snapshot contents are already
+ * loaded up.  HOWEVER: to avoid race conditions, we must check that the
+ * source xact is still running after we acquire SerializableXactHashLock.
+ * We do that by calling ProcArrayInstallImportedXmin.
+ */
 static Snapshot
-GetSerializableTransactionSnapshotInt(Snapshot snapshot)
+GetSerializableTransactionSnapshotInt(Snapshot snapshot,
+									  TransactionId sourcexid)
 {
 	PGPROC	   *proc;
 	VirtualTransactionId vxid;
@@ -1598,6 +1643,14 @@ GetSerializableTransactionSnapshotInt(Snapshot snapshot)
 	/*
 	 * First we get the sxact structure, which may involve looping and access
 	 * to the "finished" list to free a structure for use.
+	 *
+	 * We must hold SerializableXactHashLock when taking/checking the snapshot
+	 * to avoid race conditions, for much the same reasons that
+	 * GetSnapshotData takes the ProcArrayLock.  Since we might have to release
+	 * SerializableXactHashLock to call SummarizeOldestCommittedSxact, this
+	 * means we have to create the sxact first, which is a bit annoying (in
+	 * particular, an elog(ERROR) in procarray.c would cause us to leak the
+	 * sxact).  Consider refactoring to avoid this.
 	 */
 #ifdef TEST_OLDSERXID
 	SummarizeOldestCommittedSxact();
@@ -1615,8 +1668,19 @@ GetSerializableTransactionSnapshotInt(Snapshot snapshot)
 		}
 	} while (!sxact);
 
-	/* Get the snapshot */
-	snapshot = GetSnapshotData(snapshot);
+	/* Get the snapshot, or check that it's safe to use */
+	if (!TransactionIdIsValid(sourcexid))
+		snapshot = GetSnapshotData(snapshot);
+	else if (!ProcArrayInstallImportedXmin(snapshot->xmin, sourcexid))
+	{
+		ReleasePredXact(sxact);
+		LWLockRelease(SerializableXactHashLock);
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("could not import the requested snapshot"),
+				 errdetail("The source transaction %u is not running anymore.",
+						   sourcexid)));
+	}
 
 	/*
 	 * If there are no serializable transactions which are not read-only, we
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index f1d35a9a112..73e60010711 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -72,6 +72,7 @@
 #include "utils/plancache.h"
 #include "utils/portal.h"
 #include "utils/ps_status.h"
+#include "utils/snapmgr.h"
 #include "utils/tzparser.h"
 #include "utils/xml.h"
 
@@ -6093,8 +6094,11 @@ ExecSetVariableStmt(VariableSetStmt *stmt)
 		case VAR_SET_MULTI:
 
 			/*
-			 * Special case for special SQL syntax that effectively sets more
-			 * than one variable per statement.
+			 * Special-case SQL syntaxes.  The TRANSACTION and SESSION
+			 * CHARACTERISTICS cases effectively set more than one variable
+			 * per statement.  TRANSACTION SNAPSHOT only takes one argument,
+			 * but we put it here anyway since it's a special case and not
+			 * related to any GUC variable.
 			 */
 			if (strcmp(stmt->name, "TRANSACTION") == 0)
 			{
@@ -6140,6 +6144,18 @@ ExecSetVariableStmt(VariableSetStmt *stmt)
 							 item->defname);
 				}
 			}
+			else if (strcmp(stmt->name, "TRANSACTION SNAPSHOT") == 0)
+			{
+				A_Const    *con = (A_Const *) linitial(stmt->args);
+
+				if (stmt->is_local)
+					ereport(ERROR,
+							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+							 errmsg("SET LOCAL TRANSACTION SNAPSHOT is not implemented")));
+				Assert(IsA(con, A_Const));
+				Assert(nodeTag(&con->val) == T_String);
+				ImportSnapshot(strVal(&con->val));
+			}
 			else
 				elog(ERROR, "unexpected SET MULTI element: %s",
 					 stmt->name);
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 518aaf1af0c..50fb78057d8 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -15,12 +15,16 @@
  * handle this reference as an internally-tracked registration, so that this
  * module is entirely lower-level than ResourceOwners.
  *
+ * Likewise, any snapshots that have been exported by pg_export_snapshot
+ * have regd_count = 1 and are counted in RegisteredSnapshots, but are not
+ * tracked by any resource owner.
+ *
  * These arrangements let us reset MyProc->xmin when there are no snapshots
  * referenced by this transaction.	(One possible improvement would be to be
  * able to advance Xmin when the snapshot with the earliest Xmin is no longer
  * referenced.	That's a bit harder though, it requires more locking, and
- * anyway it should be rather uncommon to keep snapshots referenced for too
- * long.)
+ * anyway it should be rather uncommon to keep temporary snapshots referenced
+ * for too long.)
  *
  *
  * Portions Copyright (c) 1996-2011, PostgreSQL Global Development Group
@@ -33,12 +37,16 @@
  */
 #include "postgres.h"
 
