htup_details.h

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/*-------------------------------------------------------------------------
 *
 * htup_details.h
 *    POSTGRES heap tuple header definitions.
 *
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/access/htup_details.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef HTUP_DETAILS_H
#define HTUP_DETAILS_H

#include "access/htup.h"
#include "access/tupdesc.h"
#include "access/tupmacs.h"
#include "access/transam.h"
#include "storage/bufpage.h"

/*
 * MaxTupleAttributeNumber limits the number of (user) columns in a tuple.
 * The key limit on this value is that the size of the fixed overhead for
 * a tuple, plus the size of the null-values bitmap (at 1 bit per column),
 * plus MAXALIGN alignment, must fit into t_hoff which is uint8.  On most
 * machines the upper limit without making t_hoff wider would be a little
 * over 1700.  We use round numbers here and for MaxHeapAttributeNumber
 * so that alterations in HeapTupleHeaderData layout won't change the
 * supported max number of columns.
 */
#define MaxTupleAttributeNumber 1664    /* 8 * 208 */

/*
 * MaxHeapAttributeNumber limits the number of (user) columns in a table.
 * This should be somewhat less than MaxTupleAttributeNumber.  It must be
 * at least one less, else we will fail to do UPDATEs on a maximal-width
 * table (because UPDATE has to form working tuples that include CTID).
 * In practice we want some additional daylight so that we can gracefully
 * support operations that add hidden "resjunk" columns, for example
 * SELECT * FROM wide_table ORDER BY foo, bar, baz.
 * In any case, depending on column data types you will likely be running
 * into the disk-block-based limit on overall tuple size if you have more
 * than a thousand or so columns.  TOAST won't help.
 */
#define MaxHeapAttributeNumber  1600    /* 8 * 200 */

/*
 * Heap tuple header.  To avoid wasting space, the fields should be
 * laid out in such a way as to avoid structure padding.
 *
 * Datums of composite types (row types) share the same general structure
 * as on-disk tuples, so that the same routines can be used to build and
 * examine them.  However the requirements are slightly different: a Datum
 * does not need any transaction visibility information, and it does need
 * a length word and some embedded type information.  We can achieve this
 * by overlaying the xmin/cmin/xmax/cmax/xvac fields of a heap tuple
 * with the fields needed in the Datum case.  Typically, all tuples built
 * in-memory will be initialized with the Datum fields; but when a tuple is
 * about to be inserted in a table, the transaction fields will be filled,
 * overwriting the datum fields.
 *
 * The overall structure of a heap tuple looks like:
 *          fixed fields (HeapTupleHeaderData struct)
 *          nulls bitmap (if HEAP_HASNULL is set in t_infomask)
 *          alignment padding (as needed to make user data MAXALIGN'd)
 *          object ID (if HEAP_HASOID is set in t_infomask)
 *          user data fields
 *
 * We store five "virtual" fields Xmin, Cmin, Xmax, Cmax, and Xvac in three
 * physical fields.  Xmin and Xmax are always really stored, but Cmin, Cmax
 * and Xvac share a field.  This works because we know that Cmin and Cmax
 * are only interesting for the lifetime of the inserting and deleting
 * transaction respectively.  If a tuple is inserted and deleted in the same
 * transaction, we store a "combo" command id that can be mapped to the real
 * cmin and cmax, but only by use of local state within the originating
 * backend.  See combocid.c for more details.  Meanwhile, Xvac is only set by
 * old-style VACUUM FULL, which does not have any command sub-structure and so
 * does not need either Cmin or Cmax.  (This requires that old-style VACUUM
 * FULL never try to move a tuple whose Cmin or Cmax is still interesting,
 * ie, an insert-in-progress or delete-in-progress tuple.)
 *
 * A word about t_ctid: whenever a new tuple is stored on disk, its t_ctid
 * is initialized with its own TID (location).  If the tuple is ever updated,
 * its t_ctid is changed to point to the replacement version of the tuple.
 * Thus, a tuple is the latest version of its row iff XMAX is invalid or
 * t_ctid points to itself (in which case, if XMAX is valid, the tuple is
 * either locked or deleted).  One can follow the chain of t_ctid links
 * to find the newest version of the row.  Beware however that VACUUM might
 * erase the pointed-to (newer) tuple before erasing the pointing (older)
 * tuple.  Hence, when following a t_ctid link, it is necessary to check
 * to see if the referenced slot is empty or contains an unrelated tuple.
 * Check that the referenced tuple has XMIN equal to the referencing tuple's
 * XMAX to verify that it is actually the descendant version and not an
 * unrelated tuple stored into a slot recently freed by VACUUM.  If either
 * check fails, one may assume that there is no live descendant version.
 *
 * t_ctid is sometimes used to store a speculative insertion token, instead
 * of a real TID.  A speculative token is set on a tuple that's being
 * inserted, until the inserter is sure that it wants to go ahead with the
 * insertion.  Hence a token should only be seen on a tuple with an XMAX
 * that's still in-progress, or invalid/aborted.  The token is replaced with
 * the tuple's real TID when the insertion is confirmed.  One should never
 * see a speculative insertion token while following a chain of t_ctid links,
 * because they are not used on updates, only insertions.
 *
 * Following the fixed header fields, the nulls bitmap is stored (beginning
 * at t_bits).  The bitmap is *not* stored if t_infomask shows that there
 * are no nulls in the tuple.  If an OID field is present (as indicated by
 * t_infomask), then it is stored just before the user data, which begins at
 * the offset shown by t_hoff.  Note that t_hoff must be a multiple of
 * MAXALIGN.
 */

