Parameter toast_tuple_target controls TOAST for new rows

Specifies the point at which we try to move long column values
into TOAST tables.

No effect on existing rows.

Discussion: https://postgr.es/m/CANP8+jKsVmw6CX6YP9z7zqkTzcKV1+Uzr3XjKcZW=2Ya00OyQQ@mail.gmail.com

Author: Simon Riggs <simon@2ndQudrant.com>
Reviewed-by: Andrew Dunstan <andrew.dunstan@2ndQuadrant.com>

src/backend/access/common/reloptions.c

@@ -23,6 +23,7 @@
 #include "access/nbtree.h"
 #include "access/reloptions.h"
 #include "access/spgist.h"
+#include "access/tuptoaster.h"
 #include "catalog/pg_type.h"
 #include "commands/defrem.h"
 #include "commands/tablespace.h"
@@ -290,6 +291,15 @@ static relopt_int intRelOpts[] =
 		},
 		-1, -1, INT_MAX
 	},
+	{
+		{
+			"toast_tuple_target",
+			"Sets the target tuple length at which external columns will be toasted",
+			RELOPT_KIND_HEAP,
+			ShareUpdateExclusiveLock
+		},
+		TOAST_TUPLE_TARGET, 128, TOAST_TUPLE_TARGET_MAIN
+	},
 	{
 		{
 			"pages_per_range",
@@ -1344,6 +1354,8 @@ default_reloptions(Datum reloptions, bool validate, relopt_kind kind)
 		offsetof(StdRdOptions, autovacuum) + offsetof(AutoVacOpts, multixact_freeze_table_age)},
 		{"log_autovacuum_min_duration", RELOPT_TYPE_INT,
 		offsetof(StdRdOptions, autovacuum) + offsetof(AutoVacOpts, log_min_duration)},
+		{"toast_tuple_target", RELOPT_TYPE_INT,
+		offsetof(StdRdOptions, toast_tuple_target)},
 		{"autovacuum_vacuum_scale_factor", RELOPT_TYPE_REAL,
 		offsetof(StdRdOptions, autovacuum) + offsetof(AutoVacOpts, vacuum_scale_factor)},
 		{"autovacuum_analyze_scale_factor", RELOPT_TYPE_REAL,

src/backend/access/heap/tuptoaster.c

@@ -727,7 +727,7 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
 		hoff += sizeof(Oid);
 	hoff = MAXALIGN(hoff);
 	/* now convert to a limit on the tuple data size */
-	maxDataLen = TOAST_TUPLE_TARGET - hoff;
+	maxDataLen = RelationGetToastTupleTarget(rel, TOAST_TUPLE_TARGET) - hoff;
 
 	/*
 	 * Look for attributes with attstorage 'x' to compress.  Also find large