PostgreSQL 源码解读（235）- 查询#128(获取属性值：slot_deform_heap_tuple)

来源：这里教程网时间：2026-03-14 19:43:21 作者：

本节简单解释了PostgreSQL执行查询时获取元组属性值实现。
测试数据如下：
```
[local]:5432 pg12@testdb=# create table t_getattrs(id int,col_varchar varchar(20),col_char char(10),col_double float,col_numeric numeric);
CREATE TABLE
Time: 12425.991 ms (00:12.426)
[local]:5432 pg12@testdb=# insert into t_getattrs values(1,'test','test',1234.45,12345.77777);
INSERT 0 1
Time: 30.281 ms
[local]:5432 pg12@testdb=# select * from t_getattrs;
```
### 一、数据结构
**TupleTableSlot**
```

/*----------
 * The executor stores tuples in a "tuple table" which is a List of
 * independent TupleTableSlots.  There are several cases we need to handle:
 *      1. physical tuple in a disk buffer page
 *      2. physical tuple constructed in palloc'ed memory
 *      3. "minimal" physical tuple constructed in palloc'ed memory
 *      4. "virtual" tuple consisting of Datum/isnull arrays
 * 执行器在"tuple table"中存储元组,这个表是各自独立的TupleTableSlots链表.
 * 有以下情况需要处理:
 *      1. 磁盘缓存页中的物理元组
 *      2. 在已分配内存中构造的物理元组
 *      3. 在已分配内存中构造的"minimal"物理元组
 *      4. 含有Datum/isnull数组的"virtual"虚拟元组
 *
 * The first two cases are similar in that they both deal with "materialized"
 * tuples, but resource management is different.  For a tuple in a disk page
 * we need to hold a pin on the buffer until the TupleTableSlot's reference
 * to the tuple is dropped; while for a palloc'd tuple we usually want the
 * tuple pfree'd when the TupleTableSlot's reference is dropped.
 * 最上面2种情况跟"物化"元组的处理方式类似,但资源管理是不同的.
 * 对于在磁盘页中的元组,需要pin在缓存中直至TupleTableSlot依赖的元组被清除,
 *   而对于通过palloc分配的元组在TupleTableSlot依赖被清除后通常希望使用pfree释放
 *
 * A "minimal" tuple is handled similarly to a palloc'd regular tuple.
 * At present, minimal tuples never are stored in buffers, so there is no
 * parallel to case 1.  Note that a minimal tuple has no "system columns".
 * (Actually, it could have an OID, but we have no need to access the OID.)
 * "minimal"元组与通常的palloc分配的元组处理类似.
 * 截止目前为止,"minimal"元组不会存储在缓存中,因此对于第一种情况不会存在并行的问题.
 * 注意"minimal"没有"system columns"系统列
 * (实际上,可以有OID,但不需要访问OID列)
 *
 * A "virtual" tuple is an optimization used to minimize physical data
 * copying in a nest of plan nodes.  Any pass-by-reference Datums in the
 * tuple point to storage that is not directly associated with the
 * TupleTableSlot; generally they will point to part of a tuple stored in
 * a lower plan node's output TupleTableSlot, or to a function result
 * constructed in a plan node's per-tuple econtext.  It is the responsibility
 * of the generating plan node to be sure these resources are not released
 * for as long as the virtual tuple needs to be valid.  We only use virtual
 * tuples in the result slots of plan nodes --- tuples to be copied anywhere
 * else need to be "materialized" into physical tuples.  Note also that a
 * virtual tuple does not have any "system columns".
 * "virtual"元组是用于在嵌套计划节点中拷贝时最小化物理数据的优化.
 * 所有通过引用传递指向与TupleTableSlot非直接相关的存储的元组的Datums使用,
 *   通常它们会指向存储在低层节点输出的TupleTableSlot中的元组的一部分,
 *   或者指向在计划节点的per-tuple内存上下文econtext中构造的函数结果.
 * 产生计划节点的时候有责任确保这些资源未被释放,确保virtual元组是有效的.
 * 我们使用计划节点中的结果slots中的虚拟元组 --- 元组会拷贝到其他地方需要"物化"到物理元组中.
 * 注意virtual元组不需要有"system columns"
 *
 * It is also possible for a TupleTableSlot to hold both physical and minimal
 * copies of a tuple.  This is done when the slot is requested to provide
 * the format other than the one it currently holds.  (Originally we attempted
 * to handle such requests by replacing one format with the other, but that
 * had the fatal defect of invalidating any pass-by-reference Datums pointing
 * into the existing slot contents.)  Both copies must contain identical data
 * payloads when this is the case.
 * TupleTableSlot包含物理和minimal元组拷贝是可能的.
 * 在slot需要提供格式化而不是当前持有的格式时会出现这种情况.
 * (原始的情况是我们准备通过另外一种格式进行替换来处理这种请求,但在校验引用传递Datums时会出现致命错误)
 * 同时在这种情况下,拷贝必须含有唯一的数据payloads.
 *
 * The Datum/isnull arrays of a TupleTableSlot serve double duty.  When the
 * slot contains a virtual tuple, they are the authoritative data.  When the
 * slot contains a physical tuple, the arrays contain data extracted from
 * the tuple.  (In this state, any pass-by-reference Datums point into
 * the physical tuple.)  The extracted information is built "lazily",
 * ie, only as needed.  This serves to avoid repeated extraction of data
 * from the physical tuple.
 * TupleTableSlot中的Datum/isnull数组有双重职责.