typedef struct HeapTupleFields
{
    TransactionId t_xmin;       /* inserting xact ID */
    TransactionId t_xmax;       /* deleting or locking xact ID */

    union
    {
        CommandId   t_cid;      /* inserting or deleting command ID, or both */
        TransactionId t_xvac;   /* old-style VACUUM FULL xact ID */
    }           t_field3;
} HeapTupleFields;

typedef struct DatumTupleFields
{
    int32       datum_len_;     /* varlena header (do not touch directly!) */

    int32       datum_typmod;   /* -1, or identifier of a record type */

    Oid         datum_typeid;   /* composite type OID, or RECORDOID */

    /*
     * Note: field ordering is chosen with thought that Oid might someday
     * widen to 64 bits.
     */
} DatumTupleFields;

struct HeapTupleHeaderData
{
    union
    {
        HeapTupleFields t_heap;
        DatumTupleFields t_datum;
    }           t_choice;

    ItemPointerData t_ctid;     /* current TID of this or newer tuple (or a
                                 * speculative insertion token) */

    /* Fields below here must match MinimalTupleData! */

    uint16      t_infomask2;    /* number of attributes + various flags */

    uint16      t_infomask;     /* various flag bits, see below */

    uint8       t_hoff;         /* sizeof header incl. bitmap, padding */

    /* ^ - 23 bytes - ^ */

    bits8       t_bits[FLEXIBLE_ARRAY_MEMBER];  /* bitmap of NULLs */

    /* MORE DATA FOLLOWS AT END OF STRUCT */
};

/* typedef appears in htup.h */

#define SizeofHeapTupleHeader offsetof(HeapTupleHeaderData, t_bits)

/*
 * information stored in t_infomask:
 */
#define HEAP_HASNULL            0x0001  /* has null attribute(s) */
#define HEAP_HASVARWIDTH        0x0002  /* has variable-width attribute(s) */
#define HEAP_HASEXTERNAL        0x0004  /* has external stored attribute(s) */
#define HEAP_HASOID             0x0008  /* has an object-id field */
#define HEAP_XMAX_KEYSHR_LOCK   0x0010  /* xmax is a key-shared locker */
#define HEAP_COMBOCID           0x0020  /* t_cid is a combo cid */
#define HEAP_XMAX_EXCL_LOCK     0x0040  /* xmax is exclusive locker */
#define HEAP_XMAX_LOCK_ONLY     0x0080  /* xmax, if valid, is only a locker */

 /* xmax is a shared locker */
#define HEAP_XMAX_SHR_LOCK  (HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_KEYSHR_LOCK)

#define HEAP_LOCK_MASK  (HEAP_XMAX_SHR_LOCK | HEAP_XMAX_EXCL_LOCK | \
                         HEAP_XMAX_KEYSHR_LOCK)
#define HEAP_XMIN_COMMITTED     0x0100  /* t_xmin committed */
#define HEAP_XMIN_INVALID       0x0200  /* t_xmin invalid/aborted */
#define HEAP_XMIN_FROZEN        (HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID)
#define HEAP_XMAX_COMMITTED     0x0400  /* t_xmax committed */
#define HEAP_XMAX_INVALID       0x0800  /* t_xmax invalid/aborted */
#define HEAP_XMAX_IS_MULTI      0x1000  /* t_xmax is a MultiXactId */
#define HEAP_UPDATED            0x2000  /* this is UPDATEd version of row */
#define HEAP_MOVED_OFF          0x4000  /* moved to another place by pre-9.0
                                         * VACUUM FULL; kept for binary
                                         * upgrade support */
#define HEAP_MOVED_IN           0x8000  /* moved from another place by pre-9.0
                                         * VACUUM FULL; kept for binary
                                         * upgrade support */
#define HEAP_MOVED (HEAP_MOVED_OFF | HEAP_MOVED_IN)

