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.

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 169a385a9cc9decdfadfde16a86ec95eaea7e397..6355300d9d33e5cc332ef49f7a1415458c63bf12 100644 (file)
--- a/doc/src/sgml/func.sgml
+++ b/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>

diff --git a/src/backend/utils/adt/txid.c b/src/backend/utils/adt/txid.c
index c2069a9923b9df22b361dfbd50c1ea980e0fe458..276075e293c0242b71bf41b8c2c4c5925cbc5a0e 100644 (file)
--- a/src/backend/utils/adt/txid.c
+++ b/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
  *

diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 6fed7a0d19897940292e66971c53695369b1f68e..050a98c39720d2faeb654d1ecf9ca65014c8aedd 100644 (file)
--- a/src/include/catalog/pg_proc.h
+++ b/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_ ));

diff --git a/src/include/utils/builtins.h b/src/include/utils/builtins.h
index 40e25c8824719f3da8ff71173535ab4cb7aba7c2..2ae212a9c3f48130821fe2f5bb1f2d9fdc08c60e 100644 (file)
--- a/src/include/utils/builtins.h
+++ b/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);

diff --git a/src/test/regress/expected/txid.out b/src/test/regress/expected/txid.out
index ddd217eb1024f6ddb595155838ff698d36e4618f..802ccb949f89da3e9531ff57000805e37ac520eb 100644 (file)
--- a/src/test/regress/expected/txid.out
+++ b/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;

diff --git a/src/test/regress/sql/txid.sql b/src/test/regress/sql/txid.sql
index b6650b922e6d4b0df87fde088be491600b386e2f..4aefd9e64d1bb43ceeada58c6a19f623971f4465 100644 (file)
--- a/src/test/regress/sql/txid.sql
+++ b/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;