[Bug] Fix the incompatibility of sql mode between Doris and MySQL

fe/fe-core/src/main/java/org/apache/doris/analysis/ExpressionFunctions.java

@@ -24,7 +24,6 @@
 import org.apache.doris.catalog.Type;
 import org.apache.doris.common.AnalysisException;
 import org.apache.doris.qe.ConnectContext;
-import org.apache.doris.qe.VariableMgr;
 import org.apache.doris.rewrite.FEFunction;
 import org.apache.doris.rewrite.FEFunctionList;
 import org.apache.doris.rewrite.FEFunctions;
@@ -110,13 +109,7 @@ public Expr evalExpr(Expr constExpr) {
                 }
             }
         } else if (constExpr instanceof SysVariableDesc) {
-            try {
-                VariableMgr.fillValue(ConnectContext.get().getSessionVariable(), (SysVariableDesc) constExpr);
-                return ((SysVariableDesc) constExpr).getLiteralExpr();
-            } catch (AnalysisException e) {
-                LOG.warn("failed to get session variable value: " + ((SysVariableDesc) constExpr).getName());
-                return constExpr;
-            }
+            return ((SysVariableDesc) constExpr).getLiteralExpr();
         }
         return constExpr;
     }

fe/fe-core/src/main/java/org/apache/doris/analysis/SysVariableDesc.java

@@ -115,7 +115,6 @@ public Expr getLiteralExpr() {
 
     @Override
     public Expr getResultValue() throws AnalysisException {
-        Expr expr = super.getResultValue();
         if (!Strings.isNullOrEmpty(name) && VariableVarConverters.hasConverter(name)) {
             // Return the string type here so that it can correctly match the subsequent function signature.
             // And we also set `beConverted` to session variable name in StringLiteral, so that it can be cast back
@@ -128,7 +127,7 @@ public Expr getResultValue() throws AnalysisException {
                 throw new AnalysisException(e.getMessage());
             }
         }
-        return expr;
+        return super.getResultValue();
     }
 
     @Override

fe/fe-core/src/main/java/org/apache/doris/qe/VariableVarConverters.java

@@ -28,8 +28,8 @@
  * Each converter should put in map (variable name -> converters) and only converts the variable with specified name.
  *
  * The converted session variable is a special kind of variable.
- * It's real type is int, so for example, when querying `select @@sql_mode`, the return column
- * type is "int".
+ * It's real type is Long or Int, for example, the variable `sql_mode` is Long, when querying `select @@sql_mode`,
+ * the return column type is String.
  * But user usually set this variable by string, such as:
  * `set @@sql_mode = 'STRICT_TRANS_TABLES'`
  * or

fe/fe-core/src/test/java/org/apache/doris/analysis/SetVariableTest.java

@@ -58,7 +58,7 @@ public void testSqlMode() throws Exception {
         stmtExecutor.execute();
         Expr expr = stmtExecutor.getParsedStmt().getResultExprs().get(0);
         Assert.assertTrue(expr instanceof SlotRef);
-        Assert.assertTrue(expr.getType() == Type.BIGINT);
+        Assert.assertSame(expr.getType(), Type.VARCHAR);
     }
 
     @Test