本節簡單介紹了PostgreSQL快取管理(Buffer Manager)中的實現函式ReadBuffer_common->BufferAlloc->BufTableInsert,該函式對於給定的tag和buffer ID,插入到雜湊表中。

### 一、資料結構
**BufferDesc**
共享緩衝區的共享描述符(狀態)資料
```
/*
 * Flags for buffer descriptors
 * buffer描述器標記
 *
 * Note: TAG_VALID essentially means that there is a buffer hashtable
 * entry associated with the buffer's tag.
 * 注意:TAG_VALID本質上意味著有一個與緩衝區的標記相關聯的緩衝區雜湊表條目。
 */
//buffer header鎖定
#define BM_LOCKED               (1U << 22)  /* buffer header is locked */
//資料需要寫入(標記為DIRTY)
#define BM_DIRTY                (1U << 23)  /* data needs writing */
//資料是有效的
#define BM_VALID                (1U << 24)  /* data is valid */
//已分配buffer tag
#define BM_TAG_VALID            (1U << 25)  /* tag is assigned */
//正在R/W
#define BM_IO_IN_PROGRESS       (1U << 26)  /* read or write in progress */
//上一個I/O出現錯誤
#define BM_IO_ERROR             (1U << 27)  /* previous I/O failed */
//開始寫則變DIRTY
#define BM_JUST_DIRTIED         (1U << 28)  /* dirtied since write started */
//存在等待sole pin的其他程式
#define BM_PIN_COUNT_WAITER     (1U << 29)  /* have waiter for sole pin */
//checkpoint發生,必須刷到磁碟上
#define BM_CHECKPOINT_NEEDED    (1U << 30)  /* must write for checkpoint */
//持久化buffer(不是unlogged或者初始化fork)
#define BM_PERMANENT            (1U << 31)  /* permanent buffer (not unlogged,
                                             * or init fork) */

/*
 *  BufferDesc -- shared descriptor/state data for a single shared buffer.
 *  BufferDesc -- 共享緩衝區的共享描述符(狀態)資料
 *
 * Note: Buffer header lock (BM_LOCKED flag) must be held to examine or change
 * the tag, state or wait_backend_pid fields.  In general, buffer header lock
 * is a spinlock which is combined with flags, refcount and usagecount into
 * single atomic variable.  This layout allow us to do some operations in a
 * single atomic operation, without actually acquiring and releasing spinlock;
 * for instance, increase or decrease refcount.  buf_id field never changes
 * after initialization, so does not need locking.  freeNext is protected by
 * the buffer_strategy_lock not buffer header lock.  The LWLock can take care
 * of itself.  The buffer header lock is *not* used to control access to the
 * data in the buffer!
 * 注意:必須持有Buffer header鎖(BM_LOCKED標記)才能檢查或修改tag/state/wait_backend_pid欄位.
 * 通常來說,buffer header lock是spinlock,它與標記位/參考計數/使用計陣列合到單個原子變數中.
 * 這個佈局設計允許我們執行原子操作,而不需要實際獲得或者釋放spinlock(比如,增加或者減少參考計數).
 * buf_id欄位在初始化後不會出現變化,因此不需要鎖定.
 * freeNext透過buffer_strategy_lock鎖而不是buffer header lock保護.
 * LWLock可以很好的處理自己的狀態.
 * 務請注意的是:buffer header lock不用於控制buffer中的資料訪問!
 *
 * It's assumed that nobody changes the state field while buffer header lock
 * is held.  Thus buffer header lock holder can do complex updates of the
 * state variable in single write, simultaneously with lock release (cleaning
 * BM_LOCKED flag).  On the other hand, updating of state without holding
 * buffer header lock is restricted to CAS, which insure that BM_LOCKED flag
 * is not set.  Atomic increment/decrement, OR/AND etc. are not allowed.
 * 假定在持有buffer header lock的情況下,沒有人改變狀態欄位.
 * 持有buffer header lock的程式可以執行在單個寫操作中執行復雜的狀態變數更新,
 *   同步的釋放鎖(清除BM_LOCKED標記).
 * 換句話說,如果沒有持有buffer header lock的狀態更新,會受限於CAS,
 *   這種情況下確保BM_LOCKED沒有被設定.
 * 比如原子的增加/減少(AND/OR)等操作是不允許的.
 *
 * An exception is that if we have the buffer pinned, its tag can't change
 * underneath us, so we can examine the tag without locking the buffer header.
 * Also, in places we do one-time reads of the flags without bothering to
 * lock the buffer header; this is generally for situations where we don't
 * expect the flag bit being tested to be changing.
 * 一種例外情況是如果我們已有buffer pinned,該buffer的tag不能改變(在本程式之下),
 *   因此不需要鎖定buffer header就可以檢查tag了.
 * 同時,在執行一次性的flags讀取時不需要鎖定buffer header.
 * 這種情況通常用於我們不希望正在測試的flag bit將被改變.
 *
 * We can't physically remove items from a disk page if another backend has
 * the buffer pinned.  Hence, a backend may need to wait for all other pins
 * to go away.  This is signaled by storing its own PID into
 * wait_backend_pid and setting flag bit BM_PIN_COUNT_WAITER.  At present,
 * there can be only one such waiter per buffer.
 * 如果其他程式有buffer pinned,那麼程式不能物理的從磁碟頁面中刪除items.
 * 因此,後臺程式需要等待其他pins清除.這可以透過儲存它自己的PID到wait_backend_pid中,
 *   並設定標記位BM_PIN_COUNT_WAITER.
 * 目前,每個緩衝區只能由一個等待程式.
 *
 * We use this same struct for local buffer headers, but the locks are not
 * used and not all of the flag bits are useful either. To avoid unnecessary
 * overhead, manipulations of the state field should be done without actual
 * atomic operations (i.e. only pg_atomic_read_u32() and
 * pg_atomic_unlocked_write_u32()).
 * 本地緩衝頭部使用同樣的結構,但並不需要使用locks,而且並不是所有的標記位都使用.
 * 為了避免不必要的負載,狀態域的維護不需要實際的原子操作
 * (比如只有pg_atomic_read_u32() and pg_atomic_unlocked_write_u32())
 *
 * Be careful to avoid increasing the size of the struct when adding or
 * reordering members.  Keeping it below 64 bytes (the most common CPU
 * cache line size) is fairly important for performance.
 * 在增加或者記錄成員變數時,小心避免增加結構體的大小.
 * 保持結構體大小在64位元組內(通常的CPU快取線大小)對於效能是非常重要的.
 */
typedef struct BufferDesc
{
    //buffer tag
    BufferTag   tag;            /* ID of page contained in buffer */
    //buffer索引編號(0開始),指向相應的buffer pool slot
    int         buf_id;         /* buffer's index number (from 0) */

/* state of the tag, containing flags, refcount and usagecount */
    //tag狀態,包括flags/refcount和usagecount
    pg_atomic_uint32 state;
    //pin-count等待程式ID
    int         wait_backend_pid;   /* backend PID of pin-count waiter */
    //空閒連結串列鏈中下一個空閒的buffer
    int         freeNext;       /* link in freelist chain */

//緩衝區內容鎖
    LWLock      content_lock;   /* to lock access to buffer contents */
} BufferDesc;

