feat(query): select count return value if no rows

src/common/inc/tglobal.h

@@ -219,6 +219,7 @@ extern int32_t  cqDebugFlag;
 extern int32_t  debugFlag;
 
 extern int8_t tsClientMerge;
+extern int8_t tsCountAlwaysReturnValue;
 
 // probe alive connection
 extern int32_t tsProbeSeconds;

src/common/src/tglobal.c

@@ -273,6 +273,7 @@ int32_t  cqDebugFlag = 131;
 int32_t  fsDebugFlag = 135;
 
 int8_t tsClientMerge = 0;
+int8_t tsCountAlwaysReturnValue = 0;
 
 // probe alive connection
 int32_t tsProbeSeconds     =  5 * 60; // start probe link alive after tsProbeSeconds from starting query
@@ -1690,6 +1691,16 @@ static void doInitGlobalConfig(void) {
   cfg.unitType = TAOS_CFG_UTYPE_NONE;
   taosInitConfigOption(cfg);
 
+  cfg.option = "countAlwaysReturnValue";
+  cfg.ptr = &tsCountAlwaysReturnValue;
+  cfg.valType = TAOS_CFG_VTYPE_INT8;
+  cfg.cfgType = TSDB_CFG_CTYPE_B_CONFIG | TSDB_CFG_CTYPE_B_SHOW;
+  cfg.minValue = 0;
+  cfg.maxValue = 1;
+  cfg.ptrLength = 1;
+  cfg.unitType = TAOS_CFG_UTYPE_NONE;
+  taosInitConfigOption(cfg);
+
   // default JSON string type option "binary"/"nchar"
   cfg.option = "defaultJSONStrType";
   cfg.ptr = tsDefaultJSONStrType;

src/query/src/qAggMain.c

@@ -936,12 +936,12 @@ static void count_function(SQLFunctionCtx *pCtx) {
     }
   }
 
-  if (numOfElem > 0) {
+  if (numOfElem > 0 || tsCountAlwaysReturnValue) {
     GET_RES_INFO(pCtx)->hasResult = DATA_SET_FLAG;
+    *((int64_t *)pCtx->pOutput) += numOfElem;
+    pCtx->resultInfo->numOfRes = 1;
   }
 
-  *((int64_t *)pCtx->pOutput) += numOfElem;
-  SET_VAL(pCtx, numOfElem, 1);
 }
 
 static void count_func_merge(SQLFunctionCtx *pCtx) {

src/util/inc/tconfig.h

@@ -20,7 +20,7 @@
 extern "C" {
 #endif
 
-#define TSDB_CFG_MAX_NUM    136
+#define TSDB_CFG_MAX_NUM    137
 #define TSDB_CFG_PRINT_LEN  23
 #define TSDB_CFG_OPTION_LEN 24
 #define TSDB_CFG_VALUE_LEN  41