PostgreSQL数据库TableAM——GetTableAmRoutine函数

Love The Way You Lie 2022-10-11 01:32 123阅读 0赞

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GetTableAmRoutine函数调用指定access method handler routine以获取其TableAmRoutine结构体（**通过Oid amhandler获取相应的TableAmRoutine**），该结构体会在调用者的内存上下文中分配。主要是通过调用OidFunctionCall0(amhandler)函数获取TableAmRoutine结构体。

const TableAmRoutine *GetTableAmRoutine(Oid amhandler) { 
    	Datum		datum;
    	const TableAmRoutine *routine;
    	datum = OidFunctionCall0(amhandler);
    	routine = (TableAmRoutine *) DatumGetPointer(datum);
    	if (routine == NULL || !IsA(routine, TableAmRoutine))
    		elog(ERROR, "table access method handler %u did not return a TableAmRoutine struct", amhandler);
    	Assert(routine->scan_begin != NULL);
    	Assert(routine->scan_end != NULL);
    	Assert(routine->scan_rescan != NULL);
    	Assert(routine->scan_getnextslot != NULL);
        ...
    	Assert(routine->scan_sample_next_tuple != NULL);
    	return routine;
    }

将宏展开OidFunctionCall0(amhandler)就是OidFunctionCall0Coll(amhandler, InvalidOid)，OidFunctionCall0Coll函数详情如下：

Datum OidFunctionCall0Coll(Oid functionId, Oid collation){ 
    	FmgrInfo	flinfo;
    	fmgr_info(functionId, &flinfo);
    	return FunctionCall0Coll(&flinfo, collation);
    }
    void fmgr_info(Oid functionId, FmgrInfo *finfo) {  // 通过形参给定的函数oid填充FmgrInfo结构体
    	fmgr_info_cxt_security(functionId, finfo, CurrentMemoryContext, false);
    }

fmgr\_info\_cxt\_security函数通过形参给定的函数oid填充FmgrInfo结构体，内存上下文使用CurrentMemoryContext。

static void fmgr_info_cxt_security(Oid functionId, FmgrInfo *finfo, MemoryContext mcxt, bool ignore_security) { 
    	const FmgrBuiltin *fbp;
    	HeapTuple	procedureTuple;
    	Form_pg_proc procedureStruct;
    	Datum		prosrcdatum;
    	bool		isnull;
    	char	   *prosrc;
    	/* fn_oid *must* be filled in last. Some code assumes that if fn_oid is valid, the whole struct is valid. Some FmgrInfo struct's do survive elogs. */
    	finfo->fn_oid = InvalidOid;
    	finfo->fn_extra = NULL;
    	finfo->fn_mcxt = mcxt;
    	finfo->fn_expr = NULL;		/* caller may set this later */
    	if ((fbp = fmgr_isbuiltin(functionId)) != NULL){ 
    		/* Fast path for builtin functions: don't bother consulting pg_proc */
    		finfo->fn_nargs = fbp->nargs;
    		finfo->fn_strict = fbp->strict;
    		finfo->fn_retset = fbp->retset;
    		finfo->fn_stats = TRACK_FUNC_ALL;	/* ie, never track */
    		finfo->fn_addr = fbp->func;
    		finfo->fn_oid = functionId;
    		return;
    	}
    	/* Otherwise we need the pg_proc entry */
        ...
    }

以functionId为3为例，通过上述函数后，FmgrInfo结构体flinfo的成员变量的值为：

*  fn\_addr为heap\_tableam\_handler函数
 *  fn\_oid为3
 *  fn\_nargs为1，heap\_tableam\_handler函数参数为1个
 *  fn\_strict为true
 *  fn\_retset为false，heap\_tableam\_handler函数不返回一个set
 *  fn\_stats为TRACK\_FUNC\_ALL
 *  fn\_extra为NULL
 *  fn\_mcxt为CurrentMemoryContext
 *  fn\_expr为NULL  
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    FunctionCall0Coll函数中LOCAL\_FCINFO宏将定义一个fcinfodata联合体，并使用FunctionCallInfo指针变量fcinfo指向这个联合体；InitFunctionCallInfoData初始化fcinfodata.fcinfo。最终返回的是FmgrInfo结构体中的fn\_addr函数执行的结果，也就是返回heap\_tableam\_handler(fcinfo)。