```

**BufferTag**
Buffer tag標記了buffer儲存的是磁碟中哪個block
```
/*
 * Buffer tag identifies which disk block the buffer contains.
 * Buffer tag標記了buffer儲存的是磁碟中哪個block
 *
 * Note: the BufferTag data must be sufficient to determine where to write the
 * block, without reference to pg_class or pg_tablespace entries.  It's
 * possible that the backend flushing the buffer doesn't even believe the
 * relation is visible yet (its xact may have started before the xact that
 * created the rel).  The storage manager must be able to cope anyway.
 * 注意:BufferTag必須足以確定如何寫block而不需要參照pg_class或者pg_tablespace資料字典資訊.
 * 有可能後臺程式在重新整理緩衝區的時候深圳不相信關係是可見的(事務可能在建立rel的事務之前).
 * 儲存管理器必須可以處理這些事情.
 *
 * Note: if there's any pad bytes in the struct, INIT_BUFFERTAG will have
 * to be fixed to zero them, since this struct is used as a hash key.
 * 注意:如果在結構體中有填充的位元組,INIT_BUFFERTAG必須將它們固定為零，因為這個結構體用作雜湊鍵.
 */
typedef struct buftag
{
    //物理relation識別符號
    RelFileNode rnode;          /* physical relation identifier */
    ForkNumber  forkNum;
    //相對於relation起始的塊號
    BlockNumber blockNum;       /* blknum relative to begin of reln */
} BufferTag;

