先前三個小節的內容介紹了PostgreSQL如何構建解析表示式的步驟,本節以int4pl為例介紹PostgreSQL如何解析表示式.
SQL樣例指令碼.
```
testdb=# select 1+id,c2 from t_expr where id < 3;
```

### 一、資料結構
**FmgrInfo**
在函式透過fmgr呼叫前,該結構體持有系統目錄(字典)資訊,用於檢索相關資訊.
如果相同的函式將被呼叫多次,檢索只需要完成一次即可,該結構體會快取多次使用.
```
/*
 * This struct holds the system-catalog information that must be looked up
 * before a function can be called through fmgr.  If the same function is
 * to be called multiple times, the lookup need be done only once and the
 * info struct saved for re-use.
 * 在函式透過fmgr呼叫前,該結構體持有系統目錄(字典)資訊,用於檢索相關資訊.
 * 如果相同的函式將被呼叫多次,檢索只需要完成一次即可,該結構體會快取多次使用.
 *
 * Note that fn_expr really is parse-time-determined information about the
 * arguments, rather than about the function itself.  But it's convenient
 * to store it here rather than in FunctionCallInfoData, where it might more
 * logically belong.
 * 注意,fn_expr實際上是關於引數的解析時確定的資訊,而不是函式自身.
 * 但fn_expr在這裡儲存而不是FunctionCallInfoData中儲存,因為從邏輯上來說,它就應該屬於那.
 *
 * fn_extra is available for use by the called function; all other fields
 * should be treated as read-only after the struct is created.
 * fn_extra可用於被呼叫函式的使用;所有其他欄位應該在結構體建立後被處理為只讀.
 */
typedef struct FmgrInfo
{
    //指向函式或者將被呼叫的處理器
    PGFunction  fn_addr;        /* pointer to function or handler to be called */
    //函式的oid
    Oid         fn_oid;         /* OID of function (NOT of handler, if any) */
    //輸入引數的個數,0..FUNC_MAX_ARGS
    short       fn_nargs;       /* number of input args (0..FUNC_MAX_ARGS) */
    //函式是否嚴格(strict),輸入NULL,輸出NULL
    bool        fn_strict;      /* function is "strict" (NULL in => NULL out) */
    //函式是否返回集合
    bool        fn_retset;      /* function returns a set */
    //如track_functions > this,則收集統計資訊
    unsigned char fn_stats;     /* collect stats if track_functions > this */
    //handler使用的額外空間
    void       *fn_extra;       /* extra space for use by handler */
    //儲存fn_extra的記憶體上下文
    MemoryContext fn_mcxt;      /* memory context to store fn_extra in */
    //表示式解析樹,或者為NULL
    fmNodePtr   fn_expr;        /* expression parse tree for call, or NULL */
} FmgrInfo;

typedef struct Node *fmNodePtr;

```

**FunctionCallInfoData**
該結構體儲存了實際傳遞給fmgr-called函式的引數
```
/*
 * This struct is the data actually passed to an fmgr-called function.
 * 該結構體儲存了實際傳遞給fmgr-called函式的引數
 *
 * The called function is expected to set isnull, and possibly resultinfo or
 * fields in whatever resultinfo points to.  It should not change any other
 * fields.  (In particular, scribbling on the argument arrays is a bad idea,
 * since some callers assume they can re-call with the same arguments.)
 * 被呼叫的函式期望設定isnull以及可能的resultinfo或者resultinfo指向的域欄位.
 * 不應該改變其他欄位.
 * (特別的,在引數陣列上亂寫是個壞主意,因為某些呼叫者假定它們可以使用相同的引數重複呼叫)
 */
typedef struct FunctionCallInfoData
{
    //指向該呼叫的檢索資訊
    FmgrInfo   *flinfo;         /* ptr to lookup info used for this call */
    //呼叫上下文
    fmNodePtr   context;        /* pass info about context of call */
    //傳遞或返回關於結果的特別資訊
    fmNodePtr   resultinfo;     /* pass or return extra info about result */
    //函式的collation
    Oid         fncollation;    /* collation for function to use */
#define FIELDNO_FUNCTIONCALLINFODATA_ISNULL 4
    //如結果為NULL,則必須設定為T
    bool        isnull;         /* function must set true if result is NULL */
    //實際傳遞的引數個數
    short       nargs;          /* # arguments actually passed */
#define FIELDNO_FUNCTIONCALLINFODATA_ARG 6
    //傳遞給函式的引數
    Datum       arg[FUNC_MAX_ARGS]; /* Arguments passed to function */
#define FIELDNO_FUNCTIONCALLINFODATA_ARGNULL 7
    //如arg[i]為NULL,則對應的值為T
    bool        argnull[FUNC_MAX_ARGS]; /* T if arg[i] is actually NULL */
} FunctionCallInfoData;

/*
 * All functions that can be called directly by fmgr must have this signature.
 * (Other functions can be called by using a handler that does have this
 * signature.)
 * 所有函式可以透過fmgr直接呼叫,但必須持有簽名.
 * (其他函式可透過使用handler的方式呼叫,也有此簽名)
 */

typedef struct FunctionCallInfoData *FunctionCallInfo;

```

### 二、原始碼解讀
**ExecInterpExpr**
ExecInterpExpr中與表示式求值相關的程式碼片段如下:
```

EEO_CASE(EEOP_FUNCEXPR)
        {
            FunctionCallInfo fcinfo = op->d.func.fcinfo_data;
            Datum       d;

fcinfo->isnull = false;
            d = op->d.func.fn_addr(fcinfo);
            *op->resvalue = d;
            *op->resnull = fcinfo->isnull;

EEO_NEXT();
        }
```
如為函式表示式,則從ExecInitFunc初始化的步驟資訊中獲取統一的呼叫引數fcinfo,然後透過函式指標(用於封裝)呼叫實際的函式進行表示式求值.透過統一的引數,統一的返回值,做到了實現的統一,體現了物件導向OO中多型的思想,這再次說明了OO是思想,用過程性語言一樣可以實現.

**int4pl**
SQL樣例指令碼相應的實現函式是int4pl,其實現程式碼如下:
```
Datum
int4pl(PG_FUNCTION_ARGS)
{
    int32       arg1 = PG_GETARG_INT32(0);
    int32       arg2 = PG_GETARG_INT32(1);
    int32       result;

if (unlikely(pg_add_s32_overflow(arg1, arg2, &result)))
        ereport(ERROR,
                (errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
                 errmsg("integer out of range")));
    PG_RETURN_INT32(result);
}

/*
 * If a + b overflows, return true, otherwise store the result of a + b into
 * *result. The content of *result is implementation defined in case of
 * overflow.
 */
static inline bool
pg_add_s32_overflow(int32 a, int32 b, int32 *result)
{
#if defined(HAVE__BUILTIN_OP_OVERFLOW)
    return __builtin_add_overflow(a, b, result);
#else
    int64       res = (int64) a + (int64) b;

if (res > PG_INT32_MAX || res < PG_INT32_MIN)
    {
        *result = 0x5EED;       /* to avoid spurious warnings */
        return true;
    }
    *result = (int32) res;
    return false;
#endif
}

```
函式實現相對比較簡單,兩個數簡單相加,如溢位則返回T,否則返回F.

### 三、跟蹤分析
N/A

### 四、參考資料
[PG Source Code](https://doxygen.postgresql.org)

PostgreSQL 原始碼解讀（164）- 查詢#84(表示式求值)

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