suppress_redundant_updates_trigger function.

doc/src/sgml/func.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.452 2008/11/03 17:51:12 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.453 2008/11/03 20:17:20 adunstan Exp $ -->
 
  <chapter id="functions">
   <title>Functions and Operators</title>
@@ -12846,4 +12846,55 @@ SELECT (pg_stat_file('filename')).modification;
 
   </sect1>
 
+  <sect1 id="functions-trigger">
+   <title>Trigger Functions</title>
+
+   <indexterm>
+     <primary>suppress_redundant_updates_trigger</primary>
+   </indexterm>
+
+   <para>
+      Currently <productname>PostgreSQL</> provides one built in trigger
+	  function, <function>suppress_redundant_updates_trigger</>, 
+	  which will prevent any update
+	  that does not actually change the data in the row from taking place, in
+	  contrast to the normal behaviour which always performs the update
+	  regardless of whether or not the data has changed. (This normal behaviour
+	  makes updates run faster, since no checking is required, and is also
+	  useful in certain cases.)
+    </para>
+
+	<para>
+	  Ideally, you should normally avoid running updates that don't actually
+	  change the data in the record. Redundant updates can cost considerable
+	  unnecessary time, especially if there are lots of indexes to alter,
+	  and space in dead rows that will eventually have to be vacuumed.
+	  However, detecting such situations in client code is not
+	  always easy, or even possible, and writing expressions to detect
+	  them can be error-prone. An alternative is to use 
+	  <function>suppress_redundant_updates_trigger</>, which will skip
+	  updates that don't change the data. You should use this with care,
+	  however. The trigger takes a small but non-trivial time for each record, 
+	  so if most of the records affected by an update are actually changed,
+	  use of this trigger will actually make the update run slower.
+    </para>
+
+    <para>
+      The <function>suppress_redundant_updates_trigger</> function can be 
+	  added to a table like this:
+<programlisting>
+CREATE TRIGGER z_min_update 
+BEFORE UPDATE ON tablename
+FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();
+</programlisting>
+      In most cases, you would want to fire this trigger last for each row.
+	  Bearing in mind that triggers fire in name order, you would then
+	  choose a trigger name that comes after the name of any other trigger
+      you might have on the table.
+    </para>
+	<para>
+       For more information about creating triggers, see
+	    <xref linkend="SQL-CREATETRIGGER">.
+    </para>
+  </sect1>
 </chapter>

src/backend/utils/adt/Makefile

 #
 # Makefile for utils/adt
 #
-# $PostgreSQL: pgsql/src/backend/utils/adt/Makefile,v 1.69 2008/02/19 10:30:08 petere Exp $
+# $PostgreSQL: pgsql/src/backend/utils/adt/Makefile,v 1.70 2008/11/03 20:17:20 adunstan Exp $
 #
 
 subdir = src/backend/utils/adt
@@ -25,7 +25,7 @@ OBJS = acl.o arrayfuncs.o array_userfuncs.o arrayutils.o bool.o \
 	tid.o timestamp.o varbit.o varchar.o varlena.o version.o xid.o \
 	network.o mac.o inet_net_ntop.o inet_net_pton.o \
 	ri_triggers.o pg_lzcompress.o pg_locale.o formatting.o \
-	ascii.o quote.o pgstatfuncs.o encode.o dbsize.o genfile.o \
+	ascii.o quote.o pgstatfuncs.o encode.o dbsize.o genfile.o trigfuncs.o \
 	tsginidx.o tsgistidx.o tsquery.o tsquery_cleanup.o tsquery_gist.o \
 	tsquery_op.o tsquery_rewrite.o tsquery_util.o tsrank.o \
 	tsvector.o tsvector_op.o tsvector_parser.o \