```

**HTAB**
雜湊表的頂層控制結構.
```
/*
 * Top control structure for a hashtable --- in a shared table, each backend
 * has its own copy (OK since no fields change at runtime)
 * 雜湊表的頂層控制結構.
 * 在這個共享雜湊表中,每一個後臺程式都有自己的複製
 * (之所以沒有問題是因為fork出來後,在執行期沒有欄位會變化)
 */
struct HTAB
{
    //指向共享的控制資訊
    HASHHDR    *hctl;           /* => shared control information */
    //段開始目錄
    HASHSEGMENT *dir;           /* directory of segment starts */
    //雜湊函式
    HashValueFunc hash;         /* hash function */
    //雜湊鍵比較函式
    HashCompareFunc match;      /* key comparison function */
    //雜湊鍵複製函式
    HashCopyFunc keycopy;       /* key copying function */
    //記憶體分配器
    HashAllocFunc alloc;        /* memory allocator */
    //記憶體上下文
    MemoryContext hcxt;         /* memory context if default allocator used */
    //表名(用於錯誤資訊)
    char       *tabname;        /* table name (for error messages) */
    //如在共享記憶體中,則為T
    bool        isshared;       /* true if table is in shared memory */
    //如為T,則固定大小不能擴充套件
    bool        isfixed;        /* if true, don't enlarge */

/* freezing a shared table isn't allowed, so we can keep state here */
    //不允許凍結共享表,因此這裡會儲存相關狀態
    bool        frozen;         /* true = no more inserts allowed */

/* We keep local copies of these fixed values to reduce contention */
    //儲存這些固定值的本地複製,以減少衝突
    //雜湊鍵長度(以位元組為單位)
    Size        keysize;        /* hash key length in bytes */
    //段大小,必須為2的冪
    long        ssize;          /* segment size --- must be power of 2 */
    //段偏移,ssize的對數
    int         sshift;         /* segment shift = log2(ssize) */
};

/*
 * Header structure for a hash table --- contains all changeable info
 * 雜湊表的頭部結構 -- 儲存所有可變資訊
 *
 * In a shared-memory hash table, the HASHHDR is in shared memory, while
 * each backend has a local HTAB struct.  For a non-shared table, there isn't
 * any functional difference between HASHHDR and HTAB, but we separate them
 * anyway to share code between shared and non-shared tables.
 * 在共享記憶體雜湊表中,HASHHDR位於共享記憶體中,每一個後臺程式都有一個本地HTAB結構.
 * 對於非共享雜湊表,HASHHDR和HTAB沒有任何功能性的不同,
 * 但無論如何,我們還是把它們區分為共享和非共享表.
 */
struct HASHHDR
{
    /*
     * The freelist can become a point of contention in high-concurrency hash
     * tables, so we use an array of freelists, each with its own mutex and
     * nentries count, instead of just a single one.  Although the freelists
     * normally operate independently, we will scavenge entries from freelists
     * other than a hashcode's default freelist when necessary.
     * 在高併發的雜湊表中,空閒連結串列會成為競爭熱點,因此我們使用空閒連結串列陣列,
     *   陣列中的每一個元素都有自己的mutex和條目統計,而不是使用一個.
     *
     * If the hash table is not partitioned, only freeList[0] is used and its
     * spinlock is not used at all; callers' locking is assumed sufficient.
     * 如果雜湊表沒有分割槽,那麼只有freelist[0]元素是有用的,自旋鎖沒有任何用處;
     * 呼叫者鎖定被認為已足夠OK.
     */
    FreeListData freeList[NUM_FREELISTS];

/* These fields can change, but not in a partitioned table */
    //這些域欄位可以改變,但不適用於分割槽表
    /* Also, dsize can't change in a shared table, even if unpartitioned */
    //同時,就算是非分割槽表,共享表的dsize也不能改變
    //目錄大小
    long        dsize;          /* directory size */