 * 在slot包含虚拟元组时,它们是authoritative(权威)数据.
 * 在slot包含物理元组时,时包含从元组中提取的数据的数组.
 * (在这种情况下,所有通过引用传递的Datums指向物理元组)
 * 提取的信息通过'lazily'在需要的时候才构建.
 * 这样可以避免从物理元组的重复数据提取.
 *
 * A TupleTableSlot can also be "empty", holding no valid data.  This is
 * the only valid state for a freshly-created slot that has not yet had a
 * tuple descriptor assigned to it.  In this state, tts_isempty must be
 * true, tts_shouldFree false, tts_tuple NULL, tts_buffer InvalidBuffer,
 * and tts_nvalid zero.
 * TupleTableSlot可能为"empty",没有有效数据.
 * 对于新鲜创建仍未分配描述的的slot来说这是唯一有效的状态.
 * 在这种状态下,tts_isempty必须为T,tts_shouldFree为F, tts_tuple为NULL,
 *   tts_buffer为InvalidBuffer,tts_nvalid为0.
 *
 * The tupleDescriptor is simply referenced, not copied, by the TupleTableSlot
 * code.  The caller of ExecSetSlotDescriptor() is responsible for providing
 * a descriptor that will live as long as the slot does.  (Typically, both
 * slots and descriptors are in per-query memory and are freed by memory
 * context deallocation at query end; so it's not worth providing any extra
 * mechanism to do more.  However, the slot will increment the tupdesc
 * reference count if a reference-counted tupdesc is supplied.)
 * tupleDescriptor只是简单的引用并没有通过TupleTableSlot中的代码进行拷贝.
 * ExecSetSlotDescriptor()的调用者有责任提供与slot生命周期一样的描述符.
 * (典型的,不管是slots还是描述符会在per-query内存中,
 *  并且会在查询结束时通过内存上下文的析构器释放,因此不需要提供额外的机制来处理.
 *  但是,如果使用了引用计数型tupdesc,slot会增加tupdesc引用计数)
 *
 * When tts_shouldFree is true, the physical tuple is "owned" by the slot
 * and should be freed when the slot's reference to the tuple is dropped.
 * 在tts_shouldFree为T的情况下,物理元组由slot持有,并且在slot引用元组被清除时释放内存.
 *
 * If tts_buffer is not InvalidBuffer, then the slot is holding a pin
 * on the indicated buffer page; drop the pin when we release the
 * slot's reference to that buffer.  (tts_shouldFree should always be
 * false in such a case, since presumably tts_tuple is pointing at the
 * buffer page.)
 * 如tts_buffer不是InvalidBuffer,那么slot持有缓存页中的pin,在释放引用该buffer的slot时会清除该pin.
 * (tts_shouldFree通常来说应为F,因为tts_tuple会指向缓存页)
 *
 * tts_nvalid indicates the number of valid columns in the tts_values/isnull
 * arrays.  When the slot is holding a "virtual" tuple this must be equal
 * to the descriptor's natts.  When the slot is holding a physical tuple
 * this is equal to the number of columns we have extracted (we always
 * extract columns from left to right, so there are no holes).
 * tts_nvalid指示了tts_values/isnull数组中的有效列数.
 * 如果slot含有虚拟元组,该字段必须跟描述符的natts一样.
 * 在slot含有物理元组时,该字段等于我们提取的列数.
 * (我们通常从左到右提取列,因此不会有空洞存在)
 *
 * tts_values/tts_isnull are allocated when a descriptor is assigned to the
 * slot; they are of length equal to the descriptor's natts.
 * 在描述符分配给slot时tts_values/tts_isnull会被分配内存,长度与描述符natts长度一样.
 *
 * tts_mintuple must always be NULL if the slot does not hold a "minimal"
 * tuple.  When it does, tts_mintuple points to the actual MinimalTupleData
 * object (the thing to be pfree'd if tts_shouldFreeMin is true).  If the slot
 * has only a minimal and not also a regular physical tuple, then tts_tuple
 * points at tts_minhdr and the fields of that struct are set correctly
 * for access to the minimal tuple; in particular, tts_minhdr.t_data points
 * MINIMAL_TUPLE_OFFSET bytes before tts_mintuple.  This allows column
 * extraction to treat the case identically to regular physical tuples.
 * 如果slot没有包含minimal元组,tts_mintuple通常必须为NULL.
 * 如含有,则tts_mintuple执行实际的MinimalTupleData对象(如tts_shouldFreeMin为T,则需要通过pfree释放内存).
 * 如果slot只有一个minimal而没有通常的物理元组,那么tts_tuple指向tts_minhdr,
 *   结构体的其他字段会被正确的设置为用于访问minimal元组.
 *   特别的, tts_minhdr.t_data指向tts_mintuple前的MINIMAL_TUPLE_OFFSET字节.
 * 这可以让列提取可以独立处理通常的物理元组.
 *
 * tts_slow/tts_off are saved state for slot_deform_tuple, and should not
 * be touched by any other code.
 * tts_slow/tts_off用于存储slot_deform_tuple状态,不应通过其他代码修改.
