/src/postgres/src/include/varatt.h

Source
/*-------------------------------------------------------------------------
 *
 * varatt.h
 *    variable-length datatypes (TOAST support)
 *
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1995, Regents of the University of California
 *
 * src/include/varatt.h
 *
 *-------------------------------------------------------------------------
 */

#ifndef VARATT_H
#define VARATT_H

/*
 * varatt_external_oid is a traditional "TOAST pointer", that is, the
 * information needed to fetch a Datum stored out-of-line in a TOAST table.
 * The data is compressed if and only if the external size stored in
 * va_extinfo is less than va_rawsize - VARHDRSZ.
 *
 * This struct must not contain any padding, because we sometimes compare
 * these pointers using memcmp.
 *
 * Note that this information is stored unaligned within actual tuples, so
 * you need to memcpy from the tuple into a local struct variable before
 * you can look at these fields!  (The reason we use memcmp is to avoid
 * having to do that just to detect equality of two TOAST pointers...)
 */
typedef struct varatt_external_oid
{
  int32   va_rawsize;   /* Original data size (includes header) */
  uint32    va_extinfo;   /* External saved size (without header) and
                 * compression method */
  Oid     va_valueid;   /* Unique ID of value within TOAST table */
  Oid     va_toastrelid;  /* RelID of TOAST table containing it */
} varatt_external_oid;

StaticAssertDecl((sizeof(int32) + sizeof(uint32) + 2 * sizeof(Oid)) ==
         sizeof(varatt_external_oid),
         "varatt_external_oid should have no padding");

/*
 * varatt_external_oid8 is a "TOAST pointer" for TOAST tables that use
 * Oid8 (64-bit) as their chunk_id type.  Same layout as varatt_external_oid
 * except for the value ID, which is 8 bytes wide.  The value ID is split
 * into two uint32 to force alignment.
 *
 * This struct must not contain any padding, because we sometimes compare
 * these pointers using memcmp.
 */
typedef struct varatt_external_oid8
{
  int32   va_rawsize;   /* Original data size (includes header) */
  uint32    va_extinfo;   /* External saved size (without header) and
                 * compression method */
  uint32    va_valueid_lo;  /* Low 32 bits of value ID */
  uint32    va_valueid_hi;  /* High 32 bits of value ID */
  Oid     va_toastrelid;  /* RelID of TOAST table containing it */
} varatt_external_oid8;

StaticAssertDecl((sizeof(int32) + 3 * sizeof(uint32) + sizeof(Oid)) ==
         sizeof(varatt_external_oid8),
         "varatt_external_oid8 should have no padding");

/* Get/set the 64-bit value ID from the lo/hi halves */
static inline Oid8
VARATT_EXTERNAL_OID8_GET_VALUEID(varatt_external_oid8 toast_pointer)
{
  return ((Oid8) toast_pointer.va_valueid_lo) |
    (((Oid8) toast_pointer.va_valueid_hi) << 32);
}

static inline void
VARATT_EXTERNAL_OID8_SET_VALUEID(varatt_external_oid8 *toast_pointer, Oid8 id)
{
  toast_pointer->va_valueid_lo = (uint32) id;
  toast_pointer->va_valueid_hi = (uint32) (id >> 32);
}

/*
 * These macros define the "saved size" portion of va_extinfo.  Its remaining
 * two high-order bits identify the compression method.
 */
#define VARLENA_EXTSIZE_BITS  30
#define VARLENA_EXTSIZE_MASK  ((1U << VARLENA_EXTSIZE_BITS) - 1)

/*
 * varatt_indirect is a "TOAST pointer" representing an out-of-line
 * Datum that's stored in memory, not in an external toast relation.
 * The creator of such a Datum is entirely responsible that the referenced
 * storage survives for as long as referencing pointer Datums can exist.
 *
 * Note that just as for varatt_external_oid, this struct is stored
 * unaligned within any containing tuple.
 */
typedef struct varatt_indirect
{
  varlena    *pointer;    /* Pointer to in-memory varlena */
} varatt_indirect;