    //已分配的段大小(<= dbsize)
    long        nsegs;          /* number of allocated segments (<= dsize) */
    //正在使用的最大桶ID
    uint32      max_bucket;     /* ID of maximum bucket in use */
    //進入整個雜湊表的模掩碼
    uint32      high_mask;      /* mask to modulo into entire table */
    //進入低於半個雜湊表的模掩碼
    uint32      low_mask;       /* mask to modulo into lower half of table */

/* These fields are fixed at hashtable creation */
    //下面這些欄位在雜湊表建立時已固定
    //雜湊鍵大小(以位元組為單位)
    Size        keysize;        /* hash key length in bytes */
    //所有使用者元素大小(以位元組為單位)
    Size        entrysize;      /* total user element size in bytes */
    //分割槽個數(2的冪),或者為0
    long        num_partitions; /* # partitions (must be power of 2), or 0 */
    //目標的填充因子
    long        ffactor;        /* target fill factor */
    //如目錄是固定大小,則該值為dsize的上限值
    long        max_dsize;      /* 'dsize' limit if directory is fixed size */
    //段大小,必須是2的冪
    long        ssize;          /* segment size --- must be power of 2 */
    //端偏移,ssize的對數
    int         sshift;         /* segment shift = log2(ssize) */
    //一次性分配的條目個數
    int         nelem_alloc;    /* number of entries to allocate at once */

#ifdef HASH_STATISTICS

/*
     * Count statistics here.  NB: stats code doesn't bother with mutex, so
     * counts could be corrupted a bit in a partitioned table.
     * 統計資訊.
     * 注意:統計相關的程式碼不會影響mutex,因此對於分割槽表,統計可能有一點點問題
     */
    long        accesses;
    long        collisions;
#endif
};

/*
 * Per-freelist data.
 * 空閒連結串列資料.
 *
 * In a partitioned hash table, each freelist is associated with a specific
 * set of hashcodes, as determined by the FREELIST_IDX() macro below.
 * nentries tracks the number of live hashtable entries having those hashcodes
 * (NOT the number of entries in the freelist, as you might expect).
 * 在一個分割槽雜湊表中,每一個空閒連結串列與特定的hashcodes集合相關,透過下面的FREELIST_IDX()宏進行定義.
 * nentries跟蹤有這些hashcodes的仍存活的hashtable條目個數.
 * (注意不要搞錯,不是空閒的條目個數)
 *
 * The coverage of a freelist might be more or less than one partition, so it
 * needs its own lock rather than relying on caller locking.  Relying on that
 * wouldn't work even if the coverage was the same, because of the occasional
 * need to "borrow" entries from another freelist; see get_hash_entry().
 * 空閒連結串列的覆蓋範圍可能比一個分割槽多或少,因此需要自己的鎖而不能僅僅依賴呼叫者的鎖.
 * 依賴呼叫者鎖在覆蓋面一樣的情況下也不會起效,因為偶爾需要從另一個自由列表“借用”條目,詳細參見get_hash_entry()
 *
 * Using an array of FreeListData instead of separate arrays of mutexes,
 * nentries and freeLists helps to reduce sharing of cache lines between
 * different mutexes.
 * 使用FreeListData陣列而不是一個獨立的mutexes,nentries和freelists陣列有助於減少不同mutexes之間的快取線共享.
 */
typedef struct
{
    //該空閒連結串列的自旋鎖
    slock_t     mutex;          /* spinlock for this freelist */
    //相關桶中的條目個數
    long        nentries;       /* number of entries in associated buckets */
    //空閒元素鏈
    HASHELEMENT *freeList;      /* chain of free elements */
} FreeListData;

/*
 * HASHELEMENT is the private part of a hashtable entry.  The caller's data
 * follows the HASHELEMENT structure (on a MAXALIGN'd boundary).  The hash key
 * is expected to be at the start of the caller's hash entry data structure.