 *----------
 */
typedef struct TupleTableSlot
{
    NodeTag     type;//Node标记
    //如slot为空,则为T
    bool        tts_isempty;    /* true = slot is empty */
    //是否需要pfree tts_tuple?
    bool        tts_shouldFree; /* should pfree tts_tuple? */
    //是否需要pfree tts_mintuple?
    bool        tts_shouldFreeMin;  /* should pfree tts_mintuple? */
#define FIELDNO_TUPLETABLESLOT_SLOW 4
    //为slot_deform_tuple存储状态?
    bool        tts_slow;       /* saved state for slot_deform_tuple */
#define FIELDNO_TUPLETABLESLOT_TUPLE 5
    //物理元组,如为虚拟元组则为NULL
    HeapTuple   tts_tuple;      /* physical tuple, or NULL if virtual */
#define FIELDNO_TUPLETABLESLOT_TUPLEDESCRIPTOR 6
    //slot中的元组描述符
    TupleDesc   tts_tupleDescriptor;    /* slot's tuple descriptor */
    //slot所在的上下文
    MemoryContext tts_mcxt;     /* slot itself is in this context */
    //元组缓存,如无则为InvalidBuffer
    Buffer      tts_buffer;     /* tuple's buffer, or InvalidBuffer */
#define FIELDNO_TUPLETABLESLOT_NVALID 9
    //tts_values中的有效值
    int         tts_nvalid;     /* # of valid values in tts_values */
#define FIELDNO_TUPLETABLESLOT_VALUES 10
    //当前每个属性的值
    Datum      *tts_values;     /* current per-attribute values */
#define FIELDNO_TUPLETABLESLOT_ISNULL 11
    //isnull数组
    bool       *tts_isnull;     /* current per-attribute isnull flags */
    //minimal元组,如无则为NULL
    MinimalTuple tts_mintuple;  /* minimal tuple, or NULL if none */
    //在minimal情况下的工作空间
    HeapTupleData tts_minhdr;   /* workspace for minimal-tuple-only case */
#define FIELDNO_TUPLETABLESLOT_OFF 14
    //slot_deform_tuple的存储状态
    uint32      tts_off;        /* saved state for slot_deform_tuple */
    //不能被变更的描述符(固定描述符)
    bool        tts_fixedTupleDescriptor;   /* descriptor can't be changed */
} TupleTableSlot;

/* base tuple table slot type */
typedef struct TupleTableSlot
{
    NodeTag     type;//Node标记
#define FIELDNO_TUPLETABLESLOT_FLAGS 1
    uint16      tts_flags;      /* 布尔状态;Boolean states */
#define FIELDNO_TUPLETABLESLOT_NVALID 2
    AttrNumber  tts_nvalid;     /* 在tts_values中有多少有效的values;# of valid values in tts_values */
    const TupleTableSlotOps *const tts_ops; /* slot的实际实现;implementation of slot */
#define FIELDNO_TUPLETABLESLOT_TUPLEDESCRIPTOR 4
    TupleDesc   tts_tupleDescriptor;    /* slot的元组描述符;slot's tuple descriptor */
#define FIELDNO_TUPLETABLESLOT_VALUES 5
    Datum      *tts_values;     /* 当前属性值;current per-attribute values */
#define FIELDNO_TUPLETABLESLOT_ISNULL 6
    bool       *tts_isnull;     /* 当前属性isnull标记;current per-attribute isnull flags */
    MemoryContext tts_mcxt;     /*内存上下文; slot itself is in this context */
} TupleTableSlot;

/* routines for a TupleTableSlot implementation */
//TupleTableSlot的"小程序"
struct TupleTableSlotOps
{
    /* Minimum size of the slot */
	//slot的最小化大小
    size_t          base_slot_size;
    /* Initialization. */
    //初始化方法
    void (*init)(TupleTableSlot *slot);
    /* Destruction. */
    //析构方法
    void (*release)(TupleTableSlot *slot);
    /*
     * Clear the contents of the slot. Only the contents are expected to be
     * cleared and not the tuple descriptor. Typically an implementation of
     * this callback should free the memory allocated for the tuple contained
     * in the slot.
     * 清除slot中的内容。
     * 只希望清除内容，而不希望清除元组描述符。
     * 通常，这个回调的实现应该释放为slot中包含的元组分配的内存。
     */
    void (*clear)(TupleTableSlot *slot);
    /*
     * Fill up first natts entries of tts_values and tts_isnull arrays with
     * values from the tuple contained in the slot. The function may be called
     * with natts more than the number of attributes available in the tuple,
     * in which case it should set tts_nvalid to the number of returned
     * columns.
     * 用slot中包含的元组的值填充tts_values和tts_isnull数组的第一个natts条目。
     * 在调用该函数时，natts可能多于元组中可用属性的数量，在这种情况下，
     *   应该将tts_nvalid设置为返回列的数量。
     */
    void (*getsomeattrs)(TupleTableSlot *slot, int natts);
    /*
     * Returns value of the given system attribute as a datum and sets isnull
     * to false, if it's not NULL. Throws an error if the slot type does not
     * support system attributes.
     * 将给定系统属性的值作为基准返回，如果不为NULL，
     *   则将isnull设置为false。如果slot类型不支持系统属性，则引发错误。
     */
    Datum (*getsysattr)(TupleTableSlot *slot, int attnum, bool *isnull);
    /*
     * Make the contents of the slot solely depend on the slot, and not on
     * underlying resources (like another memory context, buffers, etc).
     * 使slot的内容完全依赖于slot，而不是底层资源(如另一个内存上下文、缓冲区等)。
     */
    void (*materialize)(TupleTableSlot *slot);
    /*
     * Copy the contents of the source slot into the destination slot's own
     * context. Invoked using callback of the destination slot.