/*
 * varatt_expanded is a "TOAST pointer" representing an out-of-line
 * Datum that is stored in memory, in some type-specific, not necessarily
 * physically contiguous format that is convenient for computation not
 * storage.  APIs for this, in particular the definition of struct
 * ExpandedObjectHeader, are in src/include/utils/expandeddatum.h.
 *
 * Note that just as for varatt_external_oid, this struct is stored
 * unaligned within any containing tuple.
 */
typedef struct ExpandedObjectHeader ExpandedObjectHeader;

typedef struct varatt_expanded
{
  ExpandedObjectHeader *eohptr;
} varatt_expanded;

/*
 * Type tag for the various sorts of "TOAST pointer" datums.  The peculiar
 * value for VARTAG_ONDISK_OID comes from a requirement for on-disk
 * compatibility with a previous notion that the tag field was the pointer
 * datum's length.
 */
typedef enum vartag_external
{
  VARTAG_INDIRECT = 1,
  VARTAG_EXPANDED_RO = 2,
  VARTAG_EXPANDED_RW = 3,
  VARTAG_ONDISK_OID8 = 4,
  VARTAG_ONDISK_OID = 18
} vartag_external;

/* Is a TOAST pointer either type of expanded-object pointer? */
/* this test relies on the specific tag values above */
static inline bool
VARTAG_IS_EXPANDED(vartag_external tag)
{
  return ((tag & ~1) == VARTAG_EXPANDED_RO);
}

/* Size of the data part of a "TOAST pointer" datum */
static inline Size
VARTAG_SIZE(vartag_external tag)
{
  if (tag == VARTAG_INDIRECT)
    return sizeof(varatt_indirect);
  else if (VARTAG_IS_EXPANDED(tag))
    return sizeof(varatt_expanded);
  else if (tag == VARTAG_ONDISK_OID)
    return sizeof(varatt_external_oid);
  else if (tag == VARTAG_ONDISK_OID8)
    return sizeof(varatt_external_oid8);
  else
  {
    Assert(false);
    return 0;
  }
}

/*
 * These structs describe the header of a varlena object that may have been
 * TOASTed.  Generally, don't reference these structs directly, but use the
 * functions and macros below.
 *
 * We use separate structs for the aligned and unaligned cases because the
 * compiler might otherwise think it could generate code that assumes
 * alignment while touching fields of a 1-byte-header varlena.
 */
typedef union
{
  struct            /* Normal varlena (4-byte length) */
  {
    uint32    va_header;
    char    va_data[FLEXIBLE_ARRAY_MEMBER];
  }     va_4byte;
  struct            /* Compressed-in-line format */
  {
    uint32    va_header;
    uint32    va_tcinfo;  /* Original data size (excludes header) and
                 * compression method; see va_extinfo */
    char    va_data[FLEXIBLE_ARRAY_MEMBER]; /* Compressed data */
  }     va_compressed;
} varattrib_4b;

typedef struct
{
  uint8   va_header;
  char    va_data[FLEXIBLE_ARRAY_MEMBER]; /* Data begins here */
} varattrib_1b;

/* TOAST pointers are a subset of varattrib_1b with an identifying tag byte */
typedef struct
{
  uint8   va_header;    /* Always 0x80 or 0x01 */
  uint8   va_tag;     /* Type of datum */
  char    va_data[FLEXIBLE_ARRAY_MEMBER]; /* Type-specific data */
} varattrib_1b_e;

/*
 * Bit layouts for varlena headers on big-endian machines:
 *
 * 00xxxxxx 4-byte length word, aligned, uncompressed data (up to 1G)
 * 01xxxxxx 4-byte length word, aligned, *compressed* data (up to 1G)
 * 10000000 1-byte length word, unaligned, TOAST pointer
 * 1xxxxxxx 1-byte length word, unaligned, uncompressed data (up to 126b)
 *
 * Bit layouts for varlena headers on little-endian machines:
 *
 * xxxxxx00 4-byte length word, aligned, uncompressed data (up to 1G)
 * xxxxxx10 4-byte length word, aligned, *compressed* data (up to 1G)
 * 00000001 1-byte length word, unaligned, TOAST pointer
 * xxxxxxx1 1-byte length word, unaligned, uncompressed data (up to 126b)
 *
 * The "xxx" bits are the length field (which includes itself in all cases).
 * In the big-endian case we mask to extract the length, in the little-endian
 * case we shift.  Note that in both cases the flag bits are in the physically
 * first byte.  Also, it is not possible for a 1-byte length word to be zero;
 * this lets us disambiguate alignment padding bytes from the start of an
 * unaligned datum.  (We now *require* pad bytes to be filled with zero!)
 *
 * In TOAST pointers the va_tag field (see varattrib_1b_e) is used to discern
 * the specific type and length of the pointer datum.
 */