 * HASHELEMENT是雜湊表條目的私有部分.
 * 呼叫者的資料按照HASHELEMENT結構組織(位於MAXALIGN的邊界).
 * 雜湊鍵應位於呼叫者hash條目資料結構的開始位置.
 */
typedef struct HASHELEMENT
{
    //連結到相同桶中的下一個條目
    struct HASHELEMENT *link;   /* link to next entry in same bucket */
    //該條目的雜湊函式結果
    uint32      hashvalue;      /* hash function result for this entry */
} HASHELEMENT;

/* Hash table header struct is an opaque type known only within dynahash.c */
//雜湊表頭部結構,非透明型別,用於dynahash.c
typedef struct HASHHDR HASHHDR;

/* Hash table control struct is an opaque type known only within dynahash.c */
//雜湊表控制結構,非透明型別,用於dynahash.c
typedef struct HTAB HTAB;

/* Parameter data structure for hash_create */
//hash_create使用的引數資料結構
/* Only those fields indicated by hash_flags need be set */
//根據hash_flags標記設定相應的欄位
typedef struct HASHCTL
{
    //分割槽個數(必須是2的冪)
    long        num_partitions; /* # partitions (must be power of 2) */
    //段大小
    long        ssize;          /* segment size */
    //初始化目錄大小
    long        dsize;          /* (initial) directory size */
    //dsize上限
    long        max_dsize;      /* limit to dsize if dir size is limited */
    //填充因子
    long        ffactor;        /* fill factor */
    //雜湊鍵大小(位元組為單位)
    Size        keysize;        /* hash key length in bytes */
    //參見上述資料結構註釋
    Size        entrysize;      /* total user element size in bytes */
    //
    HashValueFunc hash;         /* hash function */
    HashCompareFunc match;      /* key comparison function */
    HashCopyFunc keycopy;       /* key copying function */
    HashAllocFunc alloc;        /* memory allocator */
    MemoryContext hcxt;         /* memory context to use for allocations */
    //共享記憶體中的雜湊頭部結構地址
    HASHHDR    *hctl;           /* location of header in shared mem */
} HASHCTL;

/* A hash bucket is a linked list of HASHELEMENTs */
//雜湊桶是HASHELEMENTs連結串列
typedef HASHELEMENT *HASHBUCKET;

/* A hash segment is an array of bucket headers */
//hash segment是桶陣列
typedef HASHBUCKET *HASHSEGMENT;

/*
 * Hash functions must have this signature.
 * Hash函式必須有它自己的標識
 */
typedef uint32 (*HashValueFunc) (const void *key, Size keysize);

/*
 * Key comparison functions must have this signature.  Comparison functions
 * return zero for match, nonzero for no match.  (The comparison function
 * definition is designed to allow memcmp() and strncmp() to be used directly
 * as key comparison functions.)
 * 雜湊鍵對比函式必須有自己的標識.
 * 如匹配則對比函式返回0,不匹配返回非0.
 * (對比函式定義被設計為允許在對比鍵值時可直接使用memcmp()和strncmp())
 */
typedef int (*HashCompareFunc) (const void *key1, const void *key2,
 Size keysize);

/*
 * Key copying functions must have this signature.  The return value is not
 * used.  (The definition is set up to allow memcpy() and strlcpy() to be
 * used directly.)
 * 鍵複製函式必須有自己的標識.
 * 返回值無用.
 */
typedef void *(*HashCopyFunc) (void *dest, const void *src, Size keysize);

/*
 * Space allocation function for a hashtable --- designed to match malloc().
 * Note: there is no free function API; can't destroy a hashtable unless you
 * use the default allocator.
 * 雜湊表的恐懼分配函式 -- 被設計為與malloc()函式匹配.
 * 注意:這裡沒有釋放函式API;不能銷燬雜湊表,除非使用預設的分配器.
 */
typedef void *(*HashAllocFunc) (Size request);

