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一、B-tree模块的API

B-tree模块有自己的API，它们是独立于sqlite外部API的。这些API主要是针对SQLite需求量身定做的，因此对于其他的数据库并不适用。要想更加了解SQLite内部机制，先了解这些API是很有必要的。B-tree的另一个优点是本身事务机制、pager处理事务、锁以及日志文件，并为B-tree模块提供支持。B-tree API根据目的大体划分成以下函数：存取及事务函数，表函数 ，游标函数 ，记录函数，配置管理函数 。

（C API负责域值，VDBE负责处理记录，B-tree负责处理键值和数据，pager负责处理页，操作系统接口负责处理二进制数据和原始数据存储。 每个模块负责维护自身在数据库中对应的数据部分，然后依靠底层提供所需信息的初始数据，并从中提取需要的内容。）

二、API功能分析

打开数据库文件和数据库文件加锁等相关的API：

数据库建表，删表相关的API：

遍历表格的游标相关API：

增删改查相关的API:

数据库参数配置相关的API:

图源：http://huili.github.io/B-treeImplementation/b-tree_api.html

sqlite3BtreeOpen函数：

/*
** Open a database file.
** 
** zFilename is the name of the database file.  If zFilename is NULL
** then an ephemeral database is created.  The ephemeral database might
** be exclusively in memory, or it might use a disk-based memory cache.
** Either way, the ephemeral database will be automatically deleted 
** when sqlite3BtreeClose() is called.
**
** If zFilename is ":memory:" then an in-memory database is created
** that is automatically destroyed when it is closed.
**
** The "flags" parameter is a bitmask that might contain bits like
** BTREE_OMIT_JOURNAL and/or BTREE_MEMORY.
**
** If the database is already opened in the same database connection
** and we are in shared cache mode, then the open will fail with an
** SQLITE_CONSTRAINT error.  We cannot allow two or more BtShared
** objects in the same database connection since doing so will lead
** to problems with locking.
*/
int sqlite3BtreeOpen(
  sqlite3_vfs *pVfs,      /* VFS to use for this b-tree */
  const char *zFilename,  /* Name of the file containing the BTree database */
  sqlite3 *db,            /* Associated database handle */
  Btree **ppBtree,        /* Pointer to new Btree object written here */
  int flags,              /* Options */
  int vfsFlags            /* Flags passed through to sqlite3_vfs.xOpen() */
){
  BtShared *pBt = 0;             /* Shared part of btree structure */
  Btree *p;                      /* Handle to return */
  sqlite3_mutex *mutexOpen = 0;  /* Prevents a race condition. Ticket #3537 */
  int rc = SQLITE_OK;            /* Result code from this function */
  u8 nReserve;                   /* Byte of unused space on each page */
  unsigned char zDbHeader[100];  /* Database header content */

  /* True if opening an ephemeral, temporary database */
  const int isTempDb = zFilename==0 || zFilename[0]==0;

  /* Set the variable isMemdb to true for an in-memory database, or 
  ** false for a file-based database.
  */
#ifdef SQLITE_OMIT_MEMORYDB
  const int isMemdb = 0;
#else
  const int isMemdb = (zFilename && strcmp(zFilename, ":memory:")==0)
                       || (isTempDb && sqlite3TempInMemory(db))
                       || (vfsFlags & SQLITE_OPEN_MEMORY)!=0;
#endif

  assert( db!=0 );
  assert( pVfs!=0 );
  assert( sqlite3_mutex_held(db->mutex) );
  assert( (flags&0xff)==flags );   /* flags fit in 8 bits */

  /* Only a BTREE_SINGLE database can be BTREE_UNORDERED */
  assert( (flags & BTREE_UNORDERED)==0 || (flags & BTREE_SINGLE)!=0 );

