fix: modify udf document

docs/zh/07-develop/09-udf.md

@@ -10,12 +10,81 @@ description: "支持用户编码的聚合函数和标量函数，在查询中嵌
 
 用户可以通过 UDF 实现两类函数： 标量函数和聚合函数。标量函数需要实现以下标量接口函数 udf，聚合函数需要实现 udaf_start ， udaf ， udaf_finish。如果需要初始化，实现 udf_init；如果需要清理工作，实现udf_destory。
 
-## 接口函数
+## 实现标量函数
+假如我们要实现一个名为scalarfn的用户标量函数，函数实现模板如下
+```c
+#include "taos.h"
+#include "taoserror.h"
+#include "taosudf.h"
+
+// initialization function. if no initialization, we can skip definition of it. The initialization function shall be concatenation of the udf name and _init suffix
+// @return error number defined in taoserror.h
+int32_t scalarfn_init() {
+    // initialization.
+    return TSDB_CODE_SUCCESS;
+}
+
+// scalar function main computation function
+// @param inputDataBlock, input data block composed of multiple columns with each column defined by SUdfColumn
+// @param resultColumn, output column
+// @return error number defined in taoserror.h
+int32_t scalarfn(SUdfDataBlock* inputDataBlock, SUdfColumn* resultColumn) {
+    // read data from inputDataBlock and process, then output to resultColumn.
+    return TSDB_CODE_SUCCESS;
+}
+
+// cleanup function. if no cleanup related processing, we can skip definition of it. The destroy function shall be concatenation of the udf name and _destroy suffix.
+// @return error number defined in taoserror.h
+int32_t scalarfn_destroy() {
+    // clean up
+    return TSDB_CODE_SUCCESS;
+}
+```
+## 实现聚合函数
+假如实现名为aggfn的聚合函数，函数模板如下
+```c
+#include "taos.h"
+#include "taoserror.h"
+#include "taosudf.h"
+
+// Initialization function. if no initialization, we can skip definition of it. The initialization function shall be concatenation of the udf name and _init suffix
+// @return error number defined in taoserror.h
+int32_t aggfn_init() {
+    // initialization.
+    return TSDB_CODE_SUCCESS;
+}
+
+// aggregate start function. The intermediate value or the state(@interBuf) is initialized in this function. The function name shall be concatenation of udf name and _start suffix
+// @return error number defined in taoserror.h
+int32_t aggfn_start(SUdfInterBuf* interBuf) {
+    return TSDB_CODE_SUCESS;
+}
+
+// aggregate reduce function. This function aggregate old state(@interbuf) and one data bock(inputBlock) and output a new state(@newInterBuf).
+int32_t aggfn(SUdfDataBlock* inputBlock, SUdfInterBuf *interBuf, SUdfInterBuf *newInterBuf) {
+    // read from inputBlock and interBuf and output to newInterBuf
+    return TSDB_CODE_SUCCESS;
+}
+
+// aggregate function finish function. This function transforms the intermediate value(@interBuf) into the final output(@result). The function name must be concatenation of aggfn and _finish suffix.
+// @return error number defined in taoserror.h
+int32_t int32_t aggfn_finish(SUdfInterBuf* interBuf, SUdfInterBuf *result) {
+    // read data from inputDataBlock and process, then output to resultColumn
+    return TSDB_CODE_SUCCESS;
+}
+
+// cleanup function. if no cleanup related processing, we can skip definition of it. The destroy function shall be concatenation of the udf name and _destroy suffix.
+// @return error number defined in taoserror.h
+int32_t aggfn_destroy() {
+    // clean up
+    return TSDB_CODE_SUCCESS;
+}
+```
+
+## 接口函数定义
 
 ### 标量接口函数
 
-用户可以按照下列函数模板定义自己的标量计算函数
-
  `int32_t udf(SUdfDataBlock* inputDataBlock, SUdfColumn *resultColumn)` 
  
  其中 udf 是函数名的占位符，以上述模板实现的函数对行数据块进行标量计算。
@@ -26,8 +95,6 @@ description: "支持用户编码的聚合函数和标量函数，在查询中嵌
 
 ### 聚合接口函数
 
-用户可以按照如下函数模板定义自己的聚合函数。
-
 `int32_t udaf_start(SUdfInterBuf *interBuf)`
 
 `int32_t udaf(SUdfDataBlock* inputBlock, SUdfInterBuf *interBuf, SUdfInterBuf *newInterBuf)`