     * 将源slot的内容复制到目标slot自己的上下文中。
     * 使用目标slot的回调函数调用。
     */
    void (*copyslot) (TupleTableSlot *dstslot, TupleTableSlot *srcslot);
    /*
     * Return a heap tuple "owned" by the slot. It is slot's responsibility to
     * free the memory consumed by the heap tuple. If the slot can not "own" a
     * heap tuple, it should not implement this callback and should set it as
     * NULL.
     * 返回slot“拥有”的堆元组。
     * slot负责释放堆元组分配的内存。
     * 如果slot不能“拥有”堆元组，它不应该实现这个回调函数，应该将它设置为NULL。
     */
    HeapTuple (*get_heap_tuple)(TupleTableSlot *slot);
    /*
     * Return a minimal tuple "owned" by the slot. It is slot's responsibility
     * to free the memory consumed by the minimal tuple. If the slot can not
     * "own" a minimal tuple, it should not implement this callback and should
     * set it as NULL.
     * 返回slot“拥有”的最小元组。
     * slot负责释放最小元组分配的内存。
     * 如果slot不能“拥有”最小元组，它不应该实现这个回调函数，应该将它设置为NULL。
     */
    MinimalTuple (*get_minimal_tuple)(TupleTableSlot *slot);
    /*
     * Return a copy of heap tuple representing the contents of the slot. The
     * copy needs to be palloc'd in the current memory context. The slot
     * itself is expected to remain unaffected. It is *not* expected to have
     * meaningful "system columns" in the copy. The copy is not be "owned" by
     * the slot i.e. the caller has to take responsibilty to free memory
     * consumed by the slot.
     * 返回表示slot内容的堆元组副本。
     * 需要在当前内存上下文中对副本进行内存分配palloc。
     * 预计slot本身不会受到影响。
     * 它不希望在副本中有有意义的“系统列”。副本不是slot“拥有”的，即调用方必须负责释放slot消耗的内存。
     */
    HeapTuple (*copy_heap_tuple)(TupleTableSlot *slot);
    /*
     * Return a copy of minimal tuple representing the contents of the slot. The
     * copy needs to be palloc'd in the current memory context. The slot
     * itself is expected to remain unaffected. It is *not* expected to have
     * meaningful "system columns" in the copy. The copy is not be "owned" by
     * the slot i.e. the caller has to take responsibilty to free memory
     * consumed by the slot.
     * 返回表示slot内容的最小元组的副本。
     * 需要在当前内存上下文中对副本进行palloc。
     * 预计slot本身不会受到影响。
     * 它不希望在副本中有有意义的“系统列”。副本不是slot“拥有”的，即调用方必须负责释放slot消耗的内存。
     */
    MinimalTuple (*copy_minimal_tuple)(TupleTableSlot *slot);
};
typedef struct tupleDesc
{
    int         natts;          /* tuple中的属性数量;number of attributes in the tuple */
    Oid         tdtypeid;       /* tuple类型的组合类型ID;composite type ID for tuple type */
    int32       tdtypmod;       /* tuple类型的typmode;typmod for tuple type */
    int         tdrefcount;     /* 依赖计数,如为-1,则没有依赖;reference count, or -1 if not counting */
    TupleConstr *constr;        /* 约束,如无则为NULL;constraints, or NULL if none */
    /* attrs[N] is the description of Attribute Number N+1 */
	//attrs[N]是第N+1个属性的描述符
    FormData_pg_attribute attrs[FLEXIBLE_ARRAY_MEMBER];
}  *TupleDesc;
```
### 二、源码解读
```
static void
tts_heap_getsomeattrs(TupleTableSlot *slot, int natts)
{
	HeapTupleTableSlot *hslot = (HeapTupleTableSlot *) slot;
	Assert(!TTS_EMPTY(slot));
	slot_deform_heap_tuple(slot, hslot->tuple, &hslot->off, natts);
}

/*
 * slot_deform_heap_tuple
 *		Given a TupleTableSlot, extract data from the slot's physical tuple
 *		into its Datum/isnull arrays.  Data is extracted up through the
 *		natts'th column (caller must ensure this is a legal column number).
 *		给定一个TupleTableSlot，从其中提取数据到Datum/isnull数组中。
 *		数据根据第N个属性列来进行提取。
 *
 *		This is essentially an incremental version of heap_deform_tuple:
 *		on each call we extract attributes up to the one needed, without
 *		re-computing information about previously extracted attributes.
 *		slot->tts_nvalid is the number of attributes already extracted.
 *		slot->tts_nvalid是已完成提取的属性格式。
 *
 * This is marked as always inline, so the different offp for different types
 * of slots gets optimized away.
 */
static pg_attribute_always_inline void
slot_deform_heap_tuple(TupleTableSlot *slot, HeapTuple tuple, uint32 *offp,
					   int natts)
{
	//元组描述符
	TupleDesc	tupleDesc = slot->tts_tupleDescriptor;
	//列值数组
	Datum	   *values = slot->tts_values;
	//isnull标记数组
	bool	   *isnull = slot->tts_isnull;
	//头部信息
	HeapTupleHeader tup = tuple->t_data;
	bool		hasnulls = HeapTupleHasNulls(tuple);
	//属性编号
	int			attnum;
	//指向元组数据的指针
	char	   *tp;				/* ptr to tuple data */
	//偏移
	uint32		off;			/* offset in tuple data */
	//指向tuple中的null bitmap
	bits8	   *bp = tup->t_bits;	/* ptr to null bitmap in tuple */
	//是否可以使用/设置attcacheoff
	bool		slow;			/* can we use/set attcacheoff? */
	/* We can only fetch as many attributes as the tuple has. */
	//只能提取元组中有的属性,获取元组个数
	natts = Min(HeapTupleHeaderGetNatts(tuple->t_data), natts);
	/*
	 * Check whether the first call for this tuple, and initialize or restore
	 * loop state.
