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本篇內容主要講解“PostgreSQL執行查詢時獲取元組屬性值實現方法是什么”，感興趣的朋友不妨來看看。本文介紹的方法操作簡單快捷，實用性強。下面就讓小編來帶大家學習“PostgreSQL執行查詢時獲取元組屬性值實現方法是什么”吧!
測試數據如下：
[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;
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
* 執行器在"tuple table"中存儲元組,這個表是各自獨立的TupleTableSlots鏈表.
* 有以下情況需要處理:
* 1. 磁盤緩存頁中的物理元組
* 2. 在已分配內存中構造的物理元組
* 3. 在已分配內存中構造的"minimal"物理元組
* 4. 含有Datum/isnull數組的"virtual"虛擬元組
*
* 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種情況跟"物化"元組的處理方式類似,但資源管理是不同的.
* 對于在磁盤頁中的元組,需要pin在緩存中直至TupleTableSlot依賴的元組被清除,
* 而對于通過palloc分配的元組在TupleTableSlot依賴被清除后通常希望使用pfree釋放
*
* 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"元組與通常的palloc分配的元組處理類似.
* 截止目前為止,"minimal"元組不會存儲在緩存中,因此對于第一種情況不會存在并行的問題.
* 注意"minimal"沒有"system columns"系統列
* (實際上,可以有OID,但不需要訪問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"元組是用于在嵌套計劃節點中拷貝時最小化物理數據的優化.
* 所有通過引用傳遞指向與TupleTableSlot非直接相關的存儲的元組的Datums使用,
* 通常它們會指向存儲在低層節點輸出的TupleTableSlot中的元組的一部分,
* 或者指向在計劃節點的per-tuple內存上下文econtext中構造的函數結果.
* 產生計劃節點的時候有責任確保這些資源未被釋放,確保virtual元組是有效的.
* 我們使用計劃節點中的結果slots中的虛擬元組 --- 元組會拷貝到其他地方需要"物化"到物理元組中.
* 注意virtual元組不需要有"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元組拷貝是可能的.
* 在slot需要提供格式化而不是當前持有的格式時會出現這種情況.
* (原始的情況是我們準備通過另外一種格式進行替換來處理這種請求,但在校驗引用傳遞Datums時會出現致命錯誤)
* 同時在這種情況下,拷貝必須含有唯一的數據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數組有雙重職責.
* 在slot包含虛擬元組時,它們是authoritative(權威)數據.
* 在slot包含物理元組時,時包含從元組中提取的數據的數組.
* (在這種情況下,所有通過引用傳遞的Datums指向物理元組)
* 提取的信息通過'lazily'在需要的時候才構建.
* 這樣可以避免從物理元組的重復數據提取.
*
* 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可能為"empty",沒有有效數據.
* 對于新鮮創建仍未分配描述的的slot來說這是唯一有效的狀態.
* 在這種狀態下,tts_isempty必須為T,tts_shouldFree為F, tts_tuple為NULL,
* tts_buffer為InvalidBuffer,tts_nvalid為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只是簡單的引用并沒有通過TupleTableSlot中的代碼進行拷貝.
* ExecSetSlotDescriptor()的調用者有責任提供與slot生命周期一樣的描述符.
* (典型的,不管是slots還是描述符會在per-query內存中,
* 并且會在查詢結束時通過內存上下文的析構器釋放,因此不需要提供額外的機制來處理.
* 但是,如果使用了引用計數型tupdesc,slot會增加tupdesc引用計數)
*
* 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為T的情況下,物理元組由slot持有,并且在slot引用元組被清除時釋放內存.
*
* 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持有緩存頁中的pin,在釋放引用該buffer的slot時會清除該pin.
* (tts_shouldFree通常來說應為F,因為tts_tuple會指向緩存頁)
*
* 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數組中的有效列數.
* 如果slot含有虛擬元組,該字段必須跟描述符的natts一樣.
* 在slot含有物理元組時,該字段等于我們提取的列數.
* (我們通常從左到右提取列,因此不會有空洞存在)
*
* tts_values/tts_isnull are allocated when a descriptor is assigned to the
* slot; they are of length equal to the descriptor's natts.
* 在描述符分配給slot時tts_values/tts_isnull會被分配內存,長度與描述符natts長度一樣.
