PostgreSQL中有哪些窗口函数

PostgreSQL中有哪些窗口函数，很多新手对此不是很清楚，为了帮助大家解决这个难题，下面小编将为大家详细讲解，有这方面需求的人可以来学习下，希望你能有所收获。

窗口函数：

窗口函数在一组表行中执行计算，这些表行以某种方式与当前行相关。 这与使用聚合函数可以完成的计算类型相当。 但是，窗口函数不会导致行被分组到单个输出行，就像非窗口聚合调用一样。 相反，行保留其独立的身份。 在幕后，窗口功能不仅可以访问查询结果的当前行。

row_number使用示例：

[postgres@shawnpc bin]$ ./psql psql (13devel)Type "help" for help.postgres=# select row_number() over() as rownum, id from aa; rownum | id --------+----      1 |  1      2 |  2      3 |  3      4 |  4      5 |  5      6 |  6      7 |  7      8 |  8      9 |  9     10 | 10(10 rows)postgres=#

row_number代码：

/* * row_number * just increment up from 1 until current partition finishes. */Datumwindow_row_number(PG_FUNCTION_ARGS){        WindowObject winobj = PG_WINDOW_OBJECT(); //获取窗口函数内存上下文        int64           curpos = WinGetCurrentPosition(winobj); //初始化位置        WinSetMarkPosition(winobj, curpos); //将行号和位置绑定        PG_RETURN_INT64(curpos + 1); //返回行号}

看起来似乎非常简单，但是经过调试发现这里和执行计划耦合度很高： 设置函数断点：

Breakpoint 1, window_row_number (fcinfo=0x7ffc158cce90) at windowfuncs.c:8383      {(gdb) bt#0  window_row_number (fcinfo=0x7ffc158cce90) at windowfuncs.c:83#1  0x0000000000632956 in eval_windowfunction (perfuncstate=0x1ca3768, result=0x1ca3738, isnull=0x1ca3750, winstate=0x1ca23e8,     winstate=0x1ca23e8) at nodeWindowAgg.c:1056#2  0x0000000000635174 in ExecWindowAgg (pstate=0x1ca23e8) at nodeWindowAgg.c:2198#3  0x0000000000605b82 in ExecProcNode (node=0x1ca23e8) at ../../../src/include/executor/executor.h:240#4  ExecutePlan (execute_once=, dest=0x1c125e8, direction=, numberTuples=0, sendTuples=true,     operation=CMD_SELECT, use_parallel_mode=, planstate=0x1ca23e8, estate=0x1ca21c0) at execMain.c:1648#5  standard_ExecutorRun (queryDesc=0x1c0eb70, direction=, count=0, execute_once=) at execMain.c:365#6  0x000000000074c81b in PortalRunSelect (portal=portal@entry=0x1c52e90, forward=forward@entry=true, count=0, count@entry=9223372036854775807,     dest=dest@entry=0x1c125e8) at pquery.c:929#7  0x000000000074db60 in PortalRun (portal=portal@entry=0x1c52e90, count=count@entry=9223372036854775807, isTopLevel=isTopLevel@entry=true,     run_once=run_once@entry=true, dest=dest@entry=0x1c125e8, altdest=altdest@entry=0x1c125e8,     completionTag=completionTag@entry=0x7ffc158cd7e0 "") at pquery.c:770#8  0x0000000000749bc6 in exec_simple_query (query_string=0x1becfa0 "select row_number() over() as rownum, id from aa;") at postgres.c:1231#9  0x000000000074aea2 in PostgresMain (argc=, argv=argv@entry=0x1c16f70, dbname=0x1c16e98 "postgres", username=)    at postgres.c:4256#10 0x000000000047e579 in BackendRun (port=, port=) at postmaster.c:4446#11 BackendStartup (port=0x1c0ee70) at postmaster.c:4137#12 ServerLoop () at postmaster.c:1704#13 0x00000000006ddb9d in PostmasterMain (argc=argc@entry=3, argv=argv@entry=0x1be7bb0) at postmaster.c:1377#14 0x000000000047f243 in main (argc=3, argv=0x1be7bb0) at main.c:210

从上可知，首先row_number函数执行是在执行计划执行之后进行调用的。 首先进入的是ExecutePlan：

static voidExecutePlan(EState *estate,                        PlanState *planstate,                        bool use_parallel_mode,                        CmdType operation,                        bool sendTuples,                        uint64 numberTuples,                        ScanDirection direction,                        DestReceiver *dest,                        bool execute_once){        TupleTableSlot *slot;        uint64          current_tuple_count;略        for (;;)        {                /* Reset the per-output-tuple exprcontext */                ResetPerTupleExprContext(estate);                /*                 * Execute the plan and obtain a tuple                 */                slot = ExecProcNode(planstate);略}

这里调用了ExecProcNode（宏定义，调用了ExecWindowAgg），ExecWindowAgg调用了eval_windowfunction，而正是eval_windowfunction完成了row_number的调用，并且构建了相关数据。通过调试可以发现，多少行数据就会调用多少次row_number。

eval_windowfunction：

/* * eval_windowfunction * * Arguments of window functions are not evaluated here, because a window * function can need random access to arbitrary rows in the partition. * The window function uses the special WinGetFuncArgInPartition and * WinGetFuncArgInFrame functions to evaluate the arguments for the rows * it wants. */static voideval_windowfunction(WindowAggState *winstate, WindowStatePerFunc perfuncstate,                                        Datum *result, bool *isnull){        LOCAL_FCINFO(fcinfo, FUNC_MAX_ARGS);        MemoryContext oldContext;        oldContext = MemoryContextSwitchTo(winstate->ss.ps.ps_ExprContext->ecxt_per_tuple_memory); //切换至tuple的内存上下文        /*         * We don't pass any normal arguments to a window function, but we do pass         * it the number of arguments, in order to permit window function         * implementations to support varying numbers of arguments.  The real info         * goes through the WindowObject, which is passed via fcinfo->context.         */        InitFunctionCallInfoData(*fcinfo, &(perfuncstate->flinfo),                                                         perfuncstate->numArguments,                                                         perfuncstate->winCollation,                                                         (void *) perfuncstate->winobj, NULL);//初始化fcinfo，为下面调用函数使用        /* Just in case, make all the regular argument slots be null */        for (int argno = 0; argno < perfuncstate->numArguments; argno++)                fcinfo->args[argno].isnull = true;//见注释        /* Window functions don't have a current aggregate context, either */        winstate->curaggcontext = NULL;//见注释        *result = FunctionCallInvoke(fcinfo);//调用函数        *isnull = fcinfo->isnull;        /*         * Make sure pass-by-ref data is allocated in the appropriate context. (We         * need this in case the function returns a pointer into some short-lived         * tuple, as is entirely possible.)         */        if (!perfuncstate->resulttypeByVal && !fcinfo->isnull &&                !MemoryContextContains(CurrentMemoryContext,                                                           DatumGetPointer(*result)))                *result = datumCopy(*result,                                                        perfuncstate->resulttypeByVal,                                                        perfuncstate->resulttypeLen);     //见注释        MemoryContextSwitchTo(oldContext); //切换回原上下文}

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