/*
 * Endian-dependent macros.  These are considered internal --- use the
 * external functions below instead of using these directly.  All of these
 * expect an argument that is a pointer, not a Datum.  Some of them have
 * multiple-evaluation hazards, too.
 *
 * Note: IS_1B is true for external toast records but VARSIZE_1B will return 0
 * for such records. Hence you should usually check for IS_EXTERNAL before
 * checking for IS_1B.
 */

#ifdef WORDS_BIGENDIAN

#define VARATT_IS_4B(PTR) \
  ((((const varattrib_1b *) (PTR))->va_header & 0x80) == 0x00)
#define VARATT_IS_4B_U(PTR) \
  ((((const varattrib_1b *) (PTR))->va_header & 0xC0) == 0x00)
#define VARATT_IS_4B_C(PTR) \
  ((((const varattrib_1b *) (PTR))->va_header & 0xC0) == 0x40)
#define VARATT_IS_1B(PTR) \
  ((((const varattrib_1b *) (PTR))->va_header & 0x80) == 0x80)
#define VARATT_IS_1B_E(PTR) \
  ((((const varattrib_1b *) (PTR))->va_header) == 0x80)
#define VARATT_NOT_PAD_BYTE(PTR) \
  (*((const uint8 *) (PTR)) != 0)

/* VARSIZE_4B() should only be used on known-aligned data */
#define VARSIZE_4B(PTR) \
  (((const varattrib_4b *) (PTR))->va_4byte.va_header & 0x3FFFFFFF)
#define VARSIZE_1B(PTR) \
  (((const varattrib_1b *) (PTR))->va_header & 0x7F)
#define VARTAG_1B_E(PTR) \
  ((vartag_external) ((const varattrib_1b_e *) (PTR))->va_tag)

#define SET_VARSIZE_4B(PTR,len) \
  (((varattrib_4b *) (PTR))->va_4byte.va_header = (len) & 0x3FFFFFFF)
#define SET_VARSIZE_4B_C(PTR,len) \
  (((varattrib_4b *) (PTR))->va_4byte.va_header = ((len) & 0x3FFFFFFF) | 0x40000000)
#define SET_VARSIZE_1B(PTR,len) \
  (((varattrib_1b *) (PTR))->va_header = (len) | 0x80)
#define SET_VARTAG_1B_E(PTR,tag) \
  (((varattrib_1b_e *) (PTR))->va_header = 0x80, \
   ((varattrib_1b_e *) (PTR))->va_tag = (tag))

#else             /* !WORDS_BIGENDIAN */

#define VARATT_IS_4B(PTR) \
  ((((const varattrib_1b *) (PTR))->va_header & 0x01) == 0x00)
#define VARATT_IS_4B_U(PTR) \
  ((((const varattrib_1b *) (PTR))->va_header & 0x03) == 0x00)
#define VARATT_IS_4B_C(PTR) \
  ((((const varattrib_1b *) (PTR))->va_header & 0x03) == 0x02)
#define VARATT_IS_1B(PTR) \
  ((((const varattrib_1b *) (PTR))->va_header & 0x01) == 0x01)
#define VARATT_IS_1B_E(PTR) \
  ((((const varattrib_1b *) (PTR))->va_header) == 0x01)
#define VARATT_NOT_PAD_BYTE(PTR) \
  (*((const uint8 *) (PTR)) != 0)

/* VARSIZE_4B() should only be used on known-aligned data */
#define VARSIZE_4B(PTR) \
  ((((const varattrib_4b *) (PTR))->va_4byte.va_header >> 2) & 0x3FFFFFFF)
#define VARSIZE_1B(PTR) \
  ((((const varattrib_1b *) (PTR))->va_header >> 1) & 0x7F)
#define VARTAG_1B_E(PTR) \
  ((vartag_external) ((const varattrib_1b_e *) (PTR))->va_tag)

