本篇内容介绍了“PostgreSQL如何解析表达式.”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
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SQL样例脚本.
testdb=# select 1+id,c2 from t_expr where id < 3;
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;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:
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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