tableam: Move heap-specific logic from needs_toast_table below tableam.

This allows table AMs to completely suppress TOAST table creation, or
to modify the conditions under which they are created.

Patch by me.  Reviewed by Andres Freund.

Discussion: http://postgr.es/m/CA+Tgmoa4O2n=yphqD2pERUnYmUO84bH1SqMsA-nSxBGsZ7gWfA@mail.gmail.com

src/backend/access/heap/heapam_handler.c

@@ -29,6 +29,7 @@
 #include "access/rewriteheap.h"
 #include "access/tableam.h"
 #include "access/tsmapi.h"
+#include "access/tuptoaster.h"
 #include "access/xact.h"
 #include "catalog/catalog.h"
 #include "catalog/index.h"
@@ -2009,6 +2010,57 @@ heapam_relation_size(Relation rel, ForkNumber forkNumber)
 	return nblocks * BLCKSZ;
 }
 
+/*
+ * Check to see whether the table needs a TOAST table.  It does only if
+ * (1) there are any toastable attributes, and (2) the maximum length
+ * of a tuple could exceed TOAST_TUPLE_THRESHOLD.  (We don't want to
+ * create a toast table for something like "f1 varchar(20)".)
+ */
+static bool
+heapam_relation_needs_toast_table(Relation rel)
+{
+	int32		data_length = 0;
+	bool		maxlength_unknown = false;
+	bool		has_toastable_attrs = false;
+	TupleDesc	tupdesc = rel->rd_att;
+	int32		tuple_length;
+	int			i;
+
+	for (i = 0; i < tupdesc->natts; i++)
+	{
+		Form_pg_attribute att = TupleDescAttr(tupdesc, i);
+
+		if (att->attisdropped)
+			continue;
+		data_length = att_align_nominal(data_length, att->attalign);
+		if (att->attlen > 0)
+		{
+			/* Fixed-length types are never toastable */
+			data_length += att->attlen;
+		}
+		else
+		{
+			int32		maxlen = type_maximum_size(att->atttypid,
+												   att->atttypmod);
+
+			if (maxlen < 0)
+				maxlength_unknown = true;
+			else
+				data_length += maxlen;
+			if (att->attstorage != 'p')
+				has_toastable_attrs = true;
+		}
+	}
+	if (!has_toastable_attrs)
+		return false;			/* nothing to toast? */
+	if (maxlength_unknown)
+		return true;			/* any unlimited-length attrs? */
+	tuple_length = MAXALIGN(SizeofHeapTupleHeader +
+							BITMAPLEN(tupdesc->natts)) +
+		MAXALIGN(data_length);
+	return (tuple_length > TOAST_TUPLE_THRESHOLD);
+}
+
 
 /* ------------------------------------------------------------------------
  * Planner related callbacks for the heap AM
@@ -2592,6 +2644,7 @@ static const TableAmRoutine heapam_methods = {
 	.index_validate_scan = heapam_index_validate_scan,
 
 	.relation_size = heapam_relation_size,
+	.relation_needs_toast_table = heapam_relation_needs_toast_table,
 
 	.relation_estimate_size = heapam_estimate_rel_size,
 

src/backend/catalog/toasting.c

@@ -15,7 +15,6 @@
 #include "postgres.h"
 
 #include "access/heapam.h"
-#include "access/tuptoaster.h"
 #include "access/xact.h"
 #include "catalog/binary_upgrade.h"
 #include "catalog/catalog.h"
@@ -386,21 +385,11 @@ create_toast_table(Relation rel, Oid toastOid, Oid toastIndexOid,
 }
 
 /*
- * Check to see whether the table needs a TOAST table.  It does only if
- * (1) there are any toastable attributes, and (2) the maximum length
- * of a tuple could exceed TOAST_TUPLE_THRESHOLD.  (We don't want to
- * create a toast table for something like "f1 varchar(20)".)
+ * Check to see whether the table needs a TOAST table.
  */
 static bool
 needs_toast_table(Relation rel)
 {
-	int32		data_length = 0;
-	bool		maxlength_unknown = false;
-	bool		has_toastable_attrs = false;
-	TupleDesc	tupdesc;
-	int32		tuple_length;
-	int			i;
-
 	/*
 	 * No need to create a TOAST table for partitioned tables.
 	 */
@@ -423,39 +412,6 @@ needs_toast_table(Relation rel)
 	if (IsCatalogRelation(rel) && !IsBootstrapProcessingMode())
 		return false;
 
-	tupdesc = rel->rd_att;
-
-	for (i = 0; i < tupdesc->natts; i++)
-	{
-		Form_pg_attribute att = TupleDescAttr(tupdesc, i);
-
-		if (att->attisdropped)
-			continue;
-		data_length = att_align_nominal(data_length, att->attalign);
-		if (att->attlen > 0)
-		{
-			/* Fixed-length types are never toastable */
-			data_length += att->attlen;
-		}
-		else
-		{
-			int32		maxlen = type_maximum_size(att->atttypid,
-												   att->atttypmod);
-
-			if (maxlen < 0)
-				maxlength_unknown = true;
-			else
-				data_length += maxlen;
-			if (att->attstorage != 'p')
-				has_toastable_attrs = true;
-		}
-	}
-	if (!has_toastable_attrs)
-		return false;			/* nothing to toast? */
-	if (maxlength_unknown)
-		return true;			/* any unlimited-length attrs? */
-	tuple_length = MAXALIGN(SizeofHeapTupleHeader +
-							BITMAPLEN(tupdesc->natts)) +
-		MAXALIGN(data_length);
-	return (tuple_length > TOAST_TUPLE_THRESHOLD);
+	/* Otherwise, let the AM decide. */
+	return table_relation_needs_toast_table(rel);
 }

src/include/access/tableam.h

@@ -573,6 +573,16 @@ typedef struct TableAmRoutine
 	uint64		(*relation_size) (Relation rel, ForkNumber forkNumber);
 
 
+	/*
+	 * This callback should return true if the relation requires a TOAST table
+	 * and false if it does not.  It may wish to examine the relation's
+	 * tuple descriptor before making a decision, but if it uses some other
+	 * method of storing large values (or if it does not support them) it can
+	 * simply return false.
+	 */
+	bool	    (*relation_needs_toast_table) (Relation rel);
+
+
 	/* ------------------------------------------------------------------------
 	 * Planner related functions.
 	 * ------------------------------------------------------------------------
@@ -1585,6 +1595,16 @@ table_relation_size(Relation rel, ForkNumber forkNumber)
 	return rel->rd_tableam->relation_size(rel, forkNumber);
 }
 
+/*
+ * table_needs_toast_table - does this relation need a toast table?
+ */
+static inline bool
+table_relation_needs_toast_table(Relation rel)
+{
+	return rel->rd_tableam->relation_needs_toast_table(rel);
+}
+
+
 /* ----------------------------------------------------------------------------
  * Planner related functionality
  * ----------------------------------------------------------------------------