Postgresql源码（76）执行器专用元组格式TupleTableSlot

typedef struct TupleTableSlot
{
	NodeTag		type;
	uint16		tts_flags;                      /* Boolean states */
	AttrNumber	tts_nvalid;                     /* # of valid values in tts_values */
	const TupleTableSlotOps *const tts_ops;     /* implementation of slot */
	TupleDesc	tts_tupleDescriptor;            /* slot's tuple descriptor */
	Datum	   *tts_values;                     /* current per-attribute values */
	bool	   *tts_isnull;                     /* current per-attribute isnull flags */
	MemoryContext tts_mcxt;                     /* slot itself is in this context */
	ItemPointerData tts_tid;                    /* stored tuple's tid */
	Oid			tts_tableOid;                   /* table oid of tuple */
} TupleTableSlot;

const TupleTableSlotOps TTSOpsHeapTuple = {
	.base_slot_size = sizeof(HeapTupleTableSlot),
	.init = tts_heap_init,
	.release = tts_heap_release,
	.clear = tts_heap_clear,
	.getsomeattrs = tts_heap_getsomeattrs,
	.getsysattr = tts_heap_getsysattr,
	.materialize = tts_heap_materialize,
	.copyslot = tts_heap_copyslot,
	.get_heap_tuple = tts_heap_get_heap_tuple,

	/* A heap tuple table slot can not "own" a minimal tuple. */
	.get_minimal_tuple = NULL,
	.copy_heap_tuple = tts_heap_copy_heap_tuple,
	.copy_minimal_tuple = tts_heap_copy_minimal_tuple
};

const TupleTableSlotOps TTSOpsBufferHeapTuple = {
	.base_slot_size = sizeof(BufferHeapTupleTableSlot),
	.init = tts_buffer_heap_init,
	.release = tts_buffer_heap_release,
	.clear = tts_buffer_heap_clear,
	.getsomeattrs = tts_buffer_heap_getsomeattrs,
	.getsysattr = tts_buffer_heap_getsysattr,
	.materialize = tts_buffer_heap_materialize,
	.copyslot = tts_buffer_heap_copyslot,
	.get_heap_tuple = tts_buffer_heap_get_heap_tuple,

	/* A buffer heap tuple table slot can not "own" a minimal tuple. */
	.get_minimal_tuple = NULL,
	.copy_heap_tuple = tts_buffer_heap_copy_heap_tuple,
	.copy_minimal_tuple = tts_buffer_heap_copy_minimal_tuple
};

const TupleTableSlotOps TTSOpsVirtual = {
	.base_slot_size = sizeof(VirtualTupleTableSlot),
	.init = tts_virtual_init,
	.release = tts_virtual_release,
	.clear = tts_virtual_clear,
	.getsomeattrs = tts_virtual_getsomeattrs,
	.getsysattr = tts_virtual_getsysattr,
	.materialize = tts_virtual_materialize,
	.copyslot = tts_virtual_copyslot,

	/*
	 * A virtual tuple table slot can not "own" a heap tuple or a minimal
	 * tuple.
	 */
	.get_heap_tuple = NULL,
	.get_minimal_tuple = NULL,
	.copy_heap_tuple = tts_virtual_copy_heap_tuple,
	.copy_minimal_tuple = tts_virtual_copy_minimal_tuple
};

const TupleTableSlotOps TTSOpsMinimalTuple = {
	.base_slot_size = sizeof(MinimalTupleTableSlot),
	.init = tts_minimal_init,
	.release = tts_minimal_release,
	.clear = tts_minimal_clear,
	.getsomeattrs = tts_minimal_getsomeattrs,
	.getsysattr = tts_minimal_getsysattr,
	.materialize = tts_minimal_materialize,
	.copyslot = tts_minimal_copyslot,

	/* A minimal tuple table slot can not "own" a heap tuple. */
	.get_heap_tuple = NULL,
	.get_minimal_tuple = tts_minimal_get_minimal_tuple,
	.copy_heap_tuple = tts_minimal_copy_heap_tuple,
	.copy_minimal_tuple = tts_minimal_copy_minimal_tuple
};

