PostgreSQL窗口函数分析

今天看了一下PostgreSQL row_number的实现过程。之前一直好奇窗口函数是什么，原理是什么，今天稍稍解惑。下面就以row_number为例进行介绍： 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. */ Datum window_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:83 83 { (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=<optimized out>, dest=0x1c125e8, direction=<optimized out>, numberTuples=0, sendTuples=true, operation=CMD_SELECT, use_parallel_mode=<optimized out>, planstate=0x1ca23e8, estate=0x1ca21c0) at execMain.c:1648 #5 standard_ExecutorRun (queryDesc=0x1c0eb70, direction=<optimized out>, count=0, execute_once=<optimized out>) 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=<optimized out>, argv=argv@entry=0x1c16f70, dbname=0x1c16e98 "postgres", username=<optimized out>) at postgres.c:4256 #10 0x000000000047e579 in BackendRun (port=<optimized out>, port=<optimized out>) 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 void ExecutePlan(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 void eval_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 } 

 

至此分析结束。