+#include <sys/stat.h>
+#include <unistd.h>
+
 #include "access/transam.h"
 #include "access/xact.h"
+#include "miscadmin.h"
 #include "storage/predicate.h"
 #include "storage/proc.h"
 #include "storage/procarray.h"
-#include "utils/memutils.h"
+#include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/snapmgr.h"
 #include "utils/tqual.h"
@@ -111,6 +119,15 @@ bool		FirstSnapshotSet = false;
  */
 static Snapshot FirstXactSnapshot = NULL;
 
+/* Define pathname of exported-snapshot files */
+#define SNAPSHOT_EXPORT_DIR "pg_snapshots"
+#define XactExportFilePath(path, xid, num, suffix) \
+	snprintf(path, sizeof(path), SNAPSHOT_EXPORT_DIR "/%08X-%d%s", \
+			 xid, num, suffix)
+
+/* Current xact's exported snapshots (a list of Snapshot structs) */
+static List *exportedSnapshots = NIL;
+
 
 static Snapshot CopySnapshot(Snapshot snapshot);
 static void FreeSnapshot(Snapshot snapshot);
@@ -139,7 +156,8 @@ GetTransactionSnapshot(void)
 		 * In transaction-snapshot mode, the first snapshot must live until
 		 * end of xact regardless of what the caller does with it, so we must
 		 * make a copy of it rather than returning CurrentSnapshotData
-		 * directly.
+		 * directly.  Furthermore, if we're running in serializable mode,
+		 * predicate.c needs to wrap the snapshot fetch in its own processing.
 		 */
 		if (IsolationUsesXactSnapshot())
 		{
@@ -204,6 +222,88 @@ SnapshotSetCommandId(CommandId curcid)
 }
 
 /*
+ * SetTransactionSnapshot
+ *		Set the transaction's snapshot from an imported MVCC snapshot.
+ *
+ * Note that this is very closely tied to GetTransactionSnapshot --- it
+ * must take care of all the same considerations as the first-snapshot case
+ * in GetTransactionSnapshot.
+ */
+static void
+SetTransactionSnapshot(Snapshot sourcesnap, TransactionId sourcexid)
+{
+	/* Caller should have checked this already */
+	Assert(!FirstSnapshotSet);
+
+	Assert(RegisteredSnapshots == 0);
+	Assert(FirstXactSnapshot == NULL);
+
+	/*
+	 * Even though we are not going to use the snapshot it computes, we must
+	 * call GetSnapshotData, for two reasons: (1) to be sure that
+	 * CurrentSnapshotData's XID arrays have been allocated, and (2) to update
+	 * RecentXmin and RecentGlobalXmin.  (We could alternatively include those
+	 * two variables in exported snapshot files, but it seems better to have
+	 * snapshot importers compute reasonably up-to-date values for them.)
+	 */
+	CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
+
+	/*
+	 * Now copy appropriate fields from the source snapshot.
+	 */
+	CurrentSnapshot->xmin = sourcesnap->xmin;
+	CurrentSnapshot->xmax = sourcesnap->xmax;
+	CurrentSnapshot->xcnt = sourcesnap->xcnt;
+	Assert(sourcesnap->xcnt <= GetMaxSnapshotXidCount());
+	memcpy(CurrentSnapshot->xip, sourcesnap->xip,
+		   sourcesnap->xcnt * sizeof(TransactionId));
+	CurrentSnapshot->subxcnt = sourcesnap->subxcnt;
+	Assert(sourcesnap->subxcnt <= GetMaxSnapshotSubxidCount());
+	memcpy(CurrentSnapshot->subxip, sourcesnap->subxip,
+		   sourcesnap->subxcnt * sizeof(TransactionId));
+	CurrentSnapshot->suboverflowed = sourcesnap->suboverflowed;
+	CurrentSnapshot->takenDuringRecovery = sourcesnap->takenDuringRecovery;
+	/* NB: curcid should NOT be copied, it's a local matter */
+
+	/*
+	 * Now we have to fix what GetSnapshotData did with MyProc->xmin and
+	 * TransactionXmin.  There is a race condition: to make sure we are not
+	 * causing the global xmin to go backwards, we have to test that the
+	 * source transaction is still running, and that has to be done atomically.
+	 * So let procarray.c do it.
+	 *
+	 * Note: in serializable mode, predicate.c will do this a second time.
+	 * It doesn't seem worth contorting the logic here to avoid two calls,
+	 * especially since it's not clear that predicate.c *must* do this.
+	 */
+	if (!ProcArrayInstallImportedXmin(CurrentSnapshot->xmin, sourcexid))
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("could not import the requested snapshot"),
+				 errdetail("The source transaction %u is not running anymore.",
+						   sourcexid)));
+
+	/*
+	 * In transaction-snapshot mode, the first snapshot must live until end of
+	 * xact, so we must make a copy of it.  Furthermore, if we're running in
+	 * serializable mode, predicate.c needs to do its own processing.
+	 */
+	if (IsolationUsesXactSnapshot())
+	{
+		if (IsolationIsSerializable())
+			SetSerializableTransactionSnapshot(CurrentSnapshot, sourcexid);
+		/* Make a saved copy */
+		CurrentSnapshot = CopySnapshot(CurrentSnapshot);
+		FirstXactSnapshot = CurrentSnapshot;
+		/* Mark it as "registered" in FirstXactSnapshot */
+		FirstXactSnapshot->regd_count++;
+		RegisteredSnapshots++;
+	}
+
+	FirstSnapshotSet = true;
+}
+
+/*
  * CopySnapshot
  *		Copy the given snapshot.
  *
@@ -558,6 +658,42 @@ AtEOXact_Snapshot(bool isCommit)
 	}
 	FirstXactSnapshot = NULL;
 