#define HEAP_XACT_MASK          0xFFF0  /* visibility-related bits */

/*
 * A tuple is only locked (i.e. not updated by its Xmax) if the
 * HEAP_XMAX_LOCK_ONLY bit is set; or, for pg_upgrade's sake, if the Xmax is
 * not a multi and the EXCL_LOCK bit is set.
 *
 * See also HeapTupleHeaderIsOnlyLocked, which also checks for a possible
 * aborted updater transaction.
 *
 * Beware of multiple evaluations of the argument.
 */
#define HEAP_XMAX_IS_LOCKED_ONLY(infomask) \
    (((infomask) & HEAP_XMAX_LOCK_ONLY) || \
     (((infomask) & (HEAP_XMAX_IS_MULTI | HEAP_LOCK_MASK)) == HEAP_XMAX_EXCL_LOCK))

/*
 * A tuple that has HEAP_XMAX_IS_MULTI and HEAP_XMAX_LOCK_ONLY but neither of
 * XMAX_EXCL_LOCK and XMAX_KEYSHR_LOCK must come from a tuple that was
 * share-locked in 9.2 or earlier and then pg_upgrade'd.
 *
 * In 9.2 and prior, HEAP_XMAX_IS_MULTI was only set when there were multiple
 * FOR SHARE lockers of that tuple.  That set HEAP_XMAX_LOCK_ONLY (with a
 * different name back then) but neither of HEAP_XMAX_EXCL_LOCK and
 * HEAP_XMAX_KEYSHR_LOCK.  That combination is no longer possible in 9.3 and
 * up, so if we see that combination we know for certain that the tuple was
 * locked in an earlier release; since all such lockers are gone (they cannot
 * survive through pg_upgrade), such tuples can safely be considered not
 * locked.
 *
 * We must not resolve such multixacts locally, because the result would be
 * bogus, regardless of where they stand with respect to the current valid
 * multixact range.
 */
#define HEAP_LOCKED_UPGRADED(infomask) \
( \
     ((infomask) & HEAP_XMAX_IS_MULTI) != 0 && \
     ((infomask) & HEAP_XMAX_LOCK_ONLY) != 0 && \
     (((infomask) & (HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_KEYSHR_LOCK)) == 0) \
)

/*
 * Use these to test whether a particular lock is applied to a tuple
 */
#define HEAP_XMAX_IS_SHR_LOCKED(infomask) \
    (((infomask) & HEAP_LOCK_MASK) == HEAP_XMAX_SHR_LOCK)
#define HEAP_XMAX_IS_EXCL_LOCKED(infomask) \
    (((infomask) & HEAP_LOCK_MASK) == HEAP_XMAX_EXCL_LOCK)
#define HEAP_XMAX_IS_KEYSHR_LOCKED(infomask) \
    (((infomask) & HEAP_LOCK_MASK) == HEAP_XMAX_KEYSHR_LOCK)

/* turn these all off when Xmax is to change */
#define HEAP_XMAX_BITS (HEAP_XMAX_COMMITTED | HEAP_XMAX_INVALID | \
                        HEAP_XMAX_IS_MULTI | HEAP_LOCK_MASK | HEAP_XMAX_LOCK_ONLY)

/*
 * information stored in t_infomask2:
 */
#define HEAP_NATTS_MASK         0x07FF  /* 11 bits for number of attributes */
/* bits 0x1800 are available */
#define HEAP_KEYS_UPDATED       0x2000  /* tuple was updated and key cols
                                         * modified, or tuple deleted */
#define HEAP_HOT_UPDATED        0x4000  /* tuple was HOT-updated */
#define HEAP_ONLY_TUPLE         0x8000  /* this is heap-only tuple */

#define HEAP2_XACT_MASK         0xE000  /* visibility-related bits */

/*
 * HEAP_TUPLE_HAS_MATCH is a temporary flag used during hash joins.  It is
 * only used in tuples that are in the hash table, and those don't need
 * any visibility information, so we can overlay it on a visibility flag
 * instead of using up a dedicated bit.
 */
#define HEAP_TUPLE_HAS_MATCH    HEAP_ONLY_TUPLE /* tuple has a join match */