```

**BufferLookupEnt**
```
/* entry for buffer lookup hashtable */
//檢索hash表的條目
typedef struct
{
    //磁碟page的tag
    BufferTag   key;            /* Tag of a disk page */
    //相關聯的buffer ID
    int         id;             /* Associated buffer ID */
} BufferLookupEnt;

```

### 二、原始碼解讀
BufTableInsert原始碼很簡單,重點是需要理解HTAB資料結構,即全域性變數SharedBufHash的資料結構.
```
/*
 * BufTableInsert
 *      Insert a hashtable entry for given tag and buffer ID,
 *      unless an entry already exists for that tag
 * BufTableInsert
 *      給定tag和buffer ID,插入到雜湊表中,如該tag相應的條目已存在,則不處理.
 *
 * Returns -1 on successful insertion.  If a conflicting entry exists
 * already, returns the buffer ID in that entry.
 * 如成功插入,則返回-1.如衝突的條目已存在,則返回條目的buffer ID.
 *
 * Caller must hold exclusive lock on BufMappingLock for tag's partition
 * 呼叫者必須持有tag分割槽BufMappingLock獨佔鎖.
 */
int
BufTableInsert(BufferTag *tagPtr, uint32 hashcode, int buf_id)
{
    BufferLookupEnt *result;
    bool        found;

Assert(buf_id >= 0);        /* -1 is reserved for not-in-table */
    Assert(tagPtr->blockNum != P_NEW);  /* invalid tag */
   
    //static HTAB *SharedBufHash;
    result = (BufferLookupEnt *)
        hash_search_with_hash_value(SharedBufHash,
                                    (void *) tagPtr,
                                    hashcode,
                                    HASH_ENTER,
                                    &found);

if (found)                  /* found something already in the table */
        return result->id;

result->id = buf_id;

return -1;
}

```

### 三、跟蹤分析
測試指令碼,查詢資料表:
```
10:01:54 (xdb@[local]:5432)testdb=# select * from t1 limit 10;

```
啟動gdb,設定斷點
```
(gdb) 
(gdb) b BufTableInsert
Breakpoint 1 at 0x875c92: file buf_table.c, line 125.
(gdb) c
Continuing.

Breakpoint 1, BufTableInsert (tagPtr=0x7fff0cba0ef0, hashcode=1398580903, buf_id=101) at buf_table.c:125
125     Assert(buf_id >= 0);        /* -1 is reserved for not-in-table */
(gdb) 
```
輸入引數
tagPtr-BufferTag結構體
hashcode=1398580903,
buf_id=101
```
(gdb) p *tagPtr
$1 = {rnode = {spcNode = 1663, dbNode = 16402, relNode = 51439}, forkNum = MAIN_FORKNUM, blockNum = 0}
```
呼叫hash_search_with_hash_value,重點考察SharedBufHash(HTAB指標)
```
(gdb) n
129         hash_search_with_hash_value(SharedBufHash,
```
SharedBufHash
```
(gdb) p *SharedBufHash
$2 = {hctl = 0x7f5489004380, dir = 0x7f54890046d8, hash = 0xa3bf74 <tag_hash>, match = 0x4791a0 <memcmp>, 
  keycopy = 0x479690 <memcpy>, alloc = 0x89250b <shmemallocnoerror>, hcxt = 0x0, 
  tabname = 0x1fbf1d8 "Shared Buffer Lookup Table", isshared = true, isfixed = false, frozen = false, keysize = 20, 
  ssize = 256, sshift = 8}
(gdb) 
```
SharedBufHash->hctl,HASHHDR結構體
freeList是一個陣列
num_partitions是分割槽個數,預設為128
```
(gdb) p *SharedBufHash->hctl
$3 = {freeList = {{mutex = 0 '\000', nentries = 3, freeList = 0x7f5489119700}, {mutex = 0 '\000', nentries = 2, 