#define SET_VARSIZE_4B(PTR,len) \
  (((varattrib_4b *) (PTR))->va_4byte.va_header = (((uint32) (len)) << 2))
#define SET_VARSIZE_4B_C(PTR,len) \
  (((varattrib_4b *) (PTR))->va_4byte.va_header = (((uint32) (len)) << 2) | 0x02)
#define SET_VARSIZE_1B(PTR,len) \
  (((varattrib_1b *) (PTR))->va_header = (((uint8) (len)) << 1) | 0x01)
#define SET_VARTAG_1B_E(PTR,tag) \
  (((varattrib_1b_e *) (PTR))->va_header = 0x01, \
   ((varattrib_1b_e *) (PTR))->va_tag = (tag))

#endif              /* WORDS_BIGENDIAN */

#define VARDATA_4B(PTR)   (((varattrib_4b *) (PTR))->va_4byte.va_data)
#define VARDATA_4B_C(PTR) (((varattrib_4b *) (PTR))->va_compressed.va_data)
#define VARDATA_1B(PTR)   (((varattrib_1b *) (PTR))->va_data)
#define VARDATA_1B_E(PTR) (((varattrib_1b_e *) (PTR))->va_data)

/*
 * Externally visible TOAST functions and macros begin here.  All of these
 * were originally macros, accounting for the upper-case naming.
 *
 * Most of these functions accept a pointer to a value of a toastable data
 * type.  The caller's variable might be declared "text *" or the like,
 * so we use "void *" here.  Callers that are working with a Datum variable
 * must apply DatumGetPointer before calling these functions.
 */

#define VARHDRSZ_EXTERNAL   offsetof(varattrib_1b_e, va_data)
#define VARHDRSZ_COMPRESSED   offsetof(varattrib_4b, va_compressed.va_data)
#define VARHDRSZ_SHORT      offsetof(varattrib_1b, va_data)
#define VARATT_SHORT_MAX    0x7F

/*
 * In consumers oblivious to data alignment, call PG_DETOAST_DATUM_PACKED(),
 * VARDATA_ANY(), VARSIZE_ANY() and VARSIZE_ANY_EXHDR().  Elsewhere, call
 * PG_DETOAST_DATUM(), VARDATA() and VARSIZE().  Directly fetching an int16,
 * int32 or wider field in the struct representing the datum layout requires
 * aligned data.  memcpy() is alignment-oblivious, as are most operations on
 * datatypes, such as text, whose layout struct contains only char fields.
 *
 * Code assembling a new datum should call VARDATA() and SET_VARSIZE().
 * (Datums begin life untoasted.)
 *
 * Other functions here should usually be used only by tuple assembly/disassembly
 * code and code that specifically wants to work with still-toasted Datums.
 */

/* Size of a known-not-toasted varlena datum, including header */
static inline Size
VARSIZE(const void *PTR)
{
  return VARSIZE_4B(PTR);
}

/* Start of data area of a known-not-toasted varlena datum */
static inline char *
VARDATA(const void *PTR)
{
  return VARDATA_4B(PTR);
}

/* Size of a known-short-header varlena datum, including header */
static inline Size
VARSIZE_SHORT(const void *PTR)
{
  return VARSIZE_1B(PTR);
}

/* Start of data area of a known-short-header varlena datum */
static inline char *
VARDATA_SHORT(const void *PTR)
{
  return VARDATA_1B(PTR);
}

/* Type tag of a "TOAST pointer" datum */
static inline vartag_external
VARTAG_EXTERNAL(const void *PTR)
{
  return VARTAG_1B_E(PTR);
}

/* Size of a "TOAST pointer" datum, including header */
static inline Size
VARSIZE_EXTERNAL(const void *PTR)
{
  return VARHDRSZ_EXTERNAL + VARTAG_SIZE(VARTAG_EXTERNAL(PTR));
}

/* Start of data area of a "TOAST pointer" datum */
static inline char *
VARDATA_EXTERNAL(const void *PTR)
{
  return VARDATA_1B_E(PTR);
}

/* Is varlena datum in inline-compressed format? */
static inline bool
VARATT_IS_COMPRESSED(const void *PTR)
{
  return VARATT_IS_4B_C(PTR);
}

/* Is varlena datum a "TOAST pointer" datum? */
static inline bool
VARATT_IS_EXTERNAL(const void *PTR)
{
  return VARATT_IS_1B_E(PTR);
}