----------------------       <----- tuple
| HeapTuple          |
----------------------       <----- td     <----- tuple->t_data
| HeapTupleHeader    |
----------------------       <----- td + td->t_hoff    
| isnull             |
----------------------       <------ data    <------ *dataP
| Datums             |     下一列： <------ data    <------ *dataP
|                    |
|                    |     下一列： <------ data    <------ *dataP
|                    |
----------------------   

static inline void
fill_val(Form_pg_attribute att,
		 bits8 **bit,
		 int *bitmask,
		 char **dataP,          
		 uint16 *infomask,
		 Datum datum,
		 bool isnull)
{
	Size		data_length;
	char	   *data = *dataP;

	/*
	 * If we're building a null bitmap, set the appropriate bit for the
	 * current column value here.
	 */
	if (bit != NULL)
	{
		if (*bitmask != HIGHBIT)
			*bitmask <<= 1;
		else
		{
			*bit += 1;
			**bit = 0x0;
			*bitmask = 1;
		}

		if (isnull)
		{
			*infomask |= HEAP_HASNULL;
			return;
		}

		**bit |= *bitmask;
	}

	/*
	 * XXX we use the att_align macros on the pointer value itself, not on an
	 * offset.  This is a bit of a hack.
	 */
	if (att->attbyval)
	{
		/* pass-by-value */
		data = (char *) att_align_nominal(data, att->attalign);
		store_att_byval(data, datum, att->attlen);
		data_length = att->attlen;
	}
	else if (att->attlen == -1)
	{
		/* varlena */
		Pointer		val = DatumGetPointer(datum);

		*infomask |= HEAP_HASVARWIDTH;
		if (VARATT_IS_EXTERNAL(val))
		{
			if (VARATT_IS_EXTERNAL_EXPANDED(val))
			{
				/*
				 * we want to flatten the expanded value so that the
				 * constructed tuple doesn't depend on it
				 */
				ExpandedObjectHeader *eoh = DatumGetEOHP(datum);

				data = (char *) att_align_nominal(data,
												  att->attalign);
				data_length = EOH_get_flat_size(eoh);
				EOH_flatten_into(eoh, data, data_length);
			}
			else
			{
				*infomask |= HEAP_HASEXTERNAL;
				/* no alignment, since it's short by definition */
				data_length = VARSIZE_EXTERNAL(val);
				memcpy(data, val, data_length);
			}
		}
		else if (VARATT_IS_SHORT(val))
		{
			/* no alignment for short varlenas */
			data_length = VARSIZE_SHORT(val);
			memcpy(data, val, data_length);
		}
		else if (VARLENA_ATT_IS_PACKABLE(att) &&
				 VARATT_CAN_MAKE_SHORT(val))
		{
			/* convert to short varlena -- no alignment */
			data_length = VARATT_CONVERTED_SHORT_SIZE(val);
			SET_VARSIZE_SHORT(data, data_length);
			memcpy(data + 1, VARDATA(val), data_length - 1);
		}
		else
		{
			/* full 4-byte header varlena */
			data = (char *) att_align_nominal(data,
											  att->attalign);
			data_length = VARSIZE(val);
			memcpy(data, val, data_length);
		}
	}
	else if (att->attlen == -2)
	{
		/* cstring ... never needs alignment */
		*infomask |= HEAP_HASVARWIDTH;
		Assert(att->attalign == TYPALIGN_CHAR);
		data_length = strlen(DatumGetCString(datum)) + 1;
		memcpy(data, DatumGetPointer(datum), data_length);
	}
	else
	{
		/* fixed-length pass-by-reference */
		data = (char *) att_align_nominal(data, att->attalign);
		Assert(att->attlen > 0);
		data_length = att->attlen;
		memcpy(data, DatumGetPointer(datum), data_length);
	}

	data += data_length;
	*dataP = data;
}