*
* 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沒有包含minimal元組,tts_mintuple通常必須為NULL.
* 如含有,則tts_mintuple執行實際的MinimalTupleData對象(如tts_shouldFreeMin為T,則需要通過pfree釋放內存).
* 如果slot只有一個minimal而沒有通常的物理元組,那么tts_tuple指向tts_minhdr,
* 結構體的其他字段會被正確的設置為用于訪問minimal元組.
* 特別的, tts_minhdr.t_data指向tts_mintuple前的MINIMAL_TUPLE_OFFSET字節.
* 這可以讓列提取可以獨立處理通常的物理元組.
*
* tts_slow/tts_off are saved state for slot_deform_tuple, and should not
* be touched by any other code.
* tts_slow/tts_off用于存儲slot_deform_tuple狀態,不應通過其他代碼修改.
*----------
*/
typedef struct TupleTableSlot
{
NodeTag type;//Node標記
//如slot為空,則為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
//為slot_deform_tuple存儲狀態?
bool tts_slow; /* saved state for slot_deform_tuple */
#define FIELDNO_TUPLETABLESLOT_TUPLE 5
//物理元組,如為虛擬元組則為NULL
HeapTuple tts_tuple; /* physical tuple, or NULL if virtual */
#define FIELDNO_TUPLETABLESLOT_TUPLEDESCRIPTOR 6
//slot中的元組描述符
TupleDesc tts_tupleDescriptor; /* slot's tuple descriptor */
//slot所在的上下文
MemoryContext tts_mcxt; /* slot itself is in this context */
//元組緩存,如無則為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
//當前每個屬性的值
Datum *tts_values; /* current per-attribute values */
#define FIELDNO_TUPLETABLESLOT_ISNULL 11
//isnull數組
bool *tts_isnull; /* current per-attribute isnull flags */
//minimal元組,如無則為NULL
MinimalTuple tts_mintuple; /* minimal tuple, or NULL if none */
//在minimal情況下的工作空間
HeapTupleData tts_minhdr; /* workspace for minimal-tuple-only case */
#define FIELDNO_TUPLETABLESLOT_OFF 14
//slot_deform_tuple的存儲狀態
uint32 tts_off; /* saved state for slot_deform_tuple */
//不能被變更的描述符(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標記
#define FIELDNO_TUPLETABLESLOT_FLAGS 1
uint16 tts_flags; /* 布爾狀態;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的實際實現;implementation of slot */
#define FIELDNO_TUPLETABLESLOT_TUPLEDESCRIPTOR 4
TupleDesc tts_tupleDescriptor; /* slot的元組描述符;slot's tuple descriptor */
#define FIELDNO_TUPLETABLESLOT_VALUES 5
Datum *tts_values; /* 當前屬性值;current per-attribute values */
#define FIELDNO_TUPLETABLESLOT_ISNULL 6
bool *tts_isnull; /* 當前屬性isnull標記;current per-attribute isnull flags */
MemoryContext tts_mcxt; /*內存上下文; 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. */
//析構方法
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中的內容。
* 只希望清除內容，而不希望清除元組描述符。
* 通常，這個回調的實現應該釋放為slot中包含的元組分配的內存。
*/
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中包含的元組的值填充tts_values和tts_isnull數組的第一個natts條目。
* 在調用該函數時，natts可能多于元組中可用屬性的數量，在這種情況下，
* 應該將tts_nvalid設置為返回列的數量。
*/
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.
* 將給定系統屬性的值作為基準返回，如果不為NULL，
* 則將isnull設置為false。如果slot類型不支持系統屬性，則引發錯誤。
*/
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的內容完全依賴于slot，而不是底層資源(如另一個內存上下文、緩沖區等)。
*/
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.