+	/*
+	 * If we exported any snapshots, clean them up.
+	 */
+	if (exportedSnapshots != NIL)
+	{
+		TransactionId myxid = GetTopTransactionId();
+		int			i;
+		char		buf[MAXPGPATH];
+
+		/*
+		 * Get rid of the files.  Unlink failure is only a WARNING because
+		 * (1) it's too late to abort the transaction, and (2) leaving a
+		 * leaked file around has little real consequence anyway.
+		 */
+		for (i = 1; i <= list_length(exportedSnapshots); i++)
+		{
+			XactExportFilePath(buf, myxid, i, "");
+			if (unlink(buf))
+				elog(WARNING, "could not unlink file \"%s\": %m", buf);
+		}
+
+		/*
+		 * As with the FirstXactSnapshot, we needn't spend any effort on
+		 * cleaning up the per-snapshot data structures, but we do need to
+		 * adjust the RegisteredSnapshots count to prevent a warning below.
+		 *
+		 * Note: you might be thinking "why do we have the exportedSnapshots
+		 * list at all?  All we need is a counter!".  You're right, but we do
+		 * it this way in case we ever feel like improving xmin management.
+		 */
+		Assert(RegisteredSnapshots >= list_length(exportedSnapshots));
+		RegisteredSnapshots -= list_length(exportedSnapshots);
+
+		exportedSnapshots = NIL;
+	}
+
 	/* On commit, complain about leftover snapshots */
 	if (isCommit)
 	{
@@ -586,3 +722,464 @@ AtEOXact_Snapshot(bool isCommit)
 
