fea: add null tag support in schemaless

include/common/tglobal.h

@@ -128,6 +128,7 @@ extern bool tsStartUdfd;
 
 // schemaless
 extern char tsSmlChildTableName[];
+extern char tsSmlTagName[];
 extern bool tsSmlDataFormat;
 
 // internal

source/client/src/clientSml.c

@@ -63,10 +63,6 @@ for (int i = 1; i < keyLen; ++i) {      \
 
 #define TS              "_ts"
 #define TS_LEN          3
-#define TAG             "_tag"
-#define TAG_LEN         4
-#define TAG_VALUE       "NULL"
-#define TAG_VALUE_LEN   4
 #define VALUE           "value"
 #define VALUE_LEN       5
 
@@ -402,7 +398,7 @@ static int32_t smlApplySchemaAction(SSmlHandle* info, SSchemaAction* action) {
       }
       if(taosArrayGetSize(cols) == 0){
         outBytes = snprintf(pos, freeBytes,"`%s` %s(%d)",
-                               TAG, tDataTypes[TSDB_DATA_TYPE_NCHAR].name, CHAR_SAVE_LENGTH);
+                            tsSmlTagName, tDataTypes[TSDB_DATA_TYPE_NCHAR].name, CHAR_SAVE_LENGTH);
         pos += outBytes; freeBytes -= outBytes;
         *pos = ','; ++pos; --freeBytes;
       }

source/common/src/tglobal.c

@@ -78,6 +78,7 @@ char     tsTelemServer[TSDB_FQDN_LEN] = "telemetry.taosdata.com";
 uint16_t tsTelemPort = 80;
 
 // schemaless
+char tsSmlTagName[TSDB_COL_NAME_LEN] = "__tag";
 char tsSmlChildTableName[TSDB_TABLE_NAME_LEN] = ""; //user defined child table name can be specified in tag value.
                                                      //If set to empty system will generate table name using MD5 hash.
 bool tsSmlDataFormat = true;  // true means that the name and order of cols in each line are the same(only for influx protocol)
@@ -522,6 +523,7 @@ static int32_t taosSetClientCfg(SConfig *pCfg) {
   }
 
   tstrncpy(tsSmlChildTableName, cfgGetItem(pCfg, "smlChildTableName")->str, TSDB_TABLE_NAME_LEN);
+  tstrncpy(tsSmlTagName, cfgGetItem(pCfg, "smlTagName")->str, TSDB_COL_NAME_LEN);
   tsSmlDataFormat = cfgGetItem(pCfg, "smlDataFormat")->bval;
 
   tsShellActivityTimer = cfgGetItem(pCfg, "shellActivityTimer")->i32;