	 * 是否第一次调用?
	 */
	attnum = slot->tts_nvalid;
	if (attnum == 0)
	{
		/* Start from the first attribute */
		//从第一个属性开始
		off = 0;
		slow = false;
	}
	else
	{
		/* Restore state from previous execution */
		//从上一次执行中恢复状态
		off = *offp;
		slow = TTS_SLOW(slot);
	}
	//调整指针位置
	tp = (char *) tup + tup->t_hoff;
	for (; attnum < natts; attnum++)
	{
		//获取列值
		Form_pg_attribute thisatt = TupleDescAttr(tupleDesc, attnum);
		if (hasnulls && att_isnull(attnum, bp))
		{
			//null
			values[attnum] = (Datum) 0;
			isnull[attnum] = true;
			slow = true;		/* can't use attcacheoff anymore */
			continue;
		}
		isnull[attnum] = false;
		if (!slow && thisatt->attcacheoff >= 0)
			off = thisatt->attcacheoff;
		else if (thisatt->attlen == -1)
		{
			/*
			 * We can only cache the offset for a varlena attribute if the
			 * offset is already suitably aligned, so that there would be no
			 * pad bytes in any case: then the offset will be valid for either
			 * an aligned or unaligned value.
			 * varlena:无论是否对齐,偏移都是有效的.
			 */
			if (!slow &&
				off == att_align_nominal(off, thisatt->attalign))
				thisatt->attcacheoff = off;
			else
			{
				off = att_align_pointer(off, thisatt->attalign, -1,
										tp + off);
				slow = true;
			}
		}
		else
		{
			/* not varlena, so safe to use att_align_nominal */
			//非varlena:使用att_align_nominal
			off = att_align_nominal(off, thisatt->attalign);
			if (!slow)
				thisatt->attcacheoff = off;
		}
		//获取列值
		values[attnum] = fetchatt(thisatt, tp + off);
		//调整偏移
		off = att_addlength_pointer(off, thisatt->attlen, tp + off);
		if (thisatt->attlen <= 0)
			slow = true;		/* can't use attcacheoff anymore */
	}
	/*
	 * Save state for next execution
	 * 存储状态
	 */
	slot->tts_nvalid = attnum;
	*offp = off;
	if (slow)
		slot->tts_flags |= TTS_FLAG_SLOW;
	else
		slot->tts_flags &= ~TTS_FLAG_SLOW;
}
/* Accessor for the i'th attribute of tupdesc. */
#define TupleDescAttr(tupdesc, i) (&(tupdesc)->attrs[(i)])
#define fetchatt(A,T) fetch_att(T, (A)->attbyval, (A)->attlen)
/*
 * Same, but work from byval/len parameters rather than Form_pg_attribute.
 */
#if SIZEOF_DATUM == 8
#define fetch_att(T,attbyval,attlen) \
( \
	(attbyval) ? \
	( \
		(attlen) == (int) sizeof(Datum) ? \
			*((Datum *)(T)) \
		: \
	  ( \
		(attlen) == (int) sizeof(int32) ? \
			Int32GetDatum(*((int32 *)(T))) \
		: \
		( \
			(attlen) == (int) sizeof(int16) ? \
				Int16GetDatum(*((int16 *)(T))) \
			: \
			( \
				AssertMacro((attlen) == 1), \
				CharGetDatum(*((char *)(T))) \
			) \
		) \
	  ) \
	) \
	: \
	PointerGetDatum((char *) (T)) \
)
#else							/* SIZEOF_DATUM != 8 */
#define fetch_att(T,attbyval,attlen) \
( \
	(attbyval) ? \
	( \
		(attlen) == (int) sizeof(int32) ? \
			Int32GetDatum(*((int32 *)(T))) \
		: \
		( \
			(attlen) == (int) sizeof(int16) ? \
				Int16GetDatum(*((int16 *)(T))) \
			: \
			( \
				AssertMacro((attlen) == 1), \
				CharGetDatum(*((char *)(T))) \
			) \
		) \
	) \
	: \
	PointerGetDatum((char *) (T)) \
)
#endif							/* SIZEOF_DATUM == 8 */

/*
 * DatumGetPointer
 *		Returns pointer value of a datum.
 */
#define DatumGetPointer(X) ((Pointer) (X))
/*
 * PointerGetDatum
 *		Returns datum representation for a pointer.
 */
#define PointerGetDatum(X) ((Datum) (X))
```
### 三、跟踪分析
执行SQL：
```
[local]:5432 pg12@testdb=# select * from t_getattrs;
```
启动gdb，进入断点
```
(gdb) b slot_deform_heap_tuple
Breakpoint 3 at 0x6fdeac: file execTuples.c, line 892.
(gdb) c
Continuing.