 	SnapshotResetXmin();
 }
+
+
+/*
+ * ExportSnapshot
+ *		Export the snapshot to a file so that other backends can import it.
+ *		Returns the token (the file name) that can be used to import this
+ *		snapshot.
+ */
+static char *
+ExportSnapshot(Snapshot snapshot)
+{
+	TransactionId topXid;
+	TransactionId *children;
+	int			nchildren;
+	int			addTopXid;
+	StringInfoData buf;
+	FILE	   *f;
+	int			i;
+	MemoryContext oldcxt;
+	char		path[MAXPGPATH];
+	char		pathtmp[MAXPGPATH];
+
+	/*
+	 * It's tempting to call RequireTransactionChain here, since it's not
+	 * very useful to export a snapshot that will disappear immediately
+	 * afterwards.  However, we haven't got enough information to do that,
+	 * since we don't know if we're at top level or not.  For example, we
+	 * could be inside a plpgsql function that is going to fire off other
+	 * transactions via dblink.  Rather than disallow perfectly legitimate
+	 * usages, don't make a check.
+	 *
+	 * Also note that we don't make any restriction on the transaction's
+	 * isolation level; however, importers must check the level if they
+	 * are serializable.
+	 */
+
+	/*
+	 * This will assign a transaction ID if we do not yet have one.
+	 */
+	topXid = GetTopTransactionId();
+
+	/*
+	 * We cannot export a snapshot from a subtransaction because there's no
+	 * easy way for importers to verify that the same subtransaction is still
+	 * running.
+	 */
+	if (IsSubTransaction())
+		ereport(ERROR,
+				(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
+				 errmsg("cannot export a snapshot from a subtransaction")));
+
+	/*
+	 * We do however allow previous committed subtransactions to exist.
+	 * Importers of the snapshot must see them as still running, so get their
+	 * XIDs to add them to the snapshot.
+	 */
+	nchildren = xactGetCommittedChildren(&children);
+
+	/*
+	 * Copy the snapshot into TopTransactionContext, add it to the
+	 * exportedSnapshots list, and mark it pseudo-registered.  We do this to
+	 * ensure that the snapshot's xmin is honored for the rest of the
+	 * transaction.  (Right now, because SnapshotResetXmin is so stupid, this
+	 * is overkill; but later we might make that routine smarter.)
+	 */
+	snapshot = CopySnapshot(snapshot);
+
+	oldcxt = MemoryContextSwitchTo(TopTransactionContext);
+	exportedSnapshots = lappend(exportedSnapshots, snapshot);
+	MemoryContextSwitchTo(oldcxt);
+
+	snapshot->regd_count++;
+	RegisteredSnapshots++;
+
+	/*
+	 * Fill buf with a text serialization of the snapshot, plus identification
+	 * data about this transaction.  The format expected by ImportSnapshot
+	 * is pretty rigid: each line must be fieldname:value.
+	 */
+	initStringInfo(&buf);
+
+	appendStringInfo(&buf, "xid:%u\n", topXid);
+	appendStringInfo(&buf, "dbid:%u\n", MyDatabaseId);
+	appendStringInfo(&buf, "iso:%d\n", XactIsoLevel);
+	appendStringInfo(&buf, "ro:%d\n", XactReadOnly);
+
+	appendStringInfo(&buf, "xmin:%u\n", snapshot->xmin);
+	appendStringInfo(&buf, "xmax:%u\n", snapshot->xmax);
+
+	/*
+	 * We must include our own top transaction ID in the top-xid data, since
+	 * by definition we will still be running when the importing transaction
+	 * adopts the snapshot, but GetSnapshotData never includes our own XID in
+	 * the snapshot.  (There must, therefore, be enough room to add it.)
+	 *
+	 * However, it could be that our topXid is after the xmax, in which case
+	 * we shouldn't include it because xip[] members are expected to be before
+	 * xmax.  (We need not make the same check for subxip[] members, see
+	 * snapshot.h.)
+	 */
+	addTopXid = TransactionIdPrecedes(topXid, snapshot->xmax) ? 1 : 0;
+	appendStringInfo(&buf, "xcnt:%d\n", snapshot->xcnt + addTopXid);
+	for (i = 0; i < snapshot->xcnt; i++)
+		appendStringInfo(&buf, "xip:%u\n", snapshot->xip[i]);
+	if (addTopXid)
+		appendStringInfo(&buf, "xip:%u\n", topXid);
+
+	/*
+	 * Similarly, we add our subcommitted child XIDs to the subxid data.
+	 * Here, we have to cope with possible overflow.
+	 */
+	if (snapshot->suboverflowed ||
+		snapshot->subxcnt + nchildren > GetMaxSnapshotSubxidCount())
+		appendStringInfoString(&buf, "sof:1\n");
+	else
+	{
+		appendStringInfoString(&buf, "sof:0\n");
+		appendStringInfo(&buf, "sxcnt:%d\n", snapshot->subxcnt + nchildren);
+		for (i = 0; i < snapshot->subxcnt; i++)
+			appendStringInfo(&buf, "sxp:%u\n", snapshot->subxip[i]);
+		for (i = 0; i < nchildren; i++)
+			appendStringInfo(&buf, "sxp:%u\n", children[i]);
+	}
+	appendStringInfo(&buf, "rec:%u\n", snapshot->takenDuringRecovery);
+
+	/*
+	 * Now write the text representation into a file.  We first write to a
+	 * ".tmp" filename, and rename to final filename if no error.  This
+	 * ensures that no other backend can read an incomplete file
+	 * (ImportSnapshot won't allow it because of its valid-characters check).
+	 */
+	XactExportFilePath(pathtmp, topXid, list_length(exportedSnapshots), ".tmp");
+	if (!(f = AllocateFile(pathtmp, PG_BINARY_W)))
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not create file \"%s\": %m", pathtmp)));
+
+	if (fwrite(buf.data, buf.len, 1, f) != 1)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not write to file \"%s\": %m", pathtmp)));
+
+	/* no fsync() since file need not survive a system crash */
