enh(query): add selectivity support for mavg functions

TD-18050

source/libs/function/src/builtins.c

@@ -2499,7 +2499,7 @@ const SBuiltinFuncDefinition funcMgtBuiltins[] = {
   {
     .name = "mavg",
     .type = FUNCTION_TYPE_MAVG,
-    .classification = FUNC_MGT_INDEFINITE_ROWS_FUNC | FUNC_MGT_TIMELINE_FUNC | FUNC_MGT_FORBID_STREAM_FUNC,
+    .classification = FUNC_MGT_INDEFINITE_ROWS_FUNC | FUNC_MGT_SELECT_FUNC | FUNC_MGT_TIMELINE_FUNC | FUNC_MGT_FORBID_STREAM_FUNC,
     .translateFunc = translateMavg,
     .getEnvFunc   = getMavgFuncEnv,
     .initFunc     = mavgFunctionSetup,

source/libs/function/src/builtinsimpl.c

@@ -4858,6 +4858,11 @@ int32_t mavgFunction(SqlFunctionCtx* pCtx) {
         colDataAppend(pOutput, pos, (char*)&result, false);
       }
 
+      // handle selectivity
+      if (pCtx->subsidiaries.num > 0) {
+        appendSelectivityValue(pCtx, i, pos);
+      }
+
       numOfElems++;
     }