src/backend/utils/adt/trigfuncs.c

+/*-------------------------------------------------------------------------
+ *
+ * trigfuncs.c
+ *    Builtin functions for useful trigger support.
+ *
+ *
+ * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $PostgreSQL: pgsql/src/backend/utils/adt/trigfuncs.c,v 1.1 2008/11/03 20:17:20 adunstan Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+
+
+
+#include "postgres.h"
+#include "commands/trigger.h"
+#include "access/htup.h"
+
+/*
+ * suppress_redundant_updates_trigger
+ *
+ * This trigger function will inhibit an update from being done
+ * if the OLD and NEW records are identical.
+ *
+ */
+
+Datum
+suppress_redundant_updates_trigger(PG_FUNCTION_ARGS)
+{
+    TriggerData *trigdata = (TriggerData *) fcinfo->context;
+    HeapTuple   newtuple, oldtuple, rettuple;
+	HeapTupleHeader newheader, oldheader;
+
+    /* make sure it's called as a trigger */
+    if (!CALLED_AS_TRIGGER(fcinfo))
+        elog(ERROR, (errcode(ERRCODE_E_R_I_E_TRIGGER_PROTOCOL_VIOLATED),
+					 errmsg("suppress_redundant_updates_trigger: must be called as trigger")));
+	
+    /* and that it's called on update */
+    if (! TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
+        ereport(ERROR, (errcode(ERRCODE_E_R_I_E_TRIGGER_PROTOCOL_VIOLATED),
+						errmsg( "suppress_redundant_updates_trigger: may only be called on update")));
+
+    /* and that it's called before update */
+    if (! TRIGGER_FIRED_BEFORE(trigdata->tg_event))
+        ereport(ERROR, (errcode(ERRCODE_E_R_I_E_TRIGGER_PROTOCOL_VIOLATED),
+						errmsg( "suppress_redundant_updates_trigger: may only be called before update")));
+
+    /* and that it's called for each row */
+    if (! TRIGGER_FIRED_FOR_ROW(trigdata->tg_event))
+        ereport(ERROR, (errcode(ERRCODE_E_R_I_E_TRIGGER_PROTOCOL_VIOLATED),
+						errmsg( "suppress_redundant_updates_trigger: may only be called for each row")));
+
+	/* get tuple data, set default return */
+	rettuple  = newtuple = trigdata->tg_newtuple;
+	oldtuple = trigdata->tg_trigtuple;
+
+	newheader = newtuple->t_data;
+	oldheader = oldtuple->t_data;
+
+    if (newtuple->t_len == oldtuple->t_len &&
+		newheader->t_hoff == oldheader->t_hoff &&
+		(HeapTupleHeaderGetNatts(newheader) == 
+		 HeapTupleHeaderGetNatts(oldheader) ) &&
+		((newheader->t_infomask & ~HEAP_XACT_MASK) == 
+		 (oldheader->t_infomask & ~HEAP_XACT_MASK) )&&
+		memcmp(((char *)newheader) + offsetof(HeapTupleHeaderData, t_bits),
+			   ((char *)oldheader) + offsetof(HeapTupleHeaderData, t_bits),
+			   newtuple->t_len - offsetof(HeapTupleHeaderData, t_bits)) == 0)
+	{
+		rettuple = NULL;
+	}
+	
+    return PointerGetDatum(rettuple);
+}

src/include/catalog/pg_proc.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/catalog/pg_proc.h,v 1.521 2008/11/03 17:51:13 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/catalog/pg_proc.h,v 1.522 2008/11/03 20:17:20 adunstan Exp $
  *
  * NOTES
  *	  The script catalog/genbki.sh reads this file and generates .bki
@@ -1580,6 +1580,9 @@ DESCR("convert int8 to oid");
 DATA(insert OID = 1288 (  int8			   PGNSP PGUID 12 1 0 0 f f t f i 1 20 "26" _null_ _null_ _null_	oidtoi8 _null_ _null_ _null_ ));
 DESCR("convert oid to int8");
 