+
+	if (FreeFile(f))
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not write to file \"%s\": %m", pathtmp)));
+
+	/*
+	 * Now that we have written everything into a .tmp file, rename the file
+	 * to remove the .tmp suffix.
+	 */
+	XactExportFilePath(path, topXid, list_length(exportedSnapshots), "");
+
+	if (rename(pathtmp, path) < 0)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not rename file \"%s\" to \"%s\": %m",
+						pathtmp, path)));
+
+	/*
+	 * The basename of the file is what we return from pg_export_snapshot().
+	 * It's already in path in a textual format and we know that the path
+	 * starts with SNAPSHOT_EXPORT_DIR.  Skip over the prefix and the slash
+	 * and pstrdup it so as not to return the address of a local variable.
+	 */
+	return pstrdup(path + strlen(SNAPSHOT_EXPORT_DIR) + 1);
+}
+
+/*
+ * pg_export_snapshot
+ *		SQL-callable wrapper for ExportSnapshot.
+ */
+Datum
+pg_export_snapshot(PG_FUNCTION_ARGS)
+{
+	char	   *snapshotName;
+
+	snapshotName = ExportSnapshot(GetActiveSnapshot());
+	PG_RETURN_TEXT_P(cstring_to_text(snapshotName));
+}
+
+
+/*
+ * Parsing subroutines for ImportSnapshot: parse a line with the given
+ * prefix followed by a value, and advance *s to the next line.  The
+ * filename is provided for use in error messages.
+ */
+static int
+parseIntFromText(const char *prefix, char **s, const char *filename)
+{
+	char	   *ptr = *s;
+	int			prefixlen = strlen(prefix);
+	int			val;
+
+	if (strncmp(ptr, prefix, prefixlen) != 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+				 errmsg("invalid snapshot data in file \"%s\"", filename)));
+	ptr += prefixlen;
+	if (sscanf(ptr, "%d", &val) != 1)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+				 errmsg("invalid snapshot data in file \"%s\"", filename)));
+	ptr = strchr(ptr, '\n');
+	if (!ptr)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+				 errmsg("invalid snapshot data in file \"%s\"", filename)));
+	*s = ptr + 1;
+	return val;
+}
+
+static TransactionId
+parseXidFromText(const char *prefix, char **s, const char *filename)
+{
+	char	   *ptr = *s;
+	int			prefixlen = strlen(prefix);
+	TransactionId val;
+
+	if (strncmp(ptr, prefix, prefixlen) != 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+				 errmsg("invalid snapshot data in file \"%s\"", filename)));
+	ptr += prefixlen;
+	if (sscanf(ptr, "%u", &val) != 1)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+				 errmsg("invalid snapshot data in file \"%s\"", filename)));
+	ptr = strchr(ptr, '\n');
+	if (!ptr)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+				 errmsg("invalid snapshot data in file \"%s\"", filename)));
+	*s = ptr + 1;
+	return val;
+}
+
+/*
+ * ImportSnapshot
+ *      Import a previously exported snapshot.  The argument should be a
+ *      filename in SNAPSHOT_EXPORT_DIR.  Load the snapshot from that file.
+ *      This is called by "SET TRANSACTION SNAPSHOT 'foo'".
+ */
+void
+ImportSnapshot(const char *idstr)
+{
+	char		path[MAXPGPATH];
+	FILE	   *f;
+	struct stat	stat_buf;
+	char	   *filebuf;
+	int			xcnt;
+	int			i;
+	TransactionId src_xid;
+	Oid			src_dbid;
+	int			src_isolevel;
+	bool		src_readonly;
+	SnapshotData snapshot;
+
+	/*
+	 * Must be at top level of a fresh transaction.  Note in particular that
+	 * we check we haven't acquired an XID --- if we have, it's conceivable
+	 * that the snapshot would show it as not running, making for very
+	 * screwy behavior.
+	 */
+	if (FirstSnapshotSet ||
+		GetTopTransactionIdIfAny() != InvalidTransactionId ||
+		IsSubTransaction())
+		ereport(ERROR,
+				(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
+				 errmsg("SET TRANSACTION SNAPSHOT must be called before any query")));
+
+	/*
+	 * If we are in read committed mode then the next query would execute
+	 * with a new snapshot thus making this function call quite useless.
+	 */
+	if (!IsolationUsesXactSnapshot())
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ")));
+
+	/*
+	 * Verify the identifier: only 0-9, A-F and hyphens are allowed.  We do
+	 * this mainly to prevent reading arbitrary files.
+	 */
+	if (strspn(idstr, "0123456789ABCDEF-") != strlen(idstr))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("invalid snapshot identifier \"%s\"", idstr)));
+
+	/* OK, read the file */
+	snprintf(path, MAXPGPATH, SNAPSHOT_EXPORT_DIR "/%s", idstr);
+
+	f = AllocateFile(path, PG_BINARY_R);
+	if (!f)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("invalid snapshot identifier \"%s\"", idstr)));
+
+	/* get the size of the file so that we know how much memory we need */
+	if (fstat(fileno(f), &stat_buf))
+		elog(ERROR, "could not stat file \"%s\": %m", path);
+
+	/* and read the file into a palloc'd string */
+	filebuf = (char *) palloc(stat_buf.st_size + 1);
+	if (fread(filebuf, stat_buf.st_size, 1, f) != 1)
+		elog(ERROR, "could not read file \"%s\": %m", path);
+
+	filebuf[stat_buf.st_size] = '\0';
+
+	FreeFile(f);
+
+	/*
+	 * Construct a snapshot struct by parsing the file content.
+	 */
+	memset(&snapshot, 0, sizeof(snapshot));
+
+	src_xid = parseXidFromText("xid:", &filebuf, path);
+	/* we abuse parseXidFromText a bit here ... */
+	src_dbid = parseXidFromText("dbid:", &filebuf, path);