  /* A BTREE_SINGLE database is always a temporary and/or ephemeral */
  assert( (flags & BTREE_SINGLE)==0 || isTempDb );

  if( isMemdb ){
    flags |= BTREE_MEMORY;
  }
  if( (vfsFlags & SQLITE_OPEN_MAIN_DB)!=0 && (isMemdb || isTempDb) ){
    vfsFlags = (vfsFlags & ~SQLITE_OPEN_MAIN_DB) | SQLITE_OPEN_TEMP_DB;
  }
  p = sqlite3MallocZero(sizeof(Btree));
  if( !p ){
    return SQLITE_NOMEM_BKPT;
  }
  p->inTrans = TRANS_NONE;
  p->db = db;
#ifndef SQLITE_OMIT_SHARED_CACHE
  p->lock.pBtree = p;
  p->lock.iTable = 1;
#endif

#if !defined(SQLITE_OMIT_SHARED_CACHE) && !defined(SQLITE_OMIT_DISKIO)
  /*
  ** If this Btree is a candidate for shared cache, try to find an
  ** existing BtShared object that we can share with
  */
  if( isTempDb==0 && (isMemdb==0 || (vfsFlags&SQLITE_OPEN_URI)!=0) ){
    if( vfsFlags & SQLITE_OPEN_SHAREDCACHE ){
      int nFilename = sqlite3Strlen30(zFilename)+1;
      int nFullPathname = pVfs->mxPathname+1;
      char *zFullPathname = sqlite3Malloc(MAX(nFullPathname,nFilename));
      MUTEX_LOGIC( sqlite3_mutex *mutexShared; )

      p->sharable = 1;
      if( !zFullPathname ){
        sqlite3_free(p);
        return SQLITE_NOMEM_BKPT;
      }
      if( isMemdb ){
        memcpy(zFullPathname, zFilename, nFilename);
      }else{
        rc = sqlite3OsFullPathname(pVfs, zFilename,
                                   nFullPathname, zFullPathname);
        if( rc ){
          if( rc==SQLITE_OK_SYMLINK ){
            rc = SQLITE_OK;
          }else{
            sqlite3_free(zFullPathname);
            sqlite3_free(p);
            return rc;
          }
        }
      }
#if SQLITE_THREADSAFE
      mutexOpen = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_OPEN);
      sqlite3_mutex_enter(mutexOpen);
      mutexShared = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MAIN);
      sqlite3_mutex_enter(mutexShared);
#endif
      for(pBt=GLOBAL(BtShared*,sqlite3SharedCacheList); pBt; pBt=pBt->pNext){
        assert( pBt->nRef>0 );
        if( 0==strcmp(zFullPathname, sqlite3PagerFilename(pBt->pPager, 0))
                 && sqlite3PagerVfs(pBt->pPager)==pVfs ){
          int iDb;
          for(iDb=db->nDb-1; iDb>=0; iDb--){
            Btree *pExisting = db->aDb[iDb].pBt;
            if( pExisting && pExisting->pBt==pBt ){
              sqlite3_mutex_leave(mutexShared);
              sqlite3_mutex_leave(mutexOpen);
              sqlite3_free(zFullPathname);
              sqlite3_free(p);
              return SQLITE_CONSTRAINT;
            }
          }
          p->pBt = pBt;
          pBt->nRef++;
          break;
        }
      }
      sqlite3_mutex_leave(mutexShared);
      sqlite3_free(zFullPathname);
    }
#ifdef SQLITE_DEBUG
    else{
      /* In debug mode, we mark all persistent databases as sharable
      ** even when they are not.  This exercises the locking code and
      ** gives more opportunity for asserts(sqlite3_mutex_held())
      ** statements to find locking problems.
      */
      p->sharable = 1;
    }
#endif
  }
#endif
  if( pBt==0 ){
    /*
    ** The following asserts make sure that structures used by the btree are
    ** the right size.  This is to guard against size changes that result
    ** when compiling on a different architecture.
    */
    assert( sizeof(i64)==8 );
    assert( sizeof(u64)==8 );
    assert( sizeof(u32)==4 );
    assert( sizeof(u16)==2 );
    assert( sizeof(Pgno)==4 );
  