      freeList = 0x7f548912d828}, {mutex = 0 '\000', nentries = 4, freeList = 0x7f54891418d8}, {mutex = 0 '\000', 
      nentries = 3, freeList = 0x7f5489155a00}, {mutex = 0 '\000', nentries = 8, freeList = 0x7f5489169a38}, {
      mutex = 0 '\000', nentries = 3, freeList = 0x7f548917dc00}, {mutex = 0 '\000', nentries = 5, 
      freeList = 0x7f5489191cb0}, {mutex = 0 '\000', nentries = 3, freeList = 0x7f54891a5e00}, {mutex = 0 '\000', 
      nentries = 1, freeList = 0x7f54891b9f50}, {mutex = 0 '\000', nentries = 3, freeList = 0x7f54891ce000}, {
      mutex = 0 '\000', nentries = 3, freeList = 0x7f54891e2100}, {mutex = 0 '\000', nentries = 5, 
      freeList = 0x7f54891f61b0}, {mutex = 0 '\000', nentries = 4, freeList = 0x7f548920a2d8}, {mutex = 0 '\000', 
      nentries = 2, freeList = 0x7f548921e428}, {mutex = 0 '\000', nentries = 2, freeList = 0x7f5489232528}, {
      mutex = 0 '\000', nentries = 4, freeList = 0x7f54892465d8}, {mutex = 0 '\000', nentries = 3, 
      freeList = 0x7f548925a700}, {mutex = 0 '\000', nentries = 3, freeList = 0x7f548926e800}, {mutex = 0 '\000', 
      nentries = 5, freeList = 0x7f54892828b0}, {mutex = 0 '\000', nentries = 2, freeList = 0x7f5489296a28}, {
      mutex = 0 '\000', nentries = 4, freeList = 0x7f54892aaad8}, {mutex = 0 '\000', nentries = 4, 
      freeList = 0x7f54892bebd8}, {mutex = 0 '\000', nentries = 5, freeList = 0x7f54892d2cb0}, {mutex = 0 '\000', 
      nentries = 0, freeList = 0x7f54892e6e78}, {mutex = 0 '\000', nentries = 2, freeList = 0x7f54892faf28}, {
      mutex = 0 '\000', nentries = 3, freeList = 0x7f548930f000}, {mutex = 0 '\000', nentries = 4, 
      freeList = 0x7f54893230d8}, {mutex = 0 '\000', nentries = 4, freeList = 0x7f54893371d8}, {mutex = 0 '\000', 
      nentries = 2, freeList = 0x7f548934b328}, {mutex = 0 '\000', nentries = 1, freeList = 0x7f548935f450}, {
      mutex = 0 '\000', nentries = 4, freeList = 0x7f54893734d8}, {mutex = 0 '\000', nentries = 3, 
      freeList = 0x7f5489387600}}, dsize = 512, nsegs = 512, max_bucket = 131071, high_mask = 262143, low_mask = 131071, 
  keysize = 20, entrysize = 24, num_partitions = 128, ffactor = 1, max_dsize = 512, ssize = 256, sshift = 8, 
  nelem_alloc = 51}
(gdb) 
(gdb) p *SharedBufHash->hctl->freeList[0].freeList
$4 = {link = 0x7f54891196d8, hashvalue = 0}
(gdb) p *SharedBufHash->hctl->freeList[0].freeList.link
$5 = {link = 0x7f54891196b0, hashvalue = 0}
(gdb) 
```
SharedBufHash->dir,段開始目錄
```
(gdb) p *SharedBufHash->dir
$6 = (HASHSEGMENT) 0x7f5489005700
(gdb) p **SharedBufHash->dir
$7 = (HASHBUCKET) 0x0
(gdb) p *SharedBufHash->dir[0]
$8 = (HASHBUCKET) 0x0
(gdb) p *SharedBufHash->dir[1]
$9 = (HASHBUCKET) 0x0
(gdb) 
```
雜湊函式為tag_hash
雜湊鍵比較函式是memcmp@plt
雜湊鍵複製函式是memcpy@plt
記憶體分配器是ShmemAllocNoError
記憶體上下文為NULL
表名是Shared Buffer Lookup Table
共享記憶體(isshared=T)
非固定/非凍結/雜湊鍵長度為20B/段大小為256/段偏移為8