+	src_isolevel = parseIntFromText("iso:", &filebuf, path);
+	src_readonly = parseIntFromText("ro:", &filebuf, path);
+
+	snapshot.xmin = parseXidFromText("xmin:", &filebuf, path);
+	snapshot.xmax = parseXidFromText("xmax:", &filebuf, path);
+
+	snapshot.xcnt = xcnt = parseIntFromText("xcnt:", &filebuf, path);
+
+	/* sanity-check the xid count before palloc */
+	if (xcnt < 0 || xcnt > GetMaxSnapshotXidCount())
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+				 errmsg("invalid snapshot data in file \"%s\"", path)));
+
+	snapshot.xip = (TransactionId *) palloc(xcnt * sizeof(TransactionId));
+	for (i = 0; i < xcnt; i++)
+		snapshot.xip[i] = parseXidFromText("xip:", &filebuf, path);
+
+	snapshot.suboverflowed = parseIntFromText("sof:", &filebuf, path);
+
+	if (!snapshot.suboverflowed)
+	{
+		snapshot.subxcnt = xcnt = parseIntFromText("sxcnt:", &filebuf, path);
+
+		/* sanity-check the xid count before palloc */
+		if (xcnt < 0 || xcnt > GetMaxSnapshotSubxidCount())
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+					 errmsg("invalid snapshot data in file \"%s\"", path)));
+
+		snapshot.subxip = (TransactionId *) palloc(xcnt * sizeof(TransactionId));
+		for (i = 0; i < xcnt; i++)
+			snapshot.subxip[i] = parseXidFromText("sxp:", &filebuf, path);
+	}
+	else
+	{
+		snapshot.subxcnt = 0;
+		snapshot.subxip = NULL;
+	}
+
+	snapshot.takenDuringRecovery = parseIntFromText("rec:", &filebuf, path);
+
+	/*
+	 * Do some additional sanity checking, just to protect ourselves.  We
+	 * don't trouble to check the array elements, just the most critical
+	 * fields.
+	 */
+	if (!TransactionIdIsNormal(src_xid) ||
+		!OidIsValid(src_dbid) ||
+		!TransactionIdIsNormal(snapshot.xmin) ||
+		!TransactionIdIsNormal(snapshot.xmax))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
+				 errmsg("invalid snapshot data in file \"%s\"", path)));
+
+	/*
+	 * If we're serializable, the source transaction must be too, otherwise
+	 * predicate.c has problems (SxactGlobalXmin could go backwards).  Also,
+	 * a non-read-only transaction can't adopt a snapshot from a read-only
+	 * transaction, as predicate.c handles the cases very differently.
+	 */
+	if (IsolationIsSerializable())
+	{
+		if (src_isolevel != XACT_SERIALIZABLE)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("a serializable transaction cannot import a snapshot from a non-serializable transaction")));
+		if (src_readonly && !XactReadOnly)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("a non-read-only serializable transaction cannot import a snapshot from a read-only transaction")));
+	}
+
+	/*
+	 * We cannot import a snapshot that was taken in a different database,
+	 * because vacuum calculates OldestXmin on a per-database basis; so the
+	 * source transaction's xmin doesn't protect us from data loss.  This
+	 * restriction could be removed if the source transaction were to mark
+	 * its xmin as being globally applicable.  But that would require some
+	 * additional syntax, since that has to be known when the snapshot is
+	 * initially taken.  (See pgsql-hackers discussion of 2011-10-21.)
+	 */
+	if (src_dbid != MyDatabaseId)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot import a snapshot from a different database")));
+
+	/* OK, install the snapshot */
+	SetTransactionSnapshot(&snapshot, src_xid);
+}
+
+/*
+ * XactHasExportedSnapshots
+ *		Test whether current transaction has exported any snapshots.
+ */
+bool
+XactHasExportedSnapshots(void)
+{
+	return (exportedSnapshots != NIL);
+}
+
+/*
+ * DeleteAllExportedSnapshotFiles
+ *		Clean up any files that have been left behind by a crashed backend
+ *		that had exported snapshots before it died.
+ *
+ * This should be called during database startup or crash recovery.
+ */
+void
+DeleteAllExportedSnapshotFiles(void)
+{
+	char		buf[MAXPGPATH];
+	DIR		   *s_dir;
+	struct dirent *s_de;
+
+	if (!(s_dir = AllocateDir(SNAPSHOT_EXPORT_DIR)))
+	{
+		/*
+		 * We really should have that directory in a sane cluster setup. But
+		 * then again if we don't, it's not fatal enough to make it FATAL.
+		 * Since we're running in the postmaster, LOG is our best bet.
+		 */
+		elog(LOG, "could not open directory \"%s\": %m", SNAPSHOT_EXPORT_DIR);
+		return;
+	}
+
+	while ((s_de = ReadDir(s_dir, SNAPSHOT_EXPORT_DIR)) != NULL)
+	{
+		if (strcmp(s_de->d_name, ".") == 0 ||
+			strcmp(s_de->d_name, "..") == 0)
+			continue;
+
+		snprintf(buf, MAXPGPATH, SNAPSHOT_EXPORT_DIR "/%s", s_de->d_name);
+		/* Again, unlink failure is not worthy of FATAL */
+		if (unlink(buf))
+			elog(LOG, "could not unlink file \"%s\": %m", buf);
+	}
+
+	FreeDir(s_dir);
+}
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
index e535fdad1e9..29000095cba 100644
--- a/src/bin/initdb/initdb.c
+++ b/src/bin/initdb/initdb.c
@@ -2555,6 +2555,7 @@ main(int argc, char *argv[])
 		"pg_clog",
 		"pg_notify",
 		"pg_serial",
+		"pg_snapshots",
 		"pg_subtrans",
 		"pg_twophase",
 		"pg_multixact/members",
diff --git a/src/include/catalog/catversion.h b/src/include/catalog/catversion.h
index 8097545faaa..c6273c12671 100644
--- a/src/include/catalog/catversion.h
+++ b/src/include/catalog/catversion.h
@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201110161
+#define CATALOG_VERSION_NO	201110221
 