Datum FunctionCall0Coll(FmgrInfo *flinfo, Oid collation) { 
    	LOCAL_FCINFO(fcinfo, 0);
    	Datum		result;
    	InitFunctionCallInfoData(*fcinfo, flinfo, 0, collation, NULL, NULL);
    	result = FunctionCallInvoke(fcinfo);
    	/* Check for null result, since caller is clearly not expecting one */
    	if (fcinfo->isnull)
    		elog(ERROR, "function %u returned NULL", flinfo->fn_oid);
    	return result;
    }
    #define FunctionCallInvoke(fcinfo) ((* (fcinfo)->flinfo->fn_addr) (fcinfo))

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heap\_tableam\_handler函数代码如下，heapam\_methods是TableAmRoutine结构体，heap\_tableam\_handler函数将heapam\_methods用PG\_RETURN\_POINTER宏处理后进行返回。

Datum heap_tableam_handler(PG_FUNCTION_ARGS) {  PG_RETURN_POINTER(&heapam_methods); }

宏定义详解如下：

/*******************************************************************************/
    #define LOCAL_FCINFO(name, nargs) \ /* use union with FunctionCallInfoBaseData to guarantee alignment */ \
    	union \
    	{  \
    		FunctionCallInfoBaseData fcinfo; \
    		/* ensure enough space for nargs args is available */ \
    		char fcinfo_data[SizeForFunctionCallInfo(nargs)]; \
    	} name##data; \
    	FunctionCallInfo name = &name##data.fcinfo
    /*******************************************************************************/
    	LOCAL_FCINFO(fcinfo, 0);  //转变为如下联合体
    	union {  
    		FunctionCallInfoBaseData fcinfo; 
    		char fcinfo_data[SizeForFunctionCallInfo(0)]; 
    	} fcinfodata; 
    	FunctionCallInfo fcinfo = &fcinfodata.fcinfo
    /*******************************************************************************/
    #define SizeForFunctionCallInfo(nargs) \ (offsetof(FunctionCallInfoBaseData, args) + \ sizeof(NullableDatum) * (nargs))
    #define offsetof(type, field) ((long) &((type *)0)->field) 
    /*******************************************************************************/
    	LOCAL_FCINFO(fcinfo, 0);  //转变为如下联合体
    	union {  
    		FunctionCallInfoBaseData fcinfo; 
    		char fcinfo_data[(((long) &((FunctionCallInfoBaseData *)0)->0) )+sizeof(NullableDatum)*(0))]; 
    	} fcinfodata; 
    	FunctionCallInfo fcinfo = &fcinfodata.fcinfo	
    /*******************************************************************************/
    #define InitFunctionCallInfoData(Fcinfo, Flinfo, Nargs, Collation, Context, Resultinfo) \ do { \ (Fcinfo).flinfo = (Flinfo); \ (Fcinfo).context = (Context); \ (Fcinfo).resultinfo = (Resultinfo); \ (Fcinfo).fncollation = (Collation); \ (Fcinfo).isnull = false; \ (Fcinfo).nargs = (Nargs); \ } while (0)
    /*******************************************************************************/
    InitFunctionCallInfoData(*fcinfo, flinfo, 0, collation, NULL, NULL);
    	do {  
    		(*fcinfo).flinfo = (flinfo); 
    		(*fcinfo).context = (NULL); 
    		(*fcinfo).resultinfo = (NULL); 
    		(*fcinfo).fncollation = (collation); 
    		(*fcinfo).isnull = false; 
    		(*fcinfo).nargs = (0); 
    	} while (0)

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