postgresql/src/include/access/tuptoaster.h

/*-------------------------------------------------------------------------
 *
 * tuptoaster.h
 *	  POSTGRES definitions for external and compressed storage
 *	  of variable size attributes.
 *
 * Copyright (c) 2000-2014, PostgreSQL Global Development Group
 *
 * src/include/access/tuptoaster.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef TUPTOASTER_H
#define TUPTOASTER_H

#include "access/htup_details.h"
#include "utils/relcache.h"
#include "storage/lock.h"

/*
 * This enables de-toasting of index entries.  Needed until VACUUM is
 * smart enough to rebuild indexes from scratch.
 */
#define TOAST_INDEX_HACK


/*
 * 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(offsetof(HeapTupleHeaderData, t_bits)) -	\
	 sizeof(Oid) -										\
	 sizeof(int32) -									\
	 VARHDRSZ)

/* Size of an EXTERNAL datum that contains a standard TOAST pointer */
#define TOAST_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(struct varatt_external))

/* Size of an indirect datum that contains a standard TOAST pointer */
#define INDIRECT_POINTER_SIZE (VARHDRSZ_EXTERNAL + sizeof(struct varatt_indirect))

/*
 * Testing whether an externally-stored value is compressed now requires
 * comparing extsize (the actual length of the external data) to rawsize
 * (the original uncompressed datum's size).  The latter includes VARHDRSZ
 * overhead, the former doesn't.  We never use compression unless it actually
 * saves space, so we expect either equality or less-than.
 */
#define VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer) \
	((toast_pointer).va_extsize < (toast_pointer).va_rawsize - VARHDRSZ)

/*
 * Macro to fetch the possibly-unaligned contents of an EXTERNAL datum
 * into a local "struct varatt_external" toast pointer.  This should be
 * just a memcpy, but some versions of gcc seem to produce broken code
 * that assumes the datum contents are aligned.  Introducing an explicit
 * intermediate "varattrib_1b_e *" variable seems to fix it.
 */
#define VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr) \
do { \
	varattrib_1b_e *attre = (varattrib_1b_e *) (attr); \
	Assert(VARATT_IS_EXTERNAL(attre)); \
	Assert(VARSIZE_EXTERNAL(attre) == sizeof(toast_pointer) + VARHDRSZ_EXTERNAL); \
	memcpy(&(toast_pointer), VARDATA_EXTERNAL(attre), sizeof(toast_pointer)); \
} while (0)

/* ----------
 * toast_insert_or_update -
 *
 *	Called by heap_insert() and heap_update().
 * ----------
 */
extern HeapTuple toast_insert_or_update(Relation rel,
					   HeapTuple newtup, HeapTuple oldtup,
					   int options);

/* ----------
 * toast_delete -
 *
 *	Called by heap_delete().
 * ----------
 */
extern void toast_delete(Relation rel, HeapTuple oldtup);

/* ----------
 * heap_tuple_fetch_attr() -
 *
 *		Fetches an external stored attribute from the toast
 *		relation. Does NOT decompress it, if stored external
 *		in compressed format.
 * ----------
 */
extern struct varlena *heap_tuple_fetch_attr(struct varlena * attr);

/* ----------
 * heap_tuple_untoast_attr() -
 *
 *		Fully detoasts one attribute, fetching and/or decompressing
 *		it as needed.
 * ----------
 */
extern struct varlena *heap_tuple_untoast_attr(struct varlena * attr);

/* ----------
 * heap_tuple_untoast_attr_slice() -
 *
 *		Fetches only the specified portion of an attribute.
 *		(Handles all cases for attribute storage)
 * ----------
 */
extern struct varlena *heap_tuple_untoast_attr_slice(struct varlena * attr,
							  int32 sliceoffset,
							  int32 slicelength);

/* ----------
 * 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_attribute -
 *
 *	If a Datum is of composite type, "flatten" it to contain no toasted fields.
 *	This must be invoked on any potentially-composite field that is to be
 *	inserted into a tuple.	Doing this preserves the invariant that toasting
 *	goes only one level deep in a tuple.
 * ----------
 */
extern Datum toast_flatten_tuple_attribute(Datum value,
							  Oid typeId, int32 typeMod);

/* ----------
 * toast_compress_datum -
 *
 *	Create a compressed version of a varlena datum, if possible
 * ----------
 */
extern Datum toast_compress_datum(Datum value);

/* ----------
 * toast_raw_datum_size -
 *
 *	Return the raw (detoasted) size of a varlena datum
 * ----------
 */
extern Size toast_raw_datum_size(Datum value);

/* ----------
 * toast_datum_size -
 *
 *	Return the storage size of a varlena datum
 * ----------
 */
extern Size toast_datum_size(Datum value);

/* ----------
 * toast_get_valid_index -
 *
 *	Return OID of valid index associated to a toast relation
 * ----------
 */
extern Oid toast_get_valid_index(Oid toastoid, LOCKMODE lock);

#endif   /* TUPTOASTER_H */