    pBt = sqlite3MallocZero( sizeof(*pBt) );
    if( pBt==0 ){
      rc = SQLITE_NOMEM_BKPT;
      goto btree_open_out;
    }
    rc = sqlite3PagerOpen(pVfs, &pBt->pPager, zFilename,
                          sizeof(MemPage), flags, vfsFlags, pageReinit);
    if( rc==SQLITE_OK ){
      sqlite3PagerSetMmapLimit(pBt->pPager, db->szMmap);
      rc = sqlite3PagerReadFileheader(pBt->pPager,sizeof(zDbHeader),zDbHeader);
    }
    if( rc!=SQLITE_OK ){
      goto btree_open_out;
    }
    pBt->openFlags = (u8)flags;
    pBt->db = db;
    sqlite3PagerSetBusyHandler(pBt->pPager, btreeInvokeBusyHandler, pBt);
    p->pBt = pBt;
  
    pBt->pCursor = 0;
    pBt->pPage1 = 0;
    if( sqlite3PagerIsreadonly(pBt->pPager) ) pBt->btsFlags |= BTS_READ_ONLY;
#if defined(SQLITE_SECURE_DELETE)
    pBt->btsFlags |= BTS_SECURE_DELETE;
#elif defined(SQLITE_FAST_SECURE_DELETE)
    pBt->btsFlags |= BTS_OVERWRITE;
#endif
    /* EVIDENCE-OF: R-51873-39618 The page size for a database file is
    ** determined by the 2-byte integer located at an offset of 16 bytes from
    ** the beginning of the database file. */
    pBt->pageSize = (zDbHeader[16]<<8) | (zDbHeader[17]<<16);
    if( pBt->pageSize<512 || pBt->pageSize>SQLITE_MAX_PAGE_SIZE
         || ((pBt->pageSize-1)&pBt->pageSize)!=0 ){
      pBt->pageSize = 0;
#ifndef SQLITE_OMIT_AUTOVACUUM
      /* If the magic name ":memory:" will create an in-memory database, then
      ** leave the autoVacuum mode at 0 (do not auto-vacuum), even if
      ** SQLITE_DEFAULT_AUTOVACUUM is true. On the other hand, if
      ** SQLITE_OMIT_MEMORYDB has been defined, then ":memory:" is just a
      ** regular file-name. In this case the auto-vacuum applies as per normal.
      */
      if( zFilename && !isMemdb ){
        pBt->autoVacuum = (SQLITE_DEFAULT_AUTOVACUUM ? 1 : 0);
        pBt->incrVacuum = (SQLITE_DEFAULT_AUTOVACUUM==2 ? 1 : 0);
      }
#endif
      nReserve = 0;
    }else{
      /* EVIDENCE-OF: R-37497-42412 The size of the reserved region is
      ** determined by the one-byte unsigned integer found at an offset of 20
      ** into the database file header. */
      nReserve = zDbHeader[20];
      pBt->btsFlags |= BTS_PAGESIZE_FIXED;
#ifndef SQLITE_OMIT_AUTOVACUUM
      pBt->autoVacuum = (get4byte(&zDbHeader[36 + 4*4])?1:0);
      pBt->incrVacuum = (get4byte(&zDbHeader[36 + 7*4])?1:0);
#endif
    }
    rc = sqlite3PagerSetPagesize(pBt->pPager, &pBt->pageSize, nReserve);
    if( rc ) goto btree_open_out;
    pBt->usableSize = pBt->pageSize - nReserve;
    assert( (pBt->pageSize & 7)==0 );  /* 8-byte alignment of pageSize */
   
#if !defined(SQLITE_OMIT_SHARED_CACHE) && !defined(SQLITE_OMIT_DISKIO)
    /* Add the new BtShared object to the linked list sharable BtShareds.
    */
    pBt->nRef = 1;
    if( p->sharable ){
      MUTEX_LOGIC( sqlite3_mutex *mutexShared; )
      MUTEX_LOGIC( mutexShared = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MAIN);)
      if( SQLITE_THREADSAFE && sqlite3GlobalConfig.bCoreMutex ){
        pBt->mutex = sqlite3MutexAlloc(SQLITE_MUTEX_FAST);
        if( pBt->mutex==0 ){
          rc = SQLITE_NOMEM_BKPT;
          goto btree_open_out;
        }
      }
      sqlite3_mutex_enter(mutexShared);
      pBt->pNext = GLOBAL(BtShared*,sqlite3SharedCacheList);
      GLOBAL(BtShared*,sqlite3SharedCacheList) = pBt;
      sqlite3_mutex_leave(mutexShared);
    }
#endif
  }