 #endif
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 96f43fe0b1f..64b7a6a314d 100644
--- a/src/include/catalog/pg_proc.h
+++ b/src/include/catalog/pg_proc.h
@@ -2870,6 +2870,9 @@ DESCR("xlog filename and byte offset, given an xlog location");
 DATA(insert OID = 2851 ( pg_xlogfile_name			PGNSP PGUID 12 1 0 0 0 f f f t f i 1 0 25 "25" _null_ _null_ _null_ _null_ pg_xlogfile_name _null_ _null_ _null_ ));
 DESCR("xlog filename, given an xlog location");
 
+DATA(insert OID = 3809 ( pg_export_snapshot		PGNSP PGUID 12 1 0 0 0 f f f t f v 0 0 25 "" _null_ _null_ _null_ _null_ pg_export_snapshot _null_ _null_ _null_ ));
+DESCR("export a snapshot");
+
 DATA(insert OID = 3810 (  pg_is_in_recovery		PGNSP PGUID 12 1 0 0 0 f f f t f v 0 0 16 "" _null_ _null_ _null_ _null_ pg_is_in_recovery _null_ _null_ _null_ ));
 DESCR("true if server is in recovery");
 
diff --git a/src/include/parser/kwlist.h b/src/include/parser/kwlist.h
index 12c2faf3de8..3d170bc3679 100644
--- a/src/include/parser/kwlist.h
+++ b/src/include/parser/kwlist.h
@@ -337,6 +337,7 @@ PG_KEYWORD("show", SHOW, UNRESERVED_KEYWORD)
 PG_KEYWORD("similar", SIMILAR, TYPE_FUNC_NAME_KEYWORD)
 PG_KEYWORD("simple", SIMPLE, UNRESERVED_KEYWORD)
 PG_KEYWORD("smallint", SMALLINT, COL_NAME_KEYWORD)
+PG_KEYWORD("snapshot", SNAPSHOT, UNRESERVED_KEYWORD)
 PG_KEYWORD("some", SOME, RESERVED_KEYWORD)
 PG_KEYWORD("stable", STABLE, UNRESERVED_KEYWORD)
 PG_KEYWORD("standalone", STANDALONE_P, UNRESERVED_KEYWORD)
diff --git a/src/include/storage/predicate.h b/src/include/storage/predicate.h
index 9603b10ad40..7f9d5fc51c1 100644
--- a/src/include/storage/predicate.h
+++ b/src/include/storage/predicate.h
@@ -43,6 +43,8 @@ extern bool PageIsPredicateLocked(Relation relation, BlockNumber blkno);
 