/*
 * Special value used in t_ctid.ip_posid, to indicate that it holds a
 * speculative insertion token rather than a real TID.  This must be higher
 * than MaxOffsetNumber, so that it can be distinguished from a valid
 * offset number in a regular item pointer.
 */
#define SpecTokenOffsetNumber       0xfffe

/*
 * HeapTupleHeader accessor macros
 *
 * Note: beware of multiple evaluations of "tup" argument.  But the Set
 * macros evaluate their other argument only once.
 */

/*
 * HeapTupleHeaderGetRawXmin returns the "raw" xmin field, which is the xid
 * originally used to insert the tuple.  However, the tuple might actually
 * be frozen (via HeapTupleHeaderSetXminFrozen) in which case the tuple's xmin
 * is visible to every snapshot.  Prior to PostgreSQL 9.4, we actually changed
 * the xmin to FrozenTransactionId, and that value may still be encountered
 * on disk.
 */
#define HeapTupleHeaderGetRawXmin(tup) \
( \
    (tup)->t_choice.t_heap.t_xmin \
)

#define HeapTupleHeaderGetXmin(tup) \
( \
    HeapTupleHeaderXminFrozen(tup) ? \
        FrozenTransactionId : HeapTupleHeaderGetRawXmin(tup) \
)

#define HeapTupleHeaderSetXmin(tup, xid) \
( \
    (tup)->t_choice.t_heap.t_xmin = (xid) \
)

#define HeapTupleHeaderXminCommitted(tup) \
( \
    ((tup)->t_infomask & HEAP_XMIN_COMMITTED) != 0 \
)

#define HeapTupleHeaderXminInvalid(tup) \
( \
    ((tup)->t_infomask & (HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID)) == \
        HEAP_XMIN_INVALID \
)

#define HeapTupleHeaderXminFrozen(tup) \
( \
    ((tup)->t_infomask & (HEAP_XMIN_FROZEN)) == HEAP_XMIN_FROZEN \
)

#define HeapTupleHeaderSetXminCommitted(tup) \
( \
    AssertMacro(!HeapTupleHeaderXminInvalid(tup)), \
    ((tup)->t_infomask |= HEAP_XMIN_COMMITTED) \
)

#define HeapTupleHeaderSetXminInvalid(tup) \
( \
    AssertMacro(!HeapTupleHeaderXminCommitted(tup)), \
    ((tup)->t_infomask |= HEAP_XMIN_INVALID) \
)

#define HeapTupleHeaderSetXminFrozen(tup) \
( \
    AssertMacro(!HeapTupleHeaderXminInvalid(tup)), \
    ((tup)->t_infomask |= HEAP_XMIN_FROZEN) \
)

/*
 * HeapTupleHeaderGetRawXmax gets you the raw Xmax field.  To find out the Xid
 * that updated a tuple, you might need to resolve the MultiXactId if certain
 * bits are set.  HeapTupleHeaderGetUpdateXid checks those bits and takes care
 * to resolve the MultiXactId if necessary.  This might involve multixact I/O,
 * so it should only be used if absolutely necessary.
 */
#define HeapTupleHeaderGetUpdateXid(tup) \
( \
    (!((tup)->t_infomask & HEAP_XMAX_INVALID) && \
     ((tup)->t_infomask & HEAP_XMAX_IS_MULTI) && \
     !((tup)->t_infomask & HEAP_XMAX_LOCK_ONLY)) ? \
        HeapTupleGetUpdateXid(tup) \
    : \
        HeapTupleHeaderGetRawXmax(tup) \
)

#define HeapTupleHeaderGetRawXmax(tup) \
( \
    (tup)->t_choice.t_heap.t_xmax \
)

#define HeapTupleHeaderSetXmax(tup, xid) \
( \
    (tup)->t_choice.t_heap.t_xmax = (xid) \
)

/*
 * HeapTupleHeaderGetRawCommandId will give you what's in the header whether
 * it is useful or not.  Most code should use HeapTupleHeaderGetCmin or
 * HeapTupleHeaderGetCmax instead, but note that those Assert that you can
 * get a legitimate result, ie you are in the originating transaction!
 */
#define HeapTupleHeaderGetRawCommandId(tup) \
( \
    (tup)->t_choice.t_heap.t_field3.t_cid \
)

/* SetCmin is reasonably simple since we never need a combo CID */
#define HeapTupleHeaderSetCmin(tup, cid) \
do { \
    Assert(!((tup)->t_infomask & HEAP_MOVED)); \
    (tup)->t_choice.t_heap.t_field3.t_cid = (cid); \
    (tup)->t_infomask &= ~HEAP_COMBOCID; \
} while (0)