/* Is varlena datum a pointer to on-disk toasted data? */
static inline bool
VARATT_IS_EXTERNAL_ONDISK(const void *PTR)
{
  vartag_external tag;

  if (!VARATT_IS_EXTERNAL(PTR))
    return false;
  tag = VARTAG_EXTERNAL(PTR);
  return (tag == VARTAG_ONDISK_OID || tag == VARTAG_ONDISK_OID8);
}

/* Is varlena datum an indirect pointer? */
static inline bool
VARATT_IS_EXTERNAL_INDIRECT(const void *PTR)
{
  return VARATT_IS_EXTERNAL(PTR) && VARTAG_EXTERNAL(PTR) == VARTAG_INDIRECT;
}

/* Is varlena datum a read-only pointer to an expanded object? */
static inline bool
VARATT_IS_EXTERNAL_EXPANDED_RO(const void *PTR)
{
  return VARATT_IS_EXTERNAL(PTR) && VARTAG_EXTERNAL(PTR) == VARTAG_EXPANDED_RO;
}

/* Is varlena datum a read-write pointer to an expanded object? */
static inline bool
VARATT_IS_EXTERNAL_EXPANDED_RW(const void *PTR)
{
  return VARATT_IS_EXTERNAL(PTR) && VARTAG_EXTERNAL(PTR) == VARTAG_EXPANDED_RW;
}

/* Is varlena datum either type of pointer to an expanded object? */
static inline bool
VARATT_IS_EXTERNAL_EXPANDED(const void *PTR)
{
  return VARATT_IS_EXTERNAL(PTR) && VARTAG_IS_EXPANDED(VARTAG_EXTERNAL(PTR));
}

/* Is varlena datum a "TOAST pointer", but not for an expanded object? */
static inline bool
VARATT_IS_EXTERNAL_NON_EXPANDED(const void *PTR)
{
  return VARATT_IS_EXTERNAL(PTR) && !VARTAG_IS_EXPANDED(VARTAG_EXTERNAL(PTR));
}

/* Is varlena datum a short-header datum? */
static inline bool
VARATT_IS_SHORT(const void *PTR)
{
  return VARATT_IS_1B(PTR);
}

/* Is varlena datum not in traditional (4-byte-header, uncompressed) format? */
static inline bool
VARATT_IS_EXTENDED(const void *PTR)
{
  return !VARATT_IS_4B_U(PTR);
}

/* Is varlena datum short enough to convert to short-header format? */
static inline bool
VARATT_CAN_MAKE_SHORT(const void *PTR)
{
  return VARATT_IS_4B_U(PTR) &&
    (VARSIZE(PTR) - VARHDRSZ + VARHDRSZ_SHORT) <= VARATT_SHORT_MAX;
}

/* Size that datum will have in short-header format, including header */
static inline Size
VARATT_CONVERTED_SHORT_SIZE(const void *PTR)
{
  return VARSIZE(PTR) - VARHDRSZ + VARHDRSZ_SHORT;
}

/* Set the size (including header) of a 4-byte-header varlena datum */
static inline void
SET_VARSIZE(void *PTR, Size len)
{
  SET_VARSIZE_4B(PTR, len);
}

/* Set the size (including header) of a short-header varlena datum */
static inline void
SET_VARSIZE_SHORT(void *PTR, Size len)
{
  SET_VARSIZE_1B(PTR, len);
}

/* Set the size (including header) of an inline-compressed varlena datum */
static inline void
SET_VARSIZE_COMPRESSED(void *PTR, Size len)
{
  SET_VARSIZE_4B_C(PTR, len);
}

/* Set the type tag of a "TOAST pointer" datum */
static inline void
SET_VARTAG_EXTERNAL(void *PTR, vartag_external tag)
{
  SET_VARTAG_1B_E(PTR, tag);
}

/* Size of a varlena datum of any format, including header */
static inline Size
VARSIZE_ANY(const void *PTR)
{
  if (VARATT_IS_1B_E(PTR))
    return VARSIZE_EXTERNAL(PTR);
  else if (VARATT_IS_1B(PTR))
    return VARSIZE_1B(PTR);
  else
    return VARSIZE_4B(PTR);
}