Breakpoint 3, slot_deform_heap_tuple (slot=0x12312a0, tuple=0x1231880, offp=0x12312e8, natts=5)
    at execTuples.c:892
892		TupleDesc	tupleDesc = slot->tts_tupleDescriptor;
(gdb) bt
#0  slot_deform_heap_tuple (slot=0x12312a0, tuple=0x1231880, offp=0x12312e8, natts=5) at execTuples.c:892
#1  0x00000000006fd7d6 in tts_buffer_heap_getsomeattrs (slot=0x12312a0, natts=5) at execTuples.c:676
#2  0x00000000006ff7a9 in slot_getsomeattrs_int (slot=0x12312a0, attnum=5) at execTuples.c:1877
#3  0x000000000048ed13 in slot_getsomeattrs (slot=0x12312a0, attnum=5)
    at ../../../../src/include/executor/tuptable.h:345
#4  0x000000000048ed39 in slot_getallattrs (slot=0x12312a0) at ../../../../src/include/executor/tuptable.h:357
#5  0x000000000048f88a in printtup (slot=0x12312a0, self=0x1239a50) at printtup.c:392
#6  0x00000000006efc3c in ExecutePlan (estate=0x1230e38, planstate=0x1231090, use_parallel_mode=false, 
    operation=CMD_SELECT, sendTuples=true, numberTuples=0, direction=ForwardScanDirection, dest=0x1239a50, 
    execute_once=true) at execMain.c:1685
#7  0x00000000006ed9df in standard_ExecutorRun (queryDesc=0x121b978, direction=ForwardScanDirection, count=0, 
    execute_once=true) at execMain.c:364
#8  0x00000000006ed815 in ExecutorRun (queryDesc=0x121b978, direction=ForwardScanDirection, count=0, 
    execute_once=true) at execMain.c:308
#9  0x00000000008f1010 in PortalRunSelect (portal=0x11b9c08, forward=true, count=0, dest=0x1239a50)
    at pquery.c:929
#10 0x00000000008f0cae in PortalRun (portal=0x11b9c08, count=9223372036854775807, isTopLevel=true, 
    run_once=true, dest=0x1239a50, altdest=0x1239a50, completionTag=0x7ffd32962250 "") at pquery.c:770
#11 0x00000000008ead35 in exec_simple_query (query_string=0x1152d98 "select * from t_getattrs;")
    at postgres.c:1215
#12 0x00000000008eefa5 in PostgresMain (argc=1, argv=0x117fda8, dbname=0x117fbf0 "testdb", 
    username=0x114fab8 "pg12") at postgres.c:4236
#13 0x0000000000845915 in BackendRun (port=0x1175bc0) at postmaster.c:4431
#14 0x00000000008450f3 in BackendStartup (port=0x1175bc0) at postmaster.c:4122
---Type <return> to continue, or q <return> to quit---
#15 0x000000000084132f in ServerLoop () at postmaster.c:1704
#16 0x0000000000840be5 in PostmasterMain (argc=1, argv=0x114da70) at postmaster.c:1377
#17 0x0000000000761469 in main (argc=1, argv=0x114da70) at main.c:228
(gdb) 
(gdb) 
```
输入参数
```
(gdb) p *slot --> 元组slot
$1 = {type = T_TupleTableSlot, tts_flags = 16, tts_nvalid = 0, tts_ops = 0xc3e780 <TTSOpsBufferHeapTuple>, 
  tts_tupleDescriptor = 0x7fe1af2fd7a8, tts_values = 0x1231310, tts_isnull = 0x1231338, tts_mcxt = 0x1230d20, 
  tts_tid = {ip_blkid = {bi_hi = 0, bi_lo = 0}, ip_posid = 1}, tts_tableOid = 131110}
(gdb) p tuple
$2 = (HeapTuple) 0x1231880
(gdb) p *tuple --> 元组
$3 = {t_len = 67, t_self = {ip_blkid = {bi_hi = 0, bi_lo = 0}, ip_posid = 1}, t_tableOid = 131110, 
  t_data = 0x7fe18396e438}
(gdb) p *offp --> 偏移
$4 = 0
(gdb) p natts --> 5个属性
$5 = 5
(gdb) 
```
初始化相关变量
```
(gdb) n
893		Datum	   *values = slot->tts_values;
(gdb) 
894		bool	   *isnull = slot->tts_isnull;
(gdb) 
895		HeapTupleHeader tup = tuple->t_data;
(gdb) p *values
$6 = 0
(gdb) p *isnull
$7 = false
(gdb) n
896		bool		hasnulls = HeapTupleHasNulls(tuple);
(gdb) 
900		bits8	   *bp = tup->t_bits;	/* ptr to null bitmap in tuple */
(gdb) 
904		natts = Min(HeapTupleHeaderGetNatts(tuple->t_data), natts);
(gdb) p *bp
$8 = 0 '\000'
(gdb) n
910		attnum = slot->tts_nvalid;
(gdb) p natts
$9 = 5
(gdb) n
911		if (attnum == 0)
(gdb) p attnum
$10 = 0
(gdb) 
```
首次执行,设置偏移等信息以及初始化元组数据指针
```
(gdb) n
914			off = 0;
(gdb) 
915			slow = false;
(gdb) 
924		tp = (char *) tup + tup->t_hoff;
(gdb) p tup->t_hoff
$11 = 24 '\030'
(gdb) p *tup --> 元组头部信息
$12 = {t_choice = {t_heap = {t_xmin = 14764, t_xmax = 0, t_field3 = {t_cid = 0, t_xvac = 0}}, t_datum = {