#if !defined(SQLITE_OMIT_SHARED_CACHE) && !defined(SQLITE_OMIT_DISKIO)
  /* If the new Btree uses a sharable pBtShared, then link the new
  ** Btree into the list of all sharable Btrees for the same connection.
  ** The list is kept in ascending order by pBt address.
  */
  if( p->sharable ){
    int i;
    Btree *pSib;
    for(i=0; i<db->nDb; i++){
      if( (pSib = db->aDb[i].pBt)!=0 && pSib->sharable ){
        while( pSib->pPrev ){ pSib = pSib->pPrev; }
        if( (uptr)p->pBt<(uptr)pSib->pBt ){
          p->pNext = pSib;
          p->pPrev = 0;
          pSib->pPrev = p;
        }else{
          while( pSib->pNext && (uptr)pSib->pNext->pBt<(uptr)p->pBt ){
            pSib = pSib->pNext;
          }
          p->pNext = pSib->pNext;
          p->pPrev = pSib;
          if( p->pNext ){
            p->pNext->pPrev = p;
          }
          pSib->pNext = p;
        }
        break;
      }
    }
  }
#endif
  *ppBtree = p;

btree_open_out:
  if( rc!=SQLITE_OK ){
    if( pBt && pBt->pPager ){
      sqlite3PagerClose(pBt->pPager, 0);
    }
    sqlite3_free(pBt);
    sqlite3_free(p);
    *ppBtree = 0;
  }else{
    sqlite3_file *pFile;

    /* If the B-Tree was successfully opened, set the pager-cache size to the
    ** default value. Except, when opening on an existing shared pager-cache,
    ** do not change the pager-cache size.
    */
    if( sqlite3BtreeSchema(p, 0, 0)==0 ){
      sqlite3BtreeSetCacheSize(p, SQLITE_DEFAULT_CACHE_SIZE);
    }

    pFile = sqlite3PagerFile(pBt->pPager);
    if( pFile->pMethods ){
      sqlite3OsFileControlHint(pFile, SQLITE_FCNTL_PDB, (void*)&pBt->db);
    }
  }
  if( mutexOpen ){
    assert( sqlite3_mutex_held(mutexOpen) );
    sqlite3_mutex_leave(mutexOpen);
  }
  assert( rc!=SQ