* 將源slot的內容復制到目標slot自己的上下文中。
* 使用目標slot的回調函數調用。
*/
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“擁有”的堆元組。
* slot負責釋放堆元組分配的內存。
* 如果slot不能“擁有”堆元組，它不應該實現這個回調函數，應該將它設置為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“擁有”的最小元組。
* slot負責釋放最小元組分配的內存。
* 如果slot不能“擁有”最小元組，它不應該實現這個回調函數，應該將它設置為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內容的堆元組副本。
* 需要在當前內存上下文中對副本進行內存分配palloc。
* 預計slot本身不會受到影響。
* 它不希望在副本中有有意義的“系統列”。副本不是slot“擁有”的，即調用方必須負責釋放slot消耗的內存。
*/
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內容的最小元組的副本。
* 需要在當前內存上下文中對副本進行palloc。
* 預計slot本身不會受到影響。
* 它不希望在副本中有有意義的“系統列”。副本不是slot“擁有”的，即調用方必須負責釋放slot消耗的內存。
*/
MinimalTuple (*copy_minimal_tuple)(TupleTableSlot *slot);
};
typedef struct tupleDesc
{
int natts; /* tuple中的屬性數量;number of attributes in the tuple */
Oid tdtypeid; /* tuple類型的組合類型ID;composite type ID for tuple type */
int32 tdtypmod; /* tuple類型的typmode;typmod for tuple type */
int tdrefcount; /* 依賴計數,如為-1,則沒有依賴;reference count, or -1 if not counting */
TupleConstr *constr; /* 約束,如無則為NULL;constraints, or NULL if none */
/* attrs[N] is the description of Attribute Number N+1 */
//attrs[N]是第N+1個屬性的描述符
FormData_pg_attribute attrs[FLEXIBLE_ARRAY_MEMBER];
} *TupleDesc;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).
* 給定一個TupleTableSlot，從其中提取數據到Datum/isnull數組中。
* 數據根據第N個屬性列來進行提取。
*
* 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是已完成提取的屬性格式。
*
* 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)
{
//元組描述符
TupleDesc tupleDesc = slot->tts_tupleDescriptor;
//列值數組
Datum *values = slot->tts_values;
//isnull標記數組
bool *isnull = slot->tts_isnull;
//頭部信息
HeapTupleHeader tup = tuple->t_data;
bool hasnulls = HeapTupleHasNulls(tuple);
//屬性編號
int attnum;
//指向元組數據的指針
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/設置attcacheoff
bool slow; /* can we use/set attcacheoff? */
/* We can only fetch as many attributes as the tuple has. */
//只能提取元組中有的屬性,獲取元組個數
natts = Min(HeapTupleHeaderGetNatts(tuple->t_data), natts);
/*
* Check whether the first call for this tuple, and initialize or restore
* loop state.
* 是否第一次調用?
*/
attnum = slot->tts_nvalid;
if (attnum == 0)
{
/* Start from the first attribute */
//從第一個屬性開始
off = 0;
slow = false;
}
else
{
/* Restore state from previous execution */
//從上一次執行中恢復狀態
off = *offp;
slow = TTS_SLOW(slot);
}
//調整指針位置
tp = (char *) tup + tup->t_hoff;
for (; attnum < natts; attnum++)
{
//獲取列值
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:無論是否對齊,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;
}
//獲取列值
values[attnum] = fetchatt(thisatt, tp + off);
//調整偏移
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
* 存儲狀態
*/
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))執行SQL：
[local]:5432 pg12@testdb=# select * from t_getattrs;
啟動gdb，進入斷點
(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)
輸入參數
(gdb) p *slot --> 元組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 --> 元組
$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個屬性
$5 = 5
(gdb)初始化相關變量
(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)
首次執行,設置偏移等信息以及初始化元組數據指針
(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 --> 元組頭部信息
$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)開始循環獲取每個屬性的值
(gdb) n 926 for (; attnum < natts; attnum++) (gdb) 928 Form_pg_attribute thisatt = TupleDescAttr(tupleDesc, attnum); (gdb) 930 if (hasnulls && att_isnull(attnum, bp))
屬性信息
(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)獲取第1個屬性值，即id的值。注意：fetchatt執行的邏輯是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))) \ ###
獲取第2個屬性值，即col_varchar的值。注意：fetchatt執行的邏輯是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)獲取第2個屬性值，即col_char的值。注意：fetchatt執行的邏輯是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)獲取第4個屬性值，即col_double的值。注意：fetchatt執行的邏輯是*((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)獲取第5個屬性值，即col_numeric的值。注意：fetchatt執行的邏輯是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)完成函數調用
(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)
到此，相信大家對“PostgreSQL執行查詢時獲取元組屬性值實現方法是什么”有了更深的了解，不妨來實際操作一番吧！這里是億速云網站，更多相關內容可以進入相關頻道進行查詢，關注我們，繼續學習！
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