tuptoaster.h

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/*-------------------------------------------------------------------------
 *
 * tuptoaster.h
 *    POSTGRES definitions for external and compressed storage
 *    of variable size attributes.
 *
 * Copyright (c) 2000-2013, PostgreSQL Global Development Group
 *
 * src/include/access/tuptoaster.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef TUPTOASTER_H
#define TUPTOASTER_H

#include "access/htup_details.h"
#include "utils/relcache.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)


/* ----------
 * 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);

#endif   /* TUPTOASTER_H */