三 、关于游标

btreeInt.h 中 游标 的定义：

/*
** A cursor is a pointer to a particular entry within a particular
** b-tree within a database file.
**
** The entry is identified by its MemPage and the index in
** MemPage.aCell[] of the entry.
**
** A single database file can be shared by two more database connections,
** but cursors cannot be shared.  Each cursor is associated with a
** particular database connection identified BtCursor.pBtree.db.
**
** Fields in this structure are accessed under the BtShared.mutex
** found at self->pBt->mutex. 
**
** skipNext meaning:
** The meaning of skipNext depends on the value of eState:
**
**   eState            Meaning of skipNext
**   VALID             skipNext is meaningless and is ignored
**   INVALID           skipNext is meaningless and is ignored
**   SKIPNEXT          sqlite3BtreeNext() is a no-op if skipNext>0 and
**                     sqlite3BtreePrevious() is no-op if skipNext<0.
**   REQUIRESEEK       restoreCursorPosition() restores the cursor to
**                     eState=SKIPNEXT if skipNext!=0
**   FAULT             skipNext holds the cursor fault error code.
*/
struct BtCursor {
  u8 eState;                /* One of the CURSOR_XXX constants (see below) */
  u8 curFlags;              /* zero or more BTCF_* flags defined below */
  u8 curPagerFlags;         /* Flags to send to sqlite3PagerGet() */
  u8 hints;                 /* As configured by CursorSetHints() */
  int skipNext;    /* Prev() is noop if negative. Next() is noop if positive.
                   ** Error code if eState==CURSOR_FAULT */
  Btree *pBtree;            /* The Btree to which this cursor belongs */
  Pgno *aOverflow;          /* Cache of overflow page locations */
  void *pKey;               /* Saved key that was cursor last known position */
  /* All fields above are zeroed when the cursor is allocated.  See
  ** sqlite3BtreeCursorZero().  Fields that follow must be manually
  ** initialized. */
#define BTCURSOR_FIRST_UNINIT pBt   /* Name of first uninitialized field */
  BtShared *pBt;            /* The BtShared this cursor points to */
  BtCursor *pNext;          /* Forms a linked list of all cursors */
  CellInfo info;            /* A parse of the cell we are pointing at */
  i64 nKey;                 /* Size of pKey, or last integer key */
  Pgno pgnoRoot;            /* The root page of this tree */
  i8 iPage;                 /* Index of current page in apPage */
  u8 curIntKey;             /* Value of apPage[0]->intKey */
  u16 ix;                   /* Current index for apPage[iPage] */
  u16 aiIdx[BTCURSOR_MAX_DEPTH-1];     /* Current index in apPage[i] */
  struct KeyInfo *pKeyInfo;            /* Arg passed to comparison function */
  MemPage *pPage;                        /* Current page */
  MemPage *apPage[BTCURSOR_MAX_DEPTH-1]; /* Stack of parents of current page */
};

btree.c中的btreeCursor：

/*
** Create a new cursor for the BTree whose root is on the page
** iTable. If a read-only cursor is requested, it is assumed that
** the caller already has at least a read-only transaction open
** on the database already. If a write-cursor is requested, then
** the caller is assumed to have an open write transaction.
**
** If the BTREE_WRCSR bit of wrFlag is clear, then the cursor can only
** be used for reading.  If the BTREE_WRCSR bit is set, then the cursor
** can be used for reading or for writing if other conditions for writing
** are also met.  These are the conditions that must be met in order
** for writing to be allowed:
**
** 1:  The cursor must have been opened with wrFlag containing BTREE_WRCSR
**
** 2:  Other database connections that share the same pager cache
**     but which are not in the READ_UNCOMMITTED state may not have
**     cursors open with wrFlag==0 on the same table.  Otherwise
**     the changes made by this write cursor would be visible to
**     the read cursors in the other database connection.
**
** 3:  The database must be writable (not on read-only media)
**
** 4:  There must be an active transaction.
**
** The BTREE_FORDELETE bit of wrFlag may optionally be set if BTREE_WRCSR
** is set.  If FORDELETE is set, that is a hint to the implementation that
** this cursor will only be used to seek to and delete entries of an index
** as part of a larger DELETE statement.  The FORDELETE hint is not used by
** this implementation.  But in a hypothetical alternative storage engine 
** in which index entries are automatically deleted when corresponding table
** rows are deleted, the FORDELETE flag is a hint that all SEEK and DELETE
** operations on this cursor can be no-ops and all READ operations can 
** return a null row (2-bytes: 0x01 0x00).
**
** No checking is done to make sure that page iTable really is the
** root page of a b-tree.  If it is not, then the cursor acquired
** will not work correctly.
**
** It is assumed that the sqlite3BtreeCursorZero() has been called
** on pCur to initialize the memory space prior to invoking this routine.
*/
static int btreeCursor(
  Btree *p,                              /* The btree */
  Pgno iTable,                           /* Root page of table to open */
  int wrFlag,                            /* 1 to write. 0 read-only */
  struct KeyInfo *pKeyInfo,              /* First arg to comparison function */
  BtCursor *pCur                         /* Space for new cursor */
){
  BtShared *pBt = p->pBt;                /* Shared b-tree handle */
  BtCursor *pX;                          /* Looping over other all cursors */

  assert( sqlite3BtreeHoldsMutex(p) );
  assert( wrFlag==0 
       || wrFlag==BTREE_WRCSR 
       || wrFlag==(BTREE_WRCSR|BTREE_FORDELETE) 
  );