/* SetCmax must be used after HeapTupleHeaderAdjustCmax; see combocid.c */
#define HeapTupleHeaderSetCmax(tup, cid, iscombo) \
do { \
    Assert(!((tup)->t_infomask & HEAP_MOVED)); \
    (tup)->t_choice.t_heap.t_field3.t_cid = (cid); \
    if (iscombo) \
        (tup)->t_infomask |= HEAP_COMBOCID; \
    else \
        (tup)->t_infomask &= ~HEAP_COMBOCID; \
} while (0)

#define HeapTupleHeaderGetXvac(tup) \
( \
    ((tup)->t_infomask & HEAP_MOVED) ? \
        (tup)->t_choice.t_heap.t_field3.t_xvac \
    : \
        InvalidTransactionId \
)

#define HeapTupleHeaderSetXvac(tup, xid) \
do { \
    Assert((tup)->t_infomask & HEAP_MOVED); \
    (tup)->t_choice.t_heap.t_field3.t_xvac = (xid); \
} while (0)

#define HeapTupleHeaderIsSpeculative(tup) \
( \
    (ItemPointerGetOffsetNumberNoCheck(&(tup)->t_ctid) == SpecTokenOffsetNumber) \
)

#define HeapTupleHeaderGetSpeculativeToken(tup) \
( \
    AssertMacro(HeapTupleHeaderIsSpeculative(tup)), \
    ItemPointerGetBlockNumber(&(tup)->t_ctid) \
)

#define HeapTupleHeaderSetSpeculativeToken(tup, token)  \
( \
    ItemPointerSet(&(tup)->t_ctid, token, SpecTokenOffsetNumber) \
)

#define HeapTupleHeaderGetDatumLength(tup) \
    VARSIZE(tup)

#define HeapTupleHeaderSetDatumLength(tup, len) \
    SET_VARSIZE(tup, len)

#define HeapTupleHeaderGetTypeId(tup) \
( \
    (tup)->t_choice.t_datum.datum_typeid \
)

#define HeapTupleHeaderSetTypeId(tup, typeid) \
( \
    (tup)->t_choice.t_datum.datum_typeid = (typeid) \
)

#define HeapTupleHeaderGetTypMod(tup) \
( \
    (tup)->t_choice.t_datum.datum_typmod \
)

#define HeapTupleHeaderSetTypMod(tup, typmod) \
( \
    (tup)->t_choice.t_datum.datum_typmod = (typmod) \
)

#define HeapTupleHeaderGetOid(tup) \
( \
    ((tup)->t_infomask & HEAP_HASOID) ? \
        *((Oid *) ((char *)(tup) + (tup)->t_hoff - sizeof(Oid))) \
    : \
        InvalidOid \
)

#define HeapTupleHeaderSetOid(tup, oid) \
do { \
    Assert((tup)->t_infomask & HEAP_HASOID); \
    *((Oid *) ((char *)(tup) + (tup)->t_hoff - sizeof(Oid))) = (oid); \
} while (0)

/*
 * Note that we stop considering a tuple HOT-updated as soon as it is known
 * aborted or the would-be updating transaction is known aborted.  For best
 * efficiency, check tuple visibility before using this macro, so that the
 * INVALID bits will be as up to date as possible.
 */
#define HeapTupleHeaderIsHotUpdated(tup) \
( \
    ((tup)->t_infomask2 & HEAP_HOT_UPDATED) != 0 && \
    ((tup)->t_infomask & HEAP_XMAX_INVALID) == 0 && \
    !HeapTupleHeaderXminInvalid(tup) \
)

#define HeapTupleHeaderSetHotUpdated(tup) \
( \
    (tup)->t_infomask2 |= HEAP_HOT_UPDATED \
)

#define HeapTupleHeaderClearHotUpdated(tup) \
( \
    (tup)->t_infomask2 &= ~HEAP_HOT_UPDATED \
)

#define HeapTupleHeaderIsHeapOnly(tup) \
( \
  ((tup)->t_infomask2 & HEAP_ONLY_TUPLE) != 0 \
)

#define HeapTupleHeaderSetHeapOnly(tup) \
( \
  (tup)->t_infomask2 |= HEAP_ONLY_TUPLE \
)

#define HeapTupleHeaderClearHeapOnly(tup) \
( \
  (tup)->t_infomask2 &= ~HEAP_ONLY_TUPLE \
)

