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diff --git a/src/include/access/heaptoast.h b/src/include/access/heaptoast.h
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+++ b/src/include/access/heaptoast.h
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+/*-------------------------------------------------------------------------
+ *
+ * heaptoast.h
+ *	  Heap-specific definitions for external and compressed storage
+ *	  of variable size attributes.
+ *
+ * Copyright (c) 2000-2021, PostgreSQL Global Development Group
+ *
+ * src/include/access/heaptoast.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef HEAPTOAST_H
+#define HEAPTOAST_H
+
+#include "access/htup_details.h"
+#include "storage/lockdefs.h"
+#include "utils/relcache.h"
+
+/*
+ * Find the maximum size of a tuple if there are to be N tuples per page.
+ */
+#define MaximumBytesPerTuple(tuplesPerPage) \
+	MAXALIGN_DOWN((BLCKSZ - \
+				   MAXALIGN(SizeOfPageHeaderData + (tuplesPerPage) * sizeof(ItemIdData))) \
+				  / (tuplesPerPage))
+
+/*
+ * These symbols control toaster activation.  If a tuple is larger than
+ * TOAST_TUPLE_THRESHOLD, we will try to toast it down to no more than
+ * TOAST_TUPLE_TARGET bytes through compressing compressible fields and
+ * moving EXTENDED and EXTERNAL data out-of-line.
+ *
+ * The numbers need not be the same, though they currently are.  It doesn't
+ * make sense for TARGET to exceed THRESHOLD, but it could be useful to make
+ * it be smaller.
+ *
+ * Currently we choose both values to match the largest tuple size for which
+ * TOAST_TUPLES_PER_PAGE tuples can fit on a heap page.
+ *
+ * XXX while these can be modified without initdb, some thought needs to be
+ * given to needs_toast_table() in toasting.c before unleashing random
+ * changes.  Also see LOBLKSIZE in large_object.h, which can *not* be
+ * changed without initdb.
+ */
+#define TOAST_TUPLES_PER_PAGE	4
+
+#define TOAST_TUPLE_THRESHOLD	MaximumBytesPerTuple(TOAST_TUPLES_PER_PAGE)
+
+#define TOAST_TUPLE_TARGET		TOAST_TUPLE_THRESHOLD
+
+/*
+ * The code will also consider moving MAIN data out-of-line, but only as a
+ * last resort if the previous steps haven't reached the target tuple size.
+ * In this phase we use a different target size, currently equal to the
+ * largest tuple that will fit on a heap page.  This is reasonable since
+ * the user has told us to keep the data in-line if at all possible.
+ */
+#define TOAST_TUPLES_PER_PAGE_MAIN	1
+
+#define TOAST_TUPLE_TARGET_MAIN MaximumBytesPerTuple(TOAST_TUPLES_PER_PAGE_MAIN)
+
+/*
+ * If an index value is larger than TOAST_INDEX_TARGET, we will try to
+ * compress it (we can't move it out-of-line, however).  Note that this
+ * number is per-datum, not per-tuple, for simplicity in index_form_tuple().
+ */
+#define TOAST_INDEX_TARGET		(MaxHeapTupleSize / 16)
+
+/*
+ * When we store an oversize datum externally, we divide it into chunks
+ * containing at most TOAST_MAX_CHUNK_SIZE data bytes.  This number *must*
+ * be small enough that the completed toast-table tuple (including the
+ * ID and sequence fields and all overhead) will fit on a page.
+ * The coding here sets the size on the theory that we want to fit
+ * EXTERN_TUPLES_PER_PAGE tuples of maximum size onto a page.
+ *
+ * NB: Changing TOAST_MAX_CHUNK_SIZE requires an initdb.
+ */
+#define EXTERN_TUPLES_PER_PAGE	4	/* tweak only this */
+
+#define EXTERN_TUPLE_MAX_SIZE	MaximumBytesPerTuple(EXTERN_TUPLES_PER_PAGE)
+
+#define TOAST_MAX_CHUNK_SIZE	\
+	(EXTERN_TUPLE_MAX_SIZE -							\
+	 MAXALIGN(SizeofHeapTupleHeader) -					\
+	 sizeof(Oid) -										\
+	 sizeof(int32) -									\
+	 VARHDRSZ)
+
+/* ----------
+ * heap_toast_insert_or_update -
+ *
+ *	Called by heap_insert() and heap_update().
+ * ----------
+ */
+extern HeapTuple heap_toast_insert_or_update(Relation rel, HeapTuple newtup,
+											 HeapTuple oldtup, int options);
+
+/* ----------
+ * heap_toast_delete -
+ *
+ *	Called by heap_delete().
+ * ----------
+ */
+extern void heap_toast_delete(Relation rel, HeapTuple oldtup,
+							  bool is_speculative);
+
+/* ----------
+ * toast_flatten_tuple -
+ *
+ *	"Flatten" a tuple to contain no out-of-line toasted fields.
+ *	(This does not eliminate compressed or short-header datums.)
+ * ----------
+ */
+extern HeapTuple toast_flatten_tuple(HeapTuple tup, TupleDesc tupleDesc);
+
+/* ----------
+ * toast_flatten_tuple_to_datum -
+ *
+ *	"Flatten" a tuple containing out-of-line toasted fields into a Datum.
+ * ----------
+ */
+extern Datum toast_flatten_tuple_to_datum(HeapTupleHeader tup,
+										  uint32 tup_len,
+										  TupleDesc tupleDesc);
+
+/* ----------
+ * toast_build_flattened_tuple -
+ *
+ *	Build a tuple containing no out-of-line toasted fields.
+ *	(This does not eliminate compressed or short-header datums.)
+ * ----------
+ */
+extern HeapTuple toast_build_flattened_tuple(TupleDesc tupleDesc,
+											 Datum *values,
+											 bool *isnull);
+
+/* ----------
+ * heap_fetch_toast_slice
+ *
+ *	Fetch a slice from a toast value stored in a heap table.
+ * ----------
+ */
+extern void heap_fetch_toast_slice(Relation toastrel, Oid valueid,
+								   int32 attrsize, int32 sliceoffset,
+								   int32 slicelength, struct varlena *result);
+
+#endif							/* HEAPTOAST_H */