執行hash_search_with_hash_value,檢視相關資訊
```
(gdb) n
128     result = (BufferLookupEnt *)
(gdb) 
135     if (found)                  /* found something already in the table */
(gdb) p *SharedBufHash
$10 = {hctl = 0x7f5489004380, dir = 0x7f54890046d8, hash = 0xa3bf74 <tag_hash>, match = 0x4791a0 <memcmp>, 
  keycopy = 0x479690 <memcpy>, alloc = 0x89250b <shmemallocnoerror>, hcxt = 0x0, 
  tabname = 0x1fbf1d8 "Shared Buffer Lookup Table", isshared = true, isfixed = false, frozen = false, keysize = 20, 
  ssize = 256, sshift = 8}
(gdb) p *SharedBufHash->hctl
$11 = {freeList = {{mutex = 0 '\000', nentries = 3, freeList = 0x7f5489119700}, {mutex = 0 '\000', nentries = 2, 
      freeList = 0x7f548912d828}, {mutex = 0 '\000', nentries = 4, freeList = 0x7f54891418d8}, {mutex = 0 '\000', 
      nentries = 3, freeList = 0x7f5489155a00}, {mutex = 0 '\000', nentries = 8, freeList = 0x7f5489169a38}, {
      mutex = 0 '\000', nentries = 3, freeList = 0x7f548917dc00}, {mutex = 0 '\000', nentries = 5, 
      freeList = 0x7f5489191cb0}, {mutex = 0 '\000', nentries = 4, freeList = 0x7f54891a5dd8}, {mutex = 0 '\000', 
      nentries = 1, freeList = 0x7f54891b9f50}, {mutex = 0 '\000', nentries = 3, freeList = 0x7f54891ce000}, {
      mutex = 0 '\000', nentries = 3, freeList = 0x7f54891e2100}, {mutex = 0 '\000', nentries = 5, 
      freeList = 0x7f54891f61b0}, {mutex = 0 '\000', nentries = 4, freeList = 0x7f548920a2d8}, {mutex = 0 '\000', 
      nentries = 2, freeList = 0x7f548921e428}, {mutex = 0 '\000', nentries = 2, freeList = 0x7f5489232528}, {
      mutex = 0 '\000', nentries = 4, freeList = 0x7f54892465d8}, {mutex = 0 '\000', nentries = 3, 
      freeList = 0x7f548925a700}, {mutex = 0 '\000', nentries = 3, freeList = 0x7f548926e800}, {mutex = 0 '\000', 
      nentries = 5, freeList = 0x7f54892828b0}, {mutex = 0 '\000', nentries = 2, freeList = 0x7f5489296a28}, {
      mutex = 0 '\000', nentries = 4, freeList = 0x7f54892aaad8}, {mutex = 0 '\000', nentries = 4, 
      freeList = 0x7f54892bebd8}, {mutex = 0 '\000', nentries = 5, freeList = 0x7f54892d2cb0}, {mutex = 0 '\000', 
      nentries = 0, freeList = 0x7f54892e6e78}, {mutex = 0 '\000', nentries = 2, freeList = 0x7f54892faf28}, {
      mutex = 0 '\000', nentries = 3, freeList = 0x7f548930f000}, {mutex = 0 '\000', nentries = 4, 
      freeList = 0x7f54893230d8}, {mutex = 0 '\000', nentries = 4, freeList = 0x7f54893371d8}, {mutex = 0 '\000', 
      nentries = 2, freeList = 0x7f548934b328}, {mutex = 0 '\000', nentries = 1, freeList = 0x7f548935f450}, {
      mutex = 0 '\000', nentries = 4, freeList = 0x7f54893734d8}, {mutex = 0 '\000', nentries = 3, 
      freeList = 0x7f5489387600}}, dsize = 512, nsegs = 512, max_bucket = 131071, high_mask = 262143, low_mask = 131071, 
  keysize = 20, entrysize = 24, num_partitions = 128, ffactor = 1, max_dsize = 512, ssize = 256, sshift = 8, 
  nelem_alloc = 51}
(gdb) p **SharedBufHash->dir
$12 = (HASHBUCKET) 0x0
(gdb) p *SharedBufHash->dir
$13 = (HASHSEGMENT) 0x7f5489005700
(gdb) p result
$14 = (BufferLookupEnt *) 0x7f54891a5e10
(gdb) p *result
$15 = {key = {rnode = {spcNode = 1663, dbNode = 16402, relNode = 51439}, forkNum = MAIN_FORKNUM, blockNum = 0}, id = 0}
(gdb) p found
$16 = false
```
完成呼叫,返回
```
(gdb) n
138     result->id = buf_id;
(gdb) 
140     return -1;
(gdb) 
141 }
(gdb) 
BufferAlloc (smgr=0x204f430, relpersistence=112 'p', forkNum=MAIN_FORKNUM, blockNum=0, strategy=0x0, 
    foundPtr=0x7fff0cba0fa3) at bufmgr.c:1216
1216            if (buf_id >= 0)
(gdb) 
```
DONE!

### 四、參考資料
[PG Source Code](https://doxygen.postgresql.org)
[Buffer Manager](http://www.interdb.jp/pg/pgsql08.html)</shmemallocnoerror></memcpy></memcmp></tag_hash></shmemallocnoerror></memcpy></memcmp></tag_hash>

PostgreSQL 原始碼解讀（140）- Buffer Manager#5（BufTableInsert函式）

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