datum.c

关系型数据库 Postgresql 6.5.2

/*-------------------------------------------------------------------------
 *
 * datum.c
 *
 * Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $Header: /usr/local/cvsroot/pgsql/src/backend/utils/adt/datum.c,v 1.12.2.1 1999/08/02 05:24:51 scrappy Exp $
 *
 *-------------------------------------------------------------------------
 */
/*
 * In the implementation of the next routines we assume the following:
 *
 * A) if a type is "byVal" then all the information is stored in the
 * Datum itself (i.e. no pointers involved!). In this case the
 * length of the type is always greater than zero and less than
 * "sizeof(Datum)"
 * B) if a type is not "byVal" and it has a fixed length, then
 * the "Datum" always contain a pointer to a stream of bytes.
 * The number of significant bytes are always equal to the length of the
 * type.
 * C) if a type is not "byVal" and is of variable length (i.e. it has
 * length == -1) then "Datum" always points to a "struct varlena".
 * This varlena structure has information about the actual length of this
 * particular instance of the type and about its value.
 *
 */
#include "postgres.h"
#include "utils/datum.h"

/*-------------------------------------------------------------------------
 * datumGetSize
 *
 * Find the "real" size of a datum, given the datum value,
 * its type, whether it is a "by value", and its length.
 *
 * To cut a long story short, usually the real size is equal to the
 * type length, with the exception of variable length types which have
 * a length equal to -1. In this case, we have to look at the value of
 * the datum itself (which is a pointer to a 'varlena' struct) to find
 * its size.
 *-------------------------------------------------------------------------
 */
Size
datumGetSize(Datum value, Oid type, bool byVal, Size len)
{

	struct varlena *s;
	Size		size = 0;

	if (byVal)
	{
		if (len <= sizeof(Datum))
			size = len;
		else
		{
			elog(ERROR,
				 "datumGetSize: Error: type=%ld, byVaL with len=%d",
				 (long) type, len);
		}
	}
	else
	{							/* not byValue */
		if (len == -1)
		{

			/*
			 * variable length type Look at the varlena struct for its
			 * real length...
			 */
			s = (struct varlena *) DatumGetPointer(value);
			if (!PointerIsValid(s))
			{
				elog(ERROR,
					 "datumGetSize: Invalid Datum Pointer");
			}
			size = (Size) VARSIZE(s);
		}
		else
		{

			/*
			 * fixed length type
			 */
			size = len;
		}
	}

	return size;
}

/*-------------------------------------------------------------------------
 * datumCopy
 *
 * make a copy of a datum
 *
 * If the type of the datum is not passed by value (i.e. "byVal=false")
 * then we assume that the datum contains a pointer and we copy all the
 * bytes pointed by this pointer
 *-------------------------------------------------------------------------
 */
Datum
datumCopy(Datum value, Oid type, bool byVal, Size len)
{

	Size		realSize;
	Datum		res;
	char	   *s;


	if (byVal)
		res = value;
	else
	{
		if (value == 0)
			return (Datum) NULL;
		realSize = datumGetSize(value, type, byVal, len);

		/*
		 * the value is a pointer. Allocate enough space and copy the
		 * pointed data.
		 */
		s = (char *) palloc(realSize);
		if (s == NULL)
			elog(ERROR, "datumCopy: out of memory\n");
		memmove(s, DatumGetPointer(value), realSize);
		res = (Datum) s;
	}
	return res;
}

/*-------------------------------------------------------------------------
 * datumFree
 *
 * Free the space occupied by a datum CREATED BY "datumCopy"
 *
 * NOTE: DO NOT USE THIS ROUTINE with datums returned by amgetattr() etc.
 * ONLY datums created by "datumCopy" can be freed!
 *-------------------------------------------------------------------------
 */
#ifdef NOT_USED
void
datumFree(Datum value, Oid type, bool byVal, Size len)
{

	Size		realSize;
	Pointer		s;

	realSize = datumGetSize(value, type, byVal, len);

	if (!byVal)
	{

		/*
		 * free the space palloced by "datumCopy()"
		 */
		s = DatumGetPointer(value);
		pfree(s);
	}
}

#endif

/*-------------------------------------------------------------------------
 * datumIsEqual
 *
 * Return true if two datums are equal, false otherwise
 *
 * NOTE: XXX!
 * We just compare the bytes of the two values, one by one.
 * This routine will return false if there are 2 different
 * representations of the same value (something along the lines
 * of say the representation of zero in one's complement arithmetic).
 *
 *-------------------------------------------------------------------------
 */
bool
datumIsEqual(Datum value1, Datum value2, Oid type, bool byVal, Size len)
{
	Size		size1,
				size2;
	char	   *s1,
			   *s2;

	if (byVal)
	{

		/*
		 * just compare the two datums. NOTE: just comparing "len" bytes
		 * will not do the work, because we do not know how these bytes
		 * are aligned inside the "Datum".
		 */
		if (value1 == value2)
			return true;
		else
			return false;
	}
	else
	{

		/*
		 * byVal = false Compare the bytes pointed by the pointers stored
		 * in the datums.
		 */
		size1 = datumGetSize(value1, type, byVal, len);
		size2 = datumGetSize(value2, type, byVal, len);
		if (size1 != size2)
			return false;
		s1 = (char *) DatumGetPointer(value1);
		s2 = (char *) DatumGetPointer(value2);
		if (!memcmp(s1, s2, size1))
			return true;
		else
			return false;
	}
}