If a field is incompressible ('compressed' data is actually larger than

source, due to addition of header overhead), store it as plain data
rather than pseudo-compressed data.  This saves a few microseconds when
reading it out, but much more importantly guarantees that the toaster
won't actually expand tuples that contain incompressible data.  That's
essential to avoid 'Tuple too big' failures with large objects.

src/backend/access/heap/tuptoaster.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/access/heap/tuptoaster.c,v 1.12 2000/08/04 04:16:07 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/access/heap/tuptoaster.c,v 1.13 2000/10/23 23:42:04 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -247,6 +247,11 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup)
 
 	/* ----------
 	 * Then collect information about the values given
+	 *
+	 * NOTE: toast_action[i] can have these values:
+	 *		' '		default handling
+	 *		'p'		already processed --- don't touch it
+	 *		'x'		incompressible, but OK to move off
 	 * ----------
 	 */
 	memset(toast_action,    ' ', numAttrs * sizeof(char));
@@ -397,6 +402,7 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup)
 		int		biggest_attno = -1;
 		int32	biggest_size  = MAXALIGN(sizeof(varattrib));
 		Datum	old_value;
+		Datum	new_value;
 
 		/* ----------
 		 * Search for the biggest yet uncompressed internal attribute
@@ -404,7 +410,7 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup)
 		 */
 		for (i = 0; i < numAttrs; i++)
 		{
-			if (toast_action[i] == 'p')
+			if (toast_action[i] != ' ')
 				continue;
 			if (VARATT_IS_EXTENDED(toast_values[i]))
 				continue;
@@ -421,21 +427,30 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup)
 			break;
 
 		/* ----------
-		 * Compress it inline
+		 * Attempt to compress it inline
 		 * ----------
 		 */
 		i					= biggest_attno;
 		old_value			= toast_values[i];
+		new_value			= toast_compress_datum(old_value);
 
-		toast_values[i]		= toast_compress_datum(toast_values[i]);
+		if (DatumGetPointer(new_value) != NULL)
+		{
+			/* successful compression */
 			if (toast_free[i])
 				pfree(DatumGetPointer(old_value));
+			toast_values[i]	= new_value;
 			toast_free[i]	= true;
 			toast_sizes[i]	= VARATT_SIZE(toast_values[i]);
-
 			need_change		= true;
 			need_free		= true;
 		}
+		else
+		{
+			/* incompressible data, ignore on subsequent compression passes */
+			toast_action[i] = 'x';
+		}
+	}
 
 	/* ----------
 	 * Second we look for attributes of attstorage 'x' or 'e' that
@@ -504,6 +519,7 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup)
 		int		biggest_attno = -1;
 		int32	biggest_size  = MAXALIGN(sizeof(varattrib));
 		Datum	old_value;
+		Datum	new_value;
 
 		/* ----------
 		 * Search for the biggest yet uncompressed internal attribute
@@ -511,7 +527,7 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup)
 		 */
 		for (i = 0; i < numAttrs; i++)
 		{
-			if (toast_action[i] == 'p')
+			if (toast_action[i] != ' ')
 				continue;
 			if (VARATT_IS_EXTENDED(toast_values[i]))
 				continue;
@@ -528,23 +544,30 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup)
 			break;
 
 		/* ----------
-		 * Compress it inline
+		 * Attempt to compress it inline
 		 * ----------
 		 */
 		i					= biggest_attno;
 		old_value			= toast_values[i];
+		new_value			= toast_compress_datum(old_value);
 
-		toast_values[i]		= toast_compress_datum(toast_values[i]);
+		if (DatumGetPointer(new_value) != NULL)
-									
+		{
+			/* successful compression */
 			if (toast_free[i])
 				pfree(DatumGetPointer(old_value));
-
+			toast_values[i]	= new_value;
 			toast_free[i]	= true;
 			toast_sizes[i]	= VARATT_SIZE(toast_values[i]);
-
 			need_change		= true;
 			need_free		= true;
 		}
+		else
+		{
+			/* incompressible data, ignore on subsequent compression passes */
+			toast_action[i] = 'x';
+		}
+	}
 
 	/* ----------
 	 * Finally we store attributes of type 'm' external
@@ -690,6 +713,10 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup)
  * toast_compress_datum -
  *
  *	Create a compressed version of a varlena datum
+ *
+ *	If we fail (ie, compressed result is actually bigger than original)
+ *	then return NULL.  We must not use compressed data if it'd expand
+ *	the tuple!
  * ----------
  */
 static Datum
@@ -701,10 +728,19 @@ toast_compress_datum(Datum value)
 	pglz_compress(VARATT_DATA(value), VARATT_SIZE(value) - VARHDRSZ,
 				  (PGLZ_Header *) tmp,
 				  PGLZ_strategy_default);
+	if (VARATT_SIZE(tmp) < VARATT_SIZE(value))
+	{
+		/* successful compression */
 		VARATT_SIZEP(tmp) |= VARATT_FLAG_COMPRESSED;
-
 		return PointerGetDatum(tmp);
 	}
+	else
+	{
+		/* incompressible data */
+		pfree(tmp);
+		return PointerGetDatum(NULL);
+	}
+}
 
 
 /* ----------