虚拟机（Virtual Machine）

VDBE是sqlite的核心，它的上层模块和下层模块都是本质上都是为它服务的。它的实现位于vbde.c,vdbe.h,vdbeapi.c,vdbeInt.h,和vdbemem.c等几个文件中。它通过底层的基础设施B-Tree执行由编译器（Compiler）生成的字节代码，这种字节代码程序语言 (bytecode programming lauguage)是为了进行查询、读取和修改数据库而专门设计的。

字节代码在内存中被封装成sqlit_stmt对象(内部叫做Vdbe)，Vdbe（或者说statement）包含执行程序所需要的一切：

a) a bytecode program

b) names and data types for all result columns

c) values bound to input parameters

d) a program counter

e) an execution stack of operands

f) an arbitrary amount of "numbered"memory cells

g) other run-time state information (such asopen BTree objects,sorters,lists,sets)

其代码位于vdbeInt.h，如下所示：

struct Vdbe {
  sqlite3 *db;            /* The database connection that owns this statement */
  Op *aOp;                /* Space to hold the virtual machine's program */
  Mem *aMem;              /* The memory locations */
  Mem **apArg;            /* Arguments to currently executing user function */
  Mem *aColName;          /* Column names to return */
  Mem *pResultSet;        /* Pointer to an array of results */
  Parse *pParse;          /* Parsing context used to create this Vdbe */
  int nMem;               /* Number of memory locations currently allocated */
  int nOp;                /* Number of instructions in the program */
  int nCursor;            /* Number of slots in apCsr[] */
  u32 magic;              /* Magic number for sanity checking */
  char *zErrMsg;          /* Error message written here */
  Vdbe *pPrev,*pNext;     /* Linked list of VDBEs with the same Vdbe.db */
  VdbeCursor **apCsr;     /* One element of this array for each open cursor */
  Mem *aVar;              /* Values for the OP_Variable opcode. */
  char **azVar;           /* Name of variables */
  ynVar nVar;             /* Number of entries in aVar[] */
  ynVar nzVar;            /* Number of entries in azVar[] */
  u32 cacheCtr;           /* VdbeCursor row cache generation counter */
  int pc;                 /* The program counter */
  int rc;                 /* Value to return */
……
};

字节代码和汇编程序十分类似，每一条指令由操作码和五个操作数构成：P1,P2 P3,P4,和P5。Opcode为一定功能的操作码，为了理解，可以看成一个函数。P1、P2和P3都是32位有符号整数，P2通常是跳转指令的目标地址。P4是以联合体的形式进行定义，有时是32位有符号整数，有时是64有符号整数等等。P5是一个8位无符号整数，一般用来做标识。有的程序可以使用五个操作数，而有的程序一个都不用。和C API不同的是，VDBE操作码经常变化，所以不应该用字节码写程序。其代码如下所示：

struct VdbeOp {
  u8 opcode;          /* What operation to perform */
  signed char p4type; /* One of the P4_xxx constants for p4 */
  u8 opflags;         /* Mask of the OPFLG_* flags in opcodes.h */
  u8 p5;              /* Fifth parameter is an unsigned character */
  int p1;             /* First operand */
  int p2;             /* Second parameter (often the jump destination) */
  int p3;             /* The third parameter */
  union p4union {     /* fourth parameter */
    int i;                 /* Integer value if p4type==P4_INT32 */
    void *p;               /* Generic pointer */
    char *z;               /* Pointer to data for string (char array) types */
    i64 *pI64;             /* Used when p4type is P4_INT64 */
    double *pReal;         /* Used when p4type is P4_REAL */
    FuncDef *pFunc;        /* Used when p4type is P4_FUNCDEF */
    sqlite3_context *pCtx; /* Used when p4type is P4_FUNCCTX */
    CollSeq *pColl;        /* Used when p4type is P4_COLLSEQ */
    Mem *pMem;             /* Used when p4type is P4_MEM */
    VTable *pVtab;         /* Used when p4type is P4_VTAB */
    KeyInfo *pKeyInfo;     /* Used when p4type is P4_KEYINFO */
    int *ai;               /* Used when p4type is P4_INTARRAY */
    SubProgram *pProgram;  /* Used when p4type is P4_SUBPROGRAM */
    int (*xAdvance)(BtCursor *,int *);
  } p4;
#ifdef sqlITE_ENABLE_EXPLAIN_COMMENTS
  char *zComment;          /* Comment to improve readability */
#endif
#ifdef VDBE_PROFILE
  u32 cnt;                 /* Number of times this instruction was executed */
  u64 cycles;              /* Total time spent executing this instruction */
#endif
#ifdef sqlITE_VDBE_COVERAGE
  int iSrcLine;            /* Source-code line that generated this opcode */
#endif
};

下面看一个具体的字节码程序：

sqlite> CREATE TABLE examp(one text,two int);
sqlite> .explain
sqlite> EXPLAIN INSERT INTO examp VALUES('Hello,World!',99);
addr  opcode         p1    p2    p3    p4             p5  comment
----  -------------  ----  ----  ----  -------------  --  -------------
0     Init           0     8     0                    00
1     OpenWrite      0     2     0     2              00
2     NewRowid       0     1     0                    00
3     String8        0     2     0     Hello,World!  00
4     Integer        99    3     0                    00
5     MakeRecord     2     2     4     BD             00
6     Insert         0     4     1     examp          1b
7     Halt           0     0     0                    00
8     Transaction    0     1     5     0              01
9     TableLock      0     2     1     examp          00
10    Goto           0     1     0                    00

表2.1列出了部分操作码及其操作内容.