#define HeapTupleHeaderHasMatch(tup) \
( \
  ((tup)->t_infomask2 & HEAP_TUPLE_HAS_MATCH) != 0 \
)

#define HeapTupleHeaderSetMatch(tup) \
( \
  (tup)->t_infomask2 |= HEAP_TUPLE_HAS_MATCH \
)

#define HeapTupleHeaderClearMatch(tup) \
( \
  (tup)->t_infomask2 &= ~HEAP_TUPLE_HAS_MATCH \
)

#define HeapTupleHeaderGetNatts(tup) \
    ((tup)->t_infomask2 & HEAP_NATTS_MASK)

#define HeapTupleHeaderSetNatts(tup, natts) \
( \
    (tup)->t_infomask2 = ((tup)->t_infomask2 & ~HEAP_NATTS_MASK) | (natts) \
)

#define HeapTupleHeaderHasExternal(tup) \
        (((tup)->t_infomask & HEAP_HASEXTERNAL) != 0)


/*
 * BITMAPLEN(NATTS) -
 *      Computes size of null bitmap given number of data columns.
 */
#define BITMAPLEN(NATTS)    (((int)(NATTS) + 7) / 8)

/*
 * MaxHeapTupleSize is the maximum allowed size of a heap tuple, including
 * header and MAXALIGN alignment padding.  Basically it's BLCKSZ minus the
 * other stuff that has to be on a disk page.  Since heap pages use no
 * "special space", there's no deduction for that.
 *
 * NOTE: we allow for the ItemId that must point to the tuple, ensuring that
 * an otherwise-empty page can indeed hold a tuple of this size.  Because
 * ItemIds and tuples have different alignment requirements, don't assume that
 * you can, say, fit 2 tuples of size MaxHeapTupleSize/2 on the same page.
 */
#define MaxHeapTupleSize  (BLCKSZ - MAXALIGN(SizeOfPageHeaderData + sizeof(ItemIdData)))
#define MinHeapTupleSize  MAXALIGN(SizeofHeapTupleHeader)

/*
 * MaxHeapTuplesPerPage is an upper bound on the number of tuples that can
 * fit on one heap page.  (Note that indexes could have more, because they
 * use a smaller tuple header.)  We arrive at the divisor because each tuple
 * must be maxaligned, and it must have an associated item pointer.
 *
 * Note: with HOT, there could theoretically be more line pointers (not actual
 * tuples) than this on a heap page.  However we constrain the number of line
 * pointers to this anyway, to avoid excessive line-pointer bloat and not
 * require increases in the size of work arrays.
 */
#define MaxHeapTuplesPerPage    \
    ((int) ((BLCKSZ - SizeOfPageHeaderData) / \
            (MAXALIGN(SizeofHeapTupleHeader) + sizeof(ItemIdData))))

/*
 * MaxAttrSize is a somewhat arbitrary upper limit on the declared size of
 * data fields of char(n) and similar types.  It need not have anything
 * directly to do with the *actual* upper limit of varlena values, which
 * is currently 1Gb (see TOAST structures in postgres.h).  I've set it
 * at 10Mb which seems like a reasonable number --- tgl 8/6/00.
 */
#define MaxAttrSize     (10 * 1024 * 1024)


/*
 * MinimalTuple is an alternative representation that is used for transient
 * tuples inside the executor, in places where transaction status information
 * is not required, the tuple rowtype is known, and shaving off a few bytes
 * is worthwhile because we need to store many tuples.  The representation
 * is chosen so that tuple access routines can work with either full or
 * minimal tuples via a HeapTupleData pointer structure.  The access routines
 * see no difference, except that they must not access the transaction status
 * or t_ctid fields because those aren't there.
 *
 * For the most part, MinimalTuples should be accessed via TupleTableSlot
 * routines.  These routines will prevent access to the "system columns"
 * and thereby prevent accidental use of the nonexistent fields.
 *
 * MinimalTupleData contains a length word, some padding, and fields matching
 * HeapTupleHeaderData beginning with t_infomask2. The padding is chosen so
 * that offsetof(t_infomask2) is the same modulo MAXIMUM_ALIGNOF in both
 * structs.   This makes data alignment rules equivalent in both cases.
 *
 * When a minimal tuple is accessed via a HeapTupleData pointer, t_data is
 * set to point MINIMAL_TUPLE_OFFSET bytes before the actual start of the
 * minimal tuple --- that is, where a full tuple matching the minimal tuple's
 * data would start.  This trick is what makes the structs seem equivalent.
 *
 * Note that t_hoff is computed the same as in a full tuple, hence it includes
 * the MINIMAL_TUPLE_OFFSET distance.  t_len does not include that, however.
 *
 * MINIMAL_TUPLE_DATA_OFFSET is the offset to the first useful (non-pad) data
 * other than the length word.  tuplesort.c and tuplestore.c use this to avoid
 * writing the padding to disk.
 */
#define MINIMAL_TUPLE_OFFSET \
    ((offsetof(HeapTupleHeaderData, t_infomask2) - sizeof(uint32)) / MAXIMUM_ALIGNOF * MAXIMUM_ALIGNOF)
#define MINIMAL_TUPLE_PADDING \
    ((offsetof(HeapTupleHeaderData, t_infomask2) - sizeof(uint32)) % MAXIMUM_ALIGNOF)
#define MINIMAL_TUPLE_DATA_OFFSET \
    offsetof(MinimalTupleData, t_infomask2)