+DATA(insert OID = 1291 (  suppress_redundant_updates_trigger	PGNSP PGUID 12 1 0 0 f f t f v 0 2279 "" _null_ _null_ _null_ suppress_redundant_updates_trigger _null_ _null_ _null_ ));
+DESCR("trigger to suppress updates when new and old records match");
+
 DATA(insert OID = 1292 ( tideq			   PGNSP PGUID 12 1 0 0 f f t f i 2 16 "27 27" _null_ _null_ _null_ tideq _null_ _null_ _null_ ));
 DESCR("equal");
 DATA(insert OID = 1293 ( currtid		   PGNSP PGUID 12 1 0 0 f f t f v 2 27 "26 27" _null_ _null_ _null_ currtid_byreloid _null_ _null_ _null_ ));
@@ -2289,6 +2292,7 @@ DESCR("result type of a function");
 
 DATA(insert OID = 1686 (  pg_get_keywords		PGNSP PGUID 12 10 400 0 f f t t s 0 2249 "" "{25,18,25}" "{o,o,o}" "{word,catcode,catdesc}" pg_get_keywords _null_ _null_ _null_ ));
 DESCR("list of SQL keywords");
+
 DATA(insert OID = 1619 (  pg_typeof				PGNSP PGUID 12 1 0 0 f f f f i 1 2206  "2276" _null_ _null_ _null_  pg_typeof _null_ _null_ _null_ ));
 DESCR("returns the type of the argument");
 

src/include/utils/builtins.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/builtins.h,v 1.325 2008/11/03 17:51:13 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/utils/builtins.h,v 1.326 2008/11/03 20:17:20 adunstan Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -900,6 +900,9 @@ extern Datum RI_FKey_setnull_upd(PG_FUNCTION_ARGS);
 extern Datum RI_FKey_setdefault_del(PG_FUNCTION_ARGS);
 extern Datum RI_FKey_setdefault_upd(PG_FUNCTION_ARGS);
 
+/* trigfuncs.c */
+extern Datum suppress_redundant_updates_trigger(PG_FUNCTION_ARGS);
+
 /* encoding support functions */
 extern Datum getdatabaseencoding(PG_FUNCTION_ARGS);
 extern Datum database_character_set(PG_FUNCTION_ARGS);

src/test/regress/expected/triggers.out

@@ -537,3 +537,28 @@ NOTICE:  row 1 not changed
 NOTICE:  row 2 not changed
 DROP TABLE trigger_test;
 DROP FUNCTION mytrigger();
+-- minimal update trigger
+CREATE TABLE min_updates_test (
+	f1	text,
+	f2 int,
+	f3 int);
+INSERT INTO min_updates_test VALUES ('a',1,2),('b','2',null);
+CREATE TRIGGER z_min_update 
+BEFORE UPDATE ON min_updates_test
+FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();
+\set QUIET false
+UPDATE min_updates_test SET f1 = f1;
+UPDATE 0
+UPDATE min_updates_test SET f2 = f2 + 1;
+UPDATE 2
+UPDATE min_updates_test SET f3 = 2 WHERE f3 is null;
+UPDATE 1
+\set QUIET true
+SELECT * FROM min_updates_test;
+ f1 | f2 | f3 
+----+----+----
+ a  |  2 |  2
+ b  |  3 |  2
+(2 rows)
+
+DROP TABLE min_updates_test;

src/test/regress/sql/triggers.sql

@@ -415,3 +415,32 @@ UPDATE trigger_test SET f3 = NULL;
 DROP TABLE trigger_test;
 
 DROP FUNCTION mytrigger();
+
+
+-- minimal update trigger
+
+CREATE TABLE min_updates_test (
+	f1	text,
+	f2 int,
+	f3 int);
+
+INSERT INTO min_updates_test VALUES ('a',1,2),('b','2',null);
+
+CREATE TRIGGER z_min_update 
+BEFORE UPDATE ON min_updates_test
+FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();
+
+\set QUIET false
+
+UPDATE min_updates_test SET f1 = f1;
+
+UPDATE min_updates_test SET f2 = f2 + 1;
+
+UPDATE min_updates_test SET f3 = 2 WHERE f3 is null;
+
+\set QUIET true
+
+SELECT * FROM min_updates_test;
+
+DROP TABLE min_updates_test;
+