test: add udf function return with varchar

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

@@ -231,7 +231,7 @@ bit_add 实现多列的按位与功能。如果只有一列，返回这一列。
 
 </details>
 
-### 聚合函数示例 [l2norm](https://github.com/taosdata/TDengine/blob/develop/tests/script/sh/l2norm.c)
+### 聚合函数示例1 返回值为数值类型 [l2norm](https://github.com/taosdata/TDengine/blob/develop/tests/script/sh/l2norm.c)
 
 l2norm 实现了输入列的所有数据的二阶范数，即对每个数据先平方，再累加求和，最后开方。
 
@@ -243,3 +243,29 @@ l2norm 实现了输入列的所有数据的二阶范数，即对每个数据先
 ```
 
 </details>
+
+### 聚合函数示例2 返回值为字符串类型 [max_vol](https://github.com/taosdata/TDengine/blob/develop/tests/script/sh/max_vol.c)
+
+max_vol 实现了从多个输入的电压列中找到最大电压，返回由设备ID + 最大电压所在（行，列）+ 最大电压值 组成的组合字符串值
+
+创建表：
+```bash
+create table battery(ts timestamp, vol1 float, vol2 float, vol3 float, deviceId varchar(16));
+```
+创建自定义函数：
+```bash
+create aggregate function max_vol as '/root/udf/libmaxvol.so' outputtype binary(64) bufsize 10240 language 'C'; 
+```
+使用自定义函数：
+```bash
+select max_vol(vol1,vol2,vol3,deviceid) from battery;
+```
+
+<details>
+<summary>max_vol.c</summary>
+
+```c
+{{#include tests/script/sh/max_vol.c}}
+```
+
+</details>

tests/script/sh/max_vol.c

@@ -0,0 +1,101 @@
+#include <string.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <math.h>
+
+#include "taosudf.h"
+
+#define STR_MAX_LEN 256 // inter buffer length
+
+// init
+DLL_EXPORT int32_t max_vol_init()
+{
+    return 0;
+}
+
+// destory
+DLL_EXPORT int32_t max_vol_destroy()
+{
+    return 0;
+}
+
+// start 
+DLL_EXPORT int32_t max_vol_start(SUdfInterBuf *buf)
+{
+    memset(buf->buf, 0, sizeof(float) + STR_MAX_LEN);
+    // set init value
+    *((float*)buf->buf) = -10000000;
+    buf->bufLen = sizeof(float)  + STR_MAX_LEN;
+    buf->numOfResult = 0;
+    return 0;
+}
+
+DLL_EXPORT int32_t max_vol(SUdfDataBlock *block, SUdfInterBuf *interBuf, SUdfInterBuf *newInterBuf) {
+    float maxValue = *(float *)interBuf->buf;
+    char strBuff[STR_MAX_LEN] = "inter1buf";
+    
+    if (block->numOfCols < 2)
+    {
+        return TSDB_CODE_UDF_INVALID_INPUT;
+    }
+
+    // check data type
+    for (int32_t i = 0; i < block->numOfCols; ++i)
+    {
+        SUdfColumn *col = block->udfCols[i];
+        if( i == block->numOfCols - 1) {
+          // last column is device id , must varchar
+          if (col->colMeta.type != TSDB_DATA_TYPE_VARCHAR ) {
+            return TSDB_CODE_UDF_INVALID_INPUT;
+          }
+        } else {
+          if (col->colMeta.type != TSDB_DATA_TYPE_FLOAT) {
+            return TSDB_CODE_UDF_INVALID_INPUT;
+          }
+        }
+    }
+
+    // calc max voltage
+    SUdfColumn *lastCol = block->udfCols[block->numOfCols - 1];
+    for (int32_t i = 0; i < (block->numOfCols - 1); ++i) {
+        for (int32_t j = 0; j < block->numOfRows; ++j) {
+          SUdfColumn *col = block->udfCols[i];
+          if (udfColDataIsNull(col, j)) {
+            continue;
+          }
+          char *data = udfColDataGetData(col, j);
+          float voltage = *(float *)data;
+          if (voltage > maxValue) {
+            maxValue = voltage;
+            char *valData = udfColDataGetData(lastCol, j);
+            // get device id
+            char *deviceId = valData + sizeof(uint16_t);
+            sprintf(strBuff, "%s_(%d,%d)_%f", deviceId, j, i, maxValue);
+          }
+        }
+    }
+
+    *(float*)newInterBuf->buf = maxValue;
+    strcpy(newInterBuf->buf + sizeof(float), strBuff);
+    newInterBuf->bufLen = sizeof(float) + strlen(strBuff)+1;
+    newInterBuf->numOfResult = 1;
+    return 0;
+}
+
+DLL_EXPORT int32_t max_vol_finish(SUdfInterBuf *buf, SUdfInterBuf *resultData)
+{
+    char * str = buf->buf + sizeof(float);
+    // copy to des
+    char * des = resultData->buf + sizeof(uint16_t);
+    strcpy(des, str);
+
+    // set binary type len
+    uint16_t  len = strlen(str);
+    *((uint16_t*)resultData->buf) = len;
+
+    // set buf len
+    resultData->bufLen = len + sizeof(uint16_t);
+    // set row count
+    resultData->numOfResult = 1;
+    return 0;
+}