struct MinimalTupleData
{
    uint32      t_len;          /* actual length of minimal tuple */

    char        mt_padding[MINIMAL_TUPLE_PADDING];

    /* Fields below here must match HeapTupleHeaderData! */

    uint16      t_infomask2;    /* number of attributes + various flags */

    uint16      t_infomask;     /* various flag bits, see below */

    uint8       t_hoff;         /* sizeof header incl. bitmap, padding */

    /* ^ - 23 bytes - ^ */

    bits8       t_bits[FLEXIBLE_ARRAY_MEMBER];  /* bitmap of NULLs */

    /* MORE DATA FOLLOWS AT END OF STRUCT */
};

/* typedef appears in htup.h */

#define SizeofMinimalTupleHeader offsetof(MinimalTupleData, t_bits)


/*
 * GETSTRUCT - given a HeapTuple pointer, return address of the user data
 */
#define GETSTRUCT(TUP) ((char *) ((TUP)->t_data) + (TUP)->t_data->t_hoff)

/*
 * Accessor macros to be used with HeapTuple pointers.
 */

#define HeapTupleHasNulls(tuple) \
        (((tuple)->t_data->t_infomask & HEAP_HASNULL) != 0)

#define HeapTupleNoNulls(tuple) \
        (!((tuple)->t_data->t_infomask & HEAP_HASNULL))

#define HeapTupleHasVarWidth(tuple) \
        (((tuple)->t_data->t_infomask & HEAP_HASVARWIDTH) != 0)

#define HeapTupleAllFixed(tuple) \
        (!((tuple)->t_data->t_infomask & HEAP_HASVARWIDTH))

#define HeapTupleHasExternal(tuple) \
        (((tuple)->t_data->t_infomask & HEAP_HASEXTERNAL) != 0)

#define HeapTupleIsHotUpdated(tuple) \
        HeapTupleHeaderIsHotUpdated((tuple)->t_data)

#define HeapTupleSetHotUpdated(tuple) \
        HeapTupleHeaderSetHotUpdated((tuple)->t_data)

#define HeapTupleClearHotUpdated(tuple) \
        HeapTupleHeaderClearHotUpdated((tuple)->t_data)

#define HeapTupleIsHeapOnly(tuple) \
        HeapTupleHeaderIsHeapOnly((tuple)->t_data)

#define HeapTupleSetHeapOnly(tuple) \
        HeapTupleHeaderSetHeapOnly((tuple)->t_data)

#define HeapTupleClearHeapOnly(tuple) \
        HeapTupleHeaderClearHeapOnly((tuple)->t_data)

#define HeapTupleGetOid(tuple) \
        HeapTupleHeaderGetOid((tuple)->t_data)

#define HeapTupleSetOid(tuple, oid) \
        HeapTupleHeaderSetOid((tuple)->t_data, (oid))


/* ----------------
 *      fastgetattr
 *
 *      Fetch a user attribute's value as a Datum (might be either a
 *      value, or a pointer into the data area of the tuple).
 *
 *      This must not be used when a system attribute might be requested.
 *      Furthermore, the passed attnum MUST be valid.  Use heap_getattr()
 *      instead, if in doubt.
 *
 *      This gets called many times, so we macro the cacheable and NULL
 *      lookups, and call nocachegetattr() for the rest.
 * ----------------
 */

#if !defined(DISABLE_COMPLEX_MACRO)