      datum_len_ = 14764, datum_typmod = 0, datum_typeid = 0}}, t_ctid = {ip_blkid = {bi_hi = 0, bi_lo = 0}, 
    ip_posid = 1}, t_infomask2 = 5, t_infomask = 2306, t_hoff = 24 '\030', t_bits = 0x7fe18396e44f ""}
(gdb) 
```
开始循环获取每个属性的值
```
(gdb) n
926		for (; attnum < natts; attnum++)
(gdb) 
928			Form_pg_attribute thisatt = TupleDescAttr(tupleDesc, attnum);
(gdb) 
930			if (hasnulls && att_isnull(attnum, bp))
```
属性信息
```
(gdb) p thisatt
$13 = (Form_pg_attribute) 0x7fe1af2fd7c0
(gdb) p *thisatt
$14 = {attrelid = 131110, attname = {data = "id", '\000' <repeats 61 times>}, atttypid = 23, attstattarget = -1, 
  attlen = 4, attnum = 1, attndims = 0, attcacheoff = 0, atttypmod = -1, attbyval = true, attstorage = 112 'p', 
  attalign = 105 'i', attnotnull = false, atthasdef = false, atthasmissing = false, attidentity = 0 '\000', 
  attgenerated = 0 '\000', attisdropped = false, attislocal = true, attinhcount = 0, attcollation = 0}
(gdb) 
```
获取第1个属性值，即id的值。注意：fetchatt执行的逻辑是Int32GetDatum(\*((int32 \*)(T)))
```
(gdb) p thisatt->attbyval
$18 = true
(gdb) p thisatt->attlen
$19 = 4
(gdb) p SIZEOF_DATUM
$24 = 8
(gdb) p (int) sizeof(Datum)
$26 = 8
(gdb) p (int) sizeof(int32)
$27 = 4
(gdb) 
(gdb) p Int32GetDatum(*((int32 *)(tp+off)))
$25 = 1
###
(attlen) == (int) sizeof(int32) ? \
			Int32GetDatum(*((int32 *)(T))) \
###			
```
获取第2个属性值，即col_varchar的值。注意：fetchatt执行的逻辑是PointerGetDatum((char \*) (T)) 
```
(gdb) n
973			if (thisatt->attlen <= 0)
(gdb) 
926		for (; attnum < natts; attnum++)
(gdb) 
928			Form_pg_attribute thisatt = TupleDescAttr(tupleDesc, attnum);
(gdb) 
930			if (hasnulls && att_isnull(attnum, bp))
(gdb) p *thisatt
$28 = {attrelid = 131110, attname = {data = "col_varchar", '\000' <repeats 52 times>}, atttypid = 1043, 
  attstattarget = -1, attlen = -1, attnum = 2, attndims = 0, attcacheoff = 4, atttypmod = 24, attbyval = false, 
  attstorage = 120 'x', attalign = 105 'i', attnotnull = false, atthasdef = false, atthasmissing = false, 
  attidentity = 0 '\000', attgenerated = 0 '\000', attisdropped = false, attislocal = true, attinhcount = 0, 
  attcollation = 100}
(gdb) n
938			isnull[attnum] = false;
(gdb) 
940			if (!slow && thisatt->attcacheoff >= 0)
(gdb) 
941				off = thisatt->attcacheoff;
(gdb) 
969			values[attnum] = fetchatt(thisatt, tp + off);
(gdb) p off
$29 = 4
(gdb) p PointerGetDatum((char *) (tp+off)) 
$30 = 140606552073300
(gdb) x/5c PointerGetDatum((char *) (tp+off)) 
0x7fe18396e454:	11 '\v'	116 't'	101 'e'	115 's'	116 't'
(gdb) p (char *)PointerGetDatum((char *) (tp+off)) 
$32 = 0x7fe18396e454 "\vtest\027test      "
(gdb) 
```
获取第2个属性值，即col_char的值。注意：fetchatt执行的逻辑是PointerGetDatum((char \*) (T)) 
```
(gdb) n
971			off = att_addlength_pointer(off, thisatt->attlen, tp + off);
(gdb) 
973			if (thisatt->attlen <= 0)
(gdb) p off
$33 = 9
(gdb) p thisatt->attlen
$34 = -1
(gdb) n
974				slow = true;		/* can't use attcacheoff anymore */
(gdb) 
926		for (; attnum < natts; attnum++)
(gdb) 
928			Form_pg_attribute thisatt = TupleDescAttr(tupleDesc, attnum);
(gdb) 
930			if (hasnulls && att_isnull(attnum, bp))
(gdb) 
938			isnull[attnum] = false;
(gdb) 
940			if (!slow && thisatt->attcacheoff >= 0)
(gdb) 
942			else if (thisatt->attlen == -1)
(gdb) 
950				if (!slow &&
(gdb) 
955					off = att_align_pointer(off, thisatt->attalign, -1,
(gdb) 
957					slow = true;
(gdb) p off
$35 = 9
(gdb) n
969			values[attnum] = fetchatt(thisatt, tp + off);
(gdb) p *thisatt
$36 = {attrelid = 131110, attname = {data = "col_char", '\000' <repeats 55 times>}, atttypid = 1042, 
  attstattarget = -1, attlen = -1, attnum = 3, attndims = 0, attcacheoff = -1, atttypmod = 14, attbyval = false, 
  attstorage = 120 'x', attalign = 105 'i', attnotnull = false, atthasdef = false, atthasmissing = false, 
  attidentity = 0 '\000', attgenerated = 0 '\000', attisdropped = false, attislocal = true, attinhcount = 0, 
  attcollation = 100}
(gdb) p (char *)PointerGetDatum((char *) (tp+off)) 