/* Size of a varlena datum of any format, excluding header */
static inline Size
VARSIZE_ANY_EXHDR(const void *PTR)
{
  if (VARATT_IS_1B_E(PTR))
    return VARSIZE_EXTERNAL(PTR) - VARHDRSZ_EXTERNAL;
  else if (VARATT_IS_1B(PTR))
    return VARSIZE_1B(PTR) - VARHDRSZ_SHORT;
  else
    return VARSIZE_4B(PTR) - VARHDRSZ;
}

/* Start of data area of a plain or short-header varlena datum */
/* caution: this will not work on an external or compressed-in-line Datum */
/* caution: this will return a possibly unaligned pointer */
static inline char *
VARDATA_ANY(const void *PTR)
{
  return VARATT_IS_1B(PTR) ? VARDATA_1B(PTR) : VARDATA_4B(PTR);
}

/* Decompressed size of a compressed-in-line varlena datum */
static inline Size
VARDATA_COMPRESSED_GET_EXTSIZE(const void *PTR)
{
  return ((const varattrib_4b *) PTR)->va_compressed.va_tcinfo & VARLENA_EXTSIZE_MASK;
}

/* Compression method of a compressed-in-line varlena datum */
static inline uint32
VARDATA_COMPRESSED_GET_COMPRESS_METHOD(const void *PTR)
{
  return ((const varattrib_4b *) PTR)->va_compressed.va_tcinfo >> VARLENA_EXTSIZE_BITS;
}

/*
 * Set the decompressed size and the compression method of a
 * compressed-in-line varlena datum.  The length word is not touched here; the
 * caller is expected to have set it with SET_VARSIZE_COMPRESSED().
 */
static inline void
VARDATA_COMPRESSED_SET_TCINFO(void *PTR, uint32 extsize, uint32 cmethod)
{
  Assert(extsize > 0 && extsize <= VARLENA_EXTSIZE_MASK);
  Assert(cmethod < (1U << (32 - VARLENA_EXTSIZE_BITS)));

  ((varattrib_4b *) PTR)->va_compressed.va_tcinfo =
    extsize | (cmethod << VARLENA_EXTSIZE_BITS);
}

/*
 * Same for external Datums, saved into a va_extinfo.
 */
static inline Size
VARATT_EXTINFO_GET_EXTSIZE(uint32 extinfo)
{
  return extinfo & VARLENA_EXTSIZE_MASK;
}

static inline uint32
VARATT_EXTINFO_GET_COMPRESS_METHOD(uint32 extinfo)
{
  return extinfo >> VARLENA_EXTSIZE_BITS;
}

/*
 * Testing whether an externally-stored value is compressed requires comparing
 * the saved size stored in va_extinfo (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.
 */
static inline bool
VARATT_EXTINFO_IS_COMPRESSED(uint32 extinfo, int32 rawsize)
{
  return VARATT_EXTINFO_GET_EXTSIZE(extinfo) < (Size) (rawsize - VARHDRSZ);
}

#endif

Coverage Report

Created: 2026-09-28 06:55

Line	Count	Source
1		/*-------------------------------------------------------------------------
2		*
3		* varatt.h
4		* variable-length datatypes (TOAST support)
5		*
6		*
7		* Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
8		* Portions Copyright (c) 1995, Regents of the University of California
9		*
10		* src/include/varatt.h
11		*
12		*-------------------------------------------------------------------------
13		*/
14
15		#ifndef VARATT_H
16		#define VARATT_H
17
18		/*
19		* varatt_external_oid is a traditional "TOAST pointer", that is, the
20		* information needed to fetch a Datum stored out-of-line in a TOAST table.
21		* The data is compressed if and only if the external size stored in
22		* va_extinfo is less than va_rawsize - VARHDRSZ.
23		*
24		* This struct must not contain any padding, because we sometimes compare
25		* these pointers using memcmp.
26		*
27		* Note that this information is stored unaligned within actual tuples, so
28		* you need to memcpy from the tuple into a local struct variable before
29		* you can look at these fields! (The reason we use memcmp is to avoid
30		* having to do that just to detect equality of two TOAST pointers...)
31		*/
32		typedef struct varatt_external_oid
33		{
34		int32 va_rawsize; /* Original data size (includes header) */
35		uint32 va_extinfo; /* External saved size (without header) and
36		* compression method */
37		Oid va_valueid; /* Unique ID of value within TOAST table */
38		Oid va_toastrelid; /* RelID of TOAST table containing it */
39		} varatt_external_oid;