  /* The following assert statements verify that if this is a sharable 
  ** b-tree database, the connection is holding the required table locks, 
  ** and that no other connection has any open cursor that conflicts with 
  ** this lock.  The iTable<1 term disables the check for corrupt schemas. */
  assert( hasSharedCacheTableLock(p, iTable, pKeyInfo!=0, (wrFlag?2:1))
          || iTable<1 );
  assert( wrFlag==0 || !hasReadConflicts(p, iTable) );

  /* Assert that the caller has opened the required transaction. */
  assert( p->inTrans>TRANS_NONE );
  assert( wrFlag==0 || p->inTrans==TRANS_WRITE );
  assert( pBt->pPage1 && pBt->pPage1->aData );
  assert( wrFlag==0 || (pBt->btsFlags & BTS_READ_ONLY)==0 );

  if( wrFlag ){
    allocateTempSpace(pBt);
    if( pBt->pTmpSpace==0 ) return SQLITE_NOMEM_BKPT;
  }
  if( iTable<=1 ){
    if( iTable<1 ){
      return SQLITE_CORRUPT_BKPT;
    }else if( btreePagecount(pBt)==0 ){
      assert( wrFlag==0 );
      iTable = 0;
    }
  }

  /* Now that no other errors can occur, finish filling in the BtCursor
  ** variables and link the cursor into the BtShared list.  */
  pCur->pgnoRoot = iTable;
  pCur->iPage = -1;
  pCur->pKeyInfo = pKeyInfo;
  pCur->pBtree = p;
  pCur->pBt = pBt;
  pCur->curFlags = wrFlag ? BTCF_WriteFlag : 0;
  pCur->curPagerFlags = wrFlag ? 0 : PAGER_GET_READONLY;
  /* If there are two or more cursors on the same btree, then all such
  ** cursors *must* have the BTCF_Multiple flag set. */
  for(pX=pBt->pCursor; pX; pX=pX->pNext){
    if( pX->pgnoRoot==iTable ){
      pX->curFlags |= BTCF_Multiple;
      pCur->curFlags |= BTCF_Multiple;
    }
  }
  pCur->pNext = pBt->pCursor;
  pBt->pCursor = pCur;
  pCur->eState = CURSOR_INVALID;
  return SQLITE_OK;
}

游标是一种从表中检索数据并进行操作的灵活手段，游标主要用在服务器上，处理由客户端发送给服务端的sql语句，或是批处理、存储过程、触发器中的数据处理请求。
游标的优点在于它可以定位到结果集中的某一行，并可以对该行数据执行特定操作。一个完整的游标由5部分组成：
（1）声明游标 （2）打开游标 （3）从一个游标中查找信息 （4）关闭游标 （5）释放游标
2在性能上，游标会吃更多的内存，减少可用的并发，占用宽带，锁定资源还有更多的代码量。可是为什么学习游标呢？
1 现存系统有一些游标，我们查询必须通过游标来实现
2 作为一个备用方式，当我们穷尽了while循环，子查询，临时表，表变量，自建函数或其他方式仍然无法实现某些查询的时候用游标来实现。

使用Cursor对象执行select语句时，通过featchall()返回select的全部数据结果集。结果集是一个list，每个元素都是一个tuple，对应一行记录，按建表的字段顺序排列。fetchone()返回一条结果，是一个tuple，每个元素是一个字段值。

需要注意的是，SQLite游标是有状态的，只能遍历结果集一次，不能在结果集中返回移动，遍历结束返回空值。featchall()一次性返回全部结果，fetchone()依顺序每次返回一条结果，而且对fetchone()执行len()操作以获得所取得记录的条数后，相当于完成对结果集的一次遍历，结果集将被清空。因此，取得结果集后，如需对结果集进行进一步操作，可将结果集保存到变量中。

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