 /* predicate lock maintenance */
 extern Snapshot GetSerializableTransactionSnapshot(Snapshot snapshot);
+extern void SetSerializableTransactionSnapshot(Snapshot snapshot,
+											   TransactionId sourcexid);
 extern void RegisterPredicateLockingXid(TransactionId xid);
 extern void PredicateLockRelation(Relation relation, Snapshot snapshot);
 extern void PredicateLockPage(Relation relation, BlockNumber blkno, Snapshot snapshot);
diff --git a/src/include/storage/procarray.h b/src/include/storage/procarray.h
index a11d4385b7d..71c82437cdf 100644
--- a/src/include/storage/procarray.h
+++ b/src/include/storage/procarray.h
@@ -37,10 +37,16 @@ extern void ExpireTreeKnownAssignedTransactionIds(TransactionId xid,
 extern void ExpireAllKnownAssignedTransactionIds(void);
 extern void ExpireOldKnownAssignedTransactionIds(TransactionId xid);
 
-extern RunningTransactions GetRunningTransactionData(void);
+extern int	GetMaxSnapshotXidCount(void);
+extern int	GetMaxSnapshotSubxidCount(void);
 
 extern Snapshot GetSnapshotData(Snapshot snapshot);
 
+extern bool ProcArrayInstallImportedXmin(TransactionId xmin,
+										 TransactionId sourcexid);
+
+extern RunningTransactions GetRunningTransactionData(void);
+
 extern bool TransactionIdIsInProgress(TransactionId xid);
 extern bool TransactionIdIsActive(TransactionId xid);
 extern TransactionId GetOldestXmin(bool allDbs, bool ignoreVacuum);
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index e665a28aff8..1e5cb866b17 100644
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -42,4 +42,9 @@ extern void AtSubCommit_Snapshot(int level);
 extern void AtSubAbort_Snapshot(int level);
 extern void AtEOXact_Snapshot(bool isCommit);
 
+extern Datum pg_export_snapshot(PG_FUNCTION_ARGS);
+extern void ImportSnapshot(const char *idstr);
+extern bool XactHasExportedSnapshots(void);
+extern void DeleteAllExportedSnapshotFiles(void);
+
 #endif   /* SNAPMGR_H */