$37 = 0x7fe18396e459 "\027test      "
(gdb) 
```
获取第4个属性值，即col_double的值。注意：fetchatt执行的逻辑是\*((Datum \*)(T))
```
(gdb) n
971			off = att_addlength_pointer(off, thisatt->attlen, tp + off);
(gdb) 
973			if (thisatt->attlen <= 0)
(gdb) p off
$38 = 20
(gdb) n
974				slow = true;		/* can't use attcacheoff anymore */
(gdb) 
926		for (; attnum < natts; attnum++)
(gdb) 
928			Form_pg_attribute thisatt = TupleDescAttr(tupleDesc, attnum);
(gdb) 
930			if (hasnulls && att_isnull(attnum, bp))
(gdb) 
938			isnull[attnum] = false;
(gdb) 
940			if (!slow && thisatt->attcacheoff >= 0)
(gdb) 
942			else if (thisatt->attlen == -1)
(gdb) 
963				off = att_align_nominal(off, thisatt->attalign);
(gdb) 
965				if (!slow)
(gdb) p off
$39 = 24
(gdb) n
969			values[attnum] = fetchatt(thisatt, tp + off);
(gdb) p *thisatt
$40 = {attrelid = 131110, attname = {data = "col_double", '\000' <repeats 53 times>}, atttypid = 701, 
  attstattarget = -1, attlen = 8, attnum = 4, attndims = 0, attcacheoff = -1, atttypmod = -1, attbyval = true, 
  attstorage = 112 'p', attalign = 100 'd', attnotnull = false, atthasdef = false, atthasmissing = false, 
  attidentity = 0 '\000', attgenerated = 0 '\000', attisdropped = false, attislocal = true, attinhcount = 0, 
  attcollation = 0}
(gdb) p *((Datum *)(tp+off))
$41 = 4653143983961984205
(gdb) p *(double *)((Datum *)(tp+off))
$49 = 1234.45
(gdb) 
```
获取第5个属性值，即col_numeric的值。注意：fetchatt执行的逻辑是PointerGetDatum((char \*) (T)) 
```
(gdb) n
971			off = att_addlength_pointer(off, thisatt->attlen, tp + off);
(gdb) 
973			if (thisatt->attlen <= 0)
(gdb) p off
$50 = 32
(gdb) n
926		for (; attnum < natts; attnum++)
(gdb) 
928			Form_pg_attribute thisatt = TupleDescAttr(tupleDesc, attnum);
(gdb) 
930			if (hasnulls && att_isnull(attnum, bp))
(gdb) 
938			isnull[attnum] = false;
(gdb) 
940			if (!slow && thisatt->attcacheoff >= 0)
(gdb) 
942			else if (thisatt->attlen == -1)
(gdb) 
950				if (!slow &&
(gdb) 
955					off = att_align_pointer(off, thisatt->attalign, -1,
(gdb) 
957					slow = true;
(gdb) 
969			values[attnum] = fetchatt(thisatt, tp + off);
(gdb) p *thisatt
$51 = {attrelid = 131110, attname = {data = "col_numeric", '\000' <repeats 52 times>}, atttypid = 1700, 
  attstattarget = -1, attlen = -1, attnum = 5, attndims = 0, attcacheoff = -1, atttypmod = -1, attbyval = false, 
  attstorage = 109 'm', attalign = 105 'i', attnotnull = false, atthasdef = false, atthasmissing = false, 
  attidentity = 0 '\000', attgenerated = 0 '\000', attisdropped = false, attislocal = true, attinhcount = 0, 
  attcollation = 0}
(gdb) p PointerGetDatum((char *) (tp+off)) 
$52 = 140606552073328
(gdb) x/32c PointerGetDatum((char *) (tp+off)) 
0x7fe18396e470:	23 '\027'	-127 '\201'	-126 '\202'	1 '\001'	0 '\000'	41 ')'	9 '\t'	97 'a'
0x7fe18396e478:	30 '\036'	88 'X'	27 '\033'	0 '\000'	0 '\000'	0 '\000'	0 '\000' 0 '\000'
0x7fe18396e480:	0 '\000'	0 '\000'	0 '\000'	0 '\000'	0 '\000'	0 '\000'	0 '\000'	0 '\000'
0x7fe18396e488:	0 '\000'	0 '\000'	0 '\000'	0 '\000'	0 '\000'	0 '\000'	0 '\000'	0 '\000'
(gdb) x/32x PointerGetDatum((char *) (tp+off)) 
0x7fe18396e470:	0x17	0x81	0x82	0x01	0x00	0x29	0x09	0x61
0x7fe18396e478:	0x1e	0x58	0x1b	0x00	0x00	0x00	0x00	0x00
0x7fe18396e480:	0x00	0x00	0x00	0x00	0x00	0x00	0x00	0x00
0x7fe18396e488:	0x00	0x00	0x00	0x00	0x00	0x00	0x00	0x00
(gdb) 
```
完成函数调用
```
(gdb) n
971			off = att_addlength_pointer(off, thisatt->attlen, tp + off);
(gdb) p values[attnum]
$55 = 140606552073328
(gdb) n
973			if (thisatt->attlen <= 0)
(gdb) 
974				slow = true;		/* can't use attcacheoff anymore */
(gdb) 
926		for (; attnum < natts; attnum++)
(gdb) 
980		slot->tts_nvalid = attnum;
(gdb) 
981		*offp = off;
(gdb) 
982		if (slow)
(gdb) 
983			slot->tts_flags |= TTS_FLAG_SLOW;
(gdb) 
986	}
(gdb) 
tts_buffer_heap_getsomeattrs (slot=0x12312a0, natts=5) at execTuples.c:677
677	}
(gdb) 
```
numeric在PG中的内部实现有待进一步调研。
**DONE**
### 四、参考资料
N/A