#define fastgetattr(tup, attnum, tupleDesc, isnull)                 \
(                                                                   \
    AssertMacro((attnum) > 0),                                      \
    (*(isnull) = false),                                            \
    HeapTupleNoNulls(tup) ?                                         \
    (                                                               \
        (tupleDesc)->attrs[(attnum)-1]->attcacheoff >= 0 ?          \
        (                                                           \
            fetchatt((tupleDesc)->attrs[(attnum)-1],                \
                (char *) (tup)->t_data + (tup)->t_data->t_hoff +    \
                    (tupleDesc)->attrs[(attnum)-1]->attcacheoff)    \
        )                                                           \
        :                                                           \
            nocachegetattr((tup), (attnum), (tupleDesc))            \
    )                                                               \
    :                                                               \
    (                                                               \
        att_isnull((attnum)-1, (tup)->t_data->t_bits) ?             \
        (                                                           \
            (*(isnull) = true),                                     \
            (Datum)NULL                                             \
        )                                                           \
        :                                                           \
        (                                                           \
            nocachegetattr((tup), (attnum), (tupleDesc))            \
        )                                                           \
    )                                                               \
)
#else                           /* defined(DISABLE_COMPLEX_MACRO) */

extern Datum fastgetattr(HeapTuple tup, int attnum, TupleDesc tupleDesc,
            bool *isnull);
#endif                          /* defined(DISABLE_COMPLEX_MACRO) */


/* ----------------
 *      heap_getattr
 *
 *      Extract an attribute of a heap tuple and return it as a Datum.
 *      This works for either system or user attributes.  The given attnum
 *      is properly range-checked.
 *
 *      If the field in question has a NULL value, we return a zero Datum
 *      and set *isnull == true.  Otherwise, we set *isnull == false.
 *
 *      <tup> is the pointer to the heap tuple.  <attnum> is the attribute
 *      number of the column (field) caller wants.  <tupleDesc> is a
 *      pointer to the structure describing the row and all its fields.
 * ----------------
 */
#define heap_getattr(tup, attnum, tupleDesc, isnull) \
    ( \
        ((attnum) > 0) ? \
        ( \
            ((attnum) > (int) HeapTupleHeaderGetNatts((tup)->t_data)) ? \
            ( \
                (*(isnull) = true), \
                (Datum)NULL \
            ) \
            : \
                fastgetattr((tup), (attnum), (tupleDesc), (isnull)) \
        ) \
        : \
            heap_getsysattr((tup), (attnum), (tupleDesc), (isnull)) \
    )


/* prototypes for functions in common/heaptuple.c */
extern Size heap_compute_data_size(TupleDesc tupleDesc,
                       Datum *values, bool *isnull);
extern void heap_fill_tuple(TupleDesc tupleDesc,
                Datum *values, bool *isnull,
                char *data, Size data_size,
                uint16 *infomask, bits8 *bit);
extern bool heap_attisnull(HeapTuple tup, int attnum);
extern Datum nocachegetattr(HeapTuple tup, int attnum,
               TupleDesc att);
extern Datum heap_getsysattr(HeapTuple tup, int attnum, TupleDesc tupleDesc,
                bool *isnull);
extern HeapTuple heap_copytuple(HeapTuple tuple);
extern void heap_copytuple_with_tuple(HeapTuple src, HeapTuple dest);
extern Datum heap_copy_tuple_as_datum(HeapTuple tuple, TupleDesc tupleDesc);
extern HeapTuple heap_form_tuple(TupleDesc tupleDescriptor,
                Datum *values, bool *isnull);
extern HeapTuple heap_modify_tuple(HeapTuple tuple,
                  TupleDesc tupleDesc,
                  Datum *replValues,
                  bool *replIsnull,
                  bool *doReplace);
extern HeapTuple heap_modify_tuple_by_cols(HeapTuple tuple,
                          TupleDesc tupleDesc,
                          int nCols,
                          int *replCols,
                          Datum *replValues,
                          bool *replIsnull);
extern void heap_deform_tuple(HeapTuple tuple, TupleDesc tupleDesc,
                  Datum *values, bool *isnull);
extern void heap_freetuple(HeapTuple htup);
extern MinimalTuple heap_form_minimal_tuple(TupleDesc tupleDescriptor,
                        Datum *values, bool *isnull);
extern void heap_free_minimal_tuple(MinimalTuple mtup);
extern MinimalTuple heap_copy_minimal_tuple(MinimalTuple mtup);
extern HeapTuple heap_tuple_from_minimal_tuple(MinimalTuple mtup);
extern MinimalTuple minimal_tuple_from_heap_tuple(HeapTuple htup);

#endif                          /* HTUP_DETAILS_H */