Add txid_current_ifassigned().

Add a variant of txid_current() that returns NULL if no transaction ID
is assigned.  This version can be used even on a standby server,
although it will always return NULL since no transaction IDs can be
assigned during recovery.

Craig Ringer, per suggestion from Jim Nasby.  Reviewed by Petr Jelinek
and by me.

doc/src/sgml/func.sgml

@@ -17119,6 +17119,10 @@ SELECT collation for ('foo' COLLATE "de_DE");
     <primary>txid_current</primary>
    </indexterm>
 
+   <indexterm>
+    <primary>txid_current_if_assigned</primary>
+   </indexterm>
+
    <indexterm>
     <primary>txid_current_snapshot</primary>
    </indexterm>
@@ -17159,6 +17163,11 @@ SELECT collation for ('foo' COLLATE "de_DE");
        <entry><type>bigint</type></entry>
        <entry>get current transaction ID, assigning a new one if the current transaction does not have one</entry>
       </row>
+      <row>
+       <entry><literal><function>txid_current_if_assigned()</function></literal></entry>
+       <entry><type>bigint</type></entry>
+       <entry>same as <function>txid_current()</function> but returns null instead of assigning an xid if none is already assigned</entry>
+      </row>
       <row>
        <entry><literal><function>txid_current_snapshot()</function></literal></entry>
        <entry><type>txid_snapshot</type></entry>

src/backend/utils/adt/txid.c

@@ -376,6 +376,27 @@ txid_current(PG_FUNCTION_ARGS)
 	PG_RETURN_INT64(val);
 }
 
+/*
+ * Same as txid_current() but doesn't assign a new xid if there isn't one
+ * yet.
+ */
+Datum
+txid_current_if_assigned(PG_FUNCTION_ARGS)
+{
+	txid		val;
+	TxidEpoch	state;
+	TransactionId	topxid = GetTopTransactionIdIfAny();
+
+	if (topxid == InvalidTransactionId)
+		PG_RETURN_NULL();
+
+	load_xid_epoch(&state);
+
+	val = convert_xid(topxid, &state);
+
+	PG_RETURN_INT64(val);
+}
+
 /*
  * txid_current_snapshot() returns txid_snapshot
  *

src/include/catalog/pg_proc.h

@@ -4904,6 +4904,8 @@ DATA(insert OID = 2942 (  txid_snapshot_send		PGNSP PGUID 12 1  0 0 0 f f f f t
 DESCR("I/O");
 DATA(insert OID = 2943 (  txid_current				PGNSP PGUID 12 1  0 0 0 f f f f t f s u 0 0 20 "" _null_ _null_ _null_ _null_ _null_ txid_current _null_ _null_ _null_ ));
 DESCR("get current transaction ID");
+DATA(insert OID = 3348 (  txid_current_if_assigned	PGNSP PGUID 12 1  0 0 0 f f f f t f s u 0 0 20 "" _null_ _null_ _null_ _null_ _null_ txid_current_if_assigned _null_ _null_ _null_ ));
+DESCR("get current transaction ID");
 DATA(insert OID = 2944 (  txid_current_snapshot		PGNSP PGUID 12 1  0 0 0 f f f f t f s s 0 0 2970 "" _null_ _null_ _null_ _null_ _null_ txid_current_snapshot _null_ _null_ _null_ ));
 DESCR("get current snapshot");
 DATA(insert OID = 2945 (  txid_snapshot_xmin		PGNSP PGUID 12 1  0 0 0 f f f f t f i s 1 0 20 "2970" _null_ _null_ _null_ _null_ _null_ txid_snapshot_xmin _null_ _null_ _null_ ));

src/include/utils/builtins.h

@@ -1221,6 +1221,7 @@ extern Datum txid_snapshot_out(PG_FUNCTION_ARGS);
 extern Datum txid_snapshot_recv(PG_FUNCTION_ARGS);
 extern Datum txid_snapshot_send(PG_FUNCTION_ARGS);
 extern Datum txid_current(PG_FUNCTION_ARGS);
+extern Datum txid_current_if_assigned(PG_FUNCTION_ARGS);
 extern Datum txid_current_snapshot(PG_FUNCTION_ARGS);
 extern Datum txid_snapshot_xmin(PG_FUNCTION_ARGS);
 extern Datum txid_snapshot_xmax(PG_FUNCTION_ARGS);

src/test/regress/expected/txid.out

@@ -238,3 +238,19 @@ SELECT txid_snapshot '1:9223372036854775808:3';
 ERROR:  invalid input syntax for type txid_snapshot: "1:9223372036854775808:3"
 LINE 1: SELECT txid_snapshot '1:9223372036854775808:3';
                              ^
+-- test txid_current_if_assigned
+BEGIN;
+SELECT txid_current_if_assigned() IS NULL;
+ ?column? 
+----------
+ t
+(1 row)
+
+SELECT txid_current() \gset
+SELECT txid_current_if_assigned() IS NOT DISTINCT FROM BIGINT :'txid_current';
+ ?column? 
+----------
+ t
+(1 row)
+
+COMMIT;

src/test/regress/sql/txid.sql

@@ -52,3 +52,10 @@ select txid_visible_in_snapshot('1000100010001015', '1000100010001000:1000100010
 -- test 64bit overflow
 SELECT txid_snapshot '1:9223372036854775807:3';
 SELECT txid_snapshot '1:9223372036854775808:3';
+
+-- test txid_current_if_assigned
+BEGIN;
+SELECT txid_current_if_assigned() IS NULL;
+SELECT txid_current() \gset
+SELECT txid_current_if_assigned() IS NOT DISTINCT FROM BIGINT :'txid_current';
+COMMIT;