PostgreSQL如何解析表达式.

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SQL样例脚本.

testdb=# select 1+id,c2 from t_expr where id < 3;

一、数据结构

FmgrInfo
在函数通过fmgr调用前,该结构体持有系统目录(字典)信息,用于检索相关信息.
如果相同的函数将被调用多次,检索只需要完成一次即可,该结构体会缓存多次使用.

/* * 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. * 在函数通过fmgr调用前,该结构体持有系统目录(字典)信息,用于检索相关信息. * 如果相同的函数将被调用多次,检索只需要完成一次即可,该结构体会缓存多次使用. * * 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实际上是关于参数的解析时确定的信息,而不是函数自身. * 但fn_expr在这里存储而不是FunctionCallInfoData中存储,因为从逻辑上来说,它就应该属于那. * * 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可用于被调用函数的使用;所有其他字段应该在结构体创建后被处理为只读. */typedef struct FmgrInfo{    //指向函数或者将被调用的处理器    PGFunction  fn_addr;        /* pointer to function or handler to be called */    //函数的oid    Oid         fn_oid;         /* OID of function (NOT of handler, if any) */    //输入参数的个数,0..FUNC_MAX_ARGS    short       fn_nargs;       /* number of input args (0..FUNC_MAX_ARGS) */    //函数是否严格(strict),输入NULL,输出NULL    bool        fn_strict;      /* function is "strict" (NULL in => NULL out) */    //函数是否返回集合    bool        fn_retset;      /* function returns a set */    //如track_functions > this,则收集统计信息    unsigned char fn_stats;     /* collect stats if track_functions > this */    //handler使用的额外空间    void       *fn_extra;       /* extra space for use by handler */    //存储fn_extra的内存上下文    MemoryContext fn_mcxt;      /* memory context to store fn_extra in */    //表达式解析树,或者为NULL    fmNodePtr   fn_expr;        /* expression parse tree for call, or NULL */} FmgrInfo;typedef struct Node *fmNodePtr;

FunctionCallInfoData
该结构体存储了实际传递给fmgr-called函数的参数

/* * This struct is the data actually passed to an fmgr-called function. * 该结构体存储了实际传递给fmgr-called函数的参数 * * 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.) * 被调用的函数期望设置isnull以及可能的resultinfo或者resultinfo指向的域字段. * 不应该改变其他字段. * (特别的,在参数数组上乱写是个坏主意,因为某些调用者假定它们可以使用相同的参数重复调用) */typedef struct FunctionCallInfoData{    //指向该调用的检索信息    FmgrInfo   *flinfo;         /* ptr to lookup info used for this call */    //调用上下文    fmNodePtr   context;        /* pass info about context of call */    //传递或返回关于结果的特别信息    fmNodePtr   resultinfo;     /* pass or return extra info about result */    //函数的collation    Oid         fncollation;    /* collation for function to use */#define FIELDNO_FUNCTIONCALLINFODATA_ISNULL 4    //如结果为NULL,则必须设置为T    bool        isnull;         /* function must set true if result is NULL */    //实际传递的参数个数    short       nargs;          /* # arguments actually passed */#define FIELDNO_FUNCTIONCALLINFODATA_ARG 6    //传递给函数的参数    Datum       arg[FUNC_MAX_ARGS]; /* Arguments passed to function */#define FIELDNO_FUNCTIONCALLINFODATA_ARGNULL 7    //如arg[i]为NULL,则对应的值为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.) * 所有函数可以通过fmgr直接调用,但必须持有签名. * (其他函数可通过使用handler的方式调用,也有此签名) */typedef struct FunctionCallInfoData *FunctionCallInfo;

二、源码解读

ExecInterpExpr
ExecInterpExpr中与表达式求值相关的代码片段如下:

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();        }

如为函数表达式,则从ExecInitFunc初始化的步骤信息中获取统一的调用参数fcinfo,然后通过函数指针(用于封装)调用实际的函数进行表达式求值.通过统一的参数,统一的返回值,做到了实现的统一,体现了面向对象OO中多态的思想,这再次说明了OO是思想,用过程性语言一样可以实现.

int4pl
SQL样例脚本相应的实现函数是int4pl,其实现代码如下:

Datumint4pl(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 boolpg_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}

函数实现相对比较简单,两个数简单相加,如溢出则返回T,否则返回F.

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