[Bug] Fix bug of select @@sql_mode

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

@@ -20,18 +20,20 @@
 import org.apache.doris.catalog.PrimitiveType;
 import org.apache.doris.catalog.Type;
 import org.apache.doris.common.AnalysisException;
+import org.apache.doris.common.DdlException;
 import org.apache.doris.common.ErrorCode;
 import org.apache.doris.common.ErrorReport;
 import org.apache.doris.common.io.Text;
+import org.apache.doris.qe.SqlModeHelper;
 import org.apache.doris.thrift.TExprNode;
 import org.apache.doris.thrift.TExprNodeType;
 import org.apache.doris.thrift.TStringLiteral;
 
-import com.google.common.base.Preconditions;
-
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
+import com.google.common.base.Preconditions;
+
 import java.io.DataInput;
 import java.io.DataOutput;
 import java.io.IOException;
@@ -41,6 +43,18 @@
 public class StringLiteral extends LiteralExpr {
     private static final Logger LOG = LogManager.getLogger(StringLiteral.class);
     private String value;
+    /**
+     * the session variable `sql_mode` is a special kind of variable.
+     * it's real type is int, so when querying `select @@sql_mode`, the return column
+     * type is "int". but user usually set this variable by string, such as:
+     * `set @@sql_mode = 'STRICT_TRANS_TABLES'`
+     * or
+     * `set @@sql_mode = concat(@@sql_mode, 'STRICT_TRANS_TABLES')'`
+     * <p>
+     * So when it need to be cast to int, it means "cast 'STRICT_TRANS_TABLES' to Integer".
+     * To support this, we set `isSqlMode` to true, so that it can cast sql mode name to integer.
+     */
+    private boolean isSqlMode = false;
 
     public StringLiteral() {
         super();
@@ -59,6 +73,10 @@ protected StringLiteral(StringLiteral other) {
         value = other.value;
     }
 
+    public void setIsSqlMode(boolean val) {
+        this.isSqlMode = val;
+    }
+
     @Override
     public Expr clone() {
         return new StringLiteral(this);
@@ -178,8 +196,24 @@ protected Expr uncheckedCastTo(Type targetType) throws AnalysisException {
                 case SMALLINT:
                 case INT:
                 case BIGINT:
+                    if (isSqlMode) {
+                        try {
+                            long sqlMode = SqlModeHelper.encode(value);
+                            return new IntLiteral(sqlMode, targetType);
+                        } catch (DdlException e) {
+                            throw new AnalysisException(e.getMessage());
+                        }
+                    }
                     return new IntLiteral(value, targetType);
                 case LARGEINT:
+                    if (isSqlMode) {
+                        try {
+                            long sqlMode = SqlModeHelper.encode(value);
+                            return new LargeIntLiteral(String.valueOf(sqlMode));
+                        } catch (DdlException e) {
+                            throw new AnalysisException(e.getMessage());
+                        }
+                    }
                     return new LargeIntLiteral(value);
                 case FLOAT:
                 case DOUBLE:

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

@@ -122,8 +122,12 @@ public Expr getResultValue() throws AnalysisException {
             // Such as `set sql_mode =  concat(@@sql_mode, "STRICT_TRANS_TABLES");`
             // So we return the string type here so that it can correctly match the subsequent function signature.
             // We will convert the string to int in VariableMgr.
+            // And we also set `isSqlMode` to true in StringLiteral, so that it can be cast back
+            // to Integer when returning value.
             try {
-                return new StringLiteral(SqlModeHelper.decode(intValue));
+                StringLiteral s = new StringLiteral(SqlModeHelper.decode(intValue));
+                s.setIsSqlMode(true);
+                return s;
             } catch (DdlException e) {
                 throw new AnalysisException(e.getMessage());
             }

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

@@ -17,6 +17,7 @@
 
 package org.apache.doris.analysis;
 
+import org.apache.doris.catalog.Type;
 import org.apache.doris.qe.ConnectContext;
 import org.apache.doris.qe.SqlModeHelper;
 import org.apache.doris.qe.StmtExecutor;
@@ -50,6 +51,14 @@ public void testSqlMode() throws Exception {
         stmtExecutor.execute();
         Assert.assertEquals("STRICT_TRANS_TABLES",
                 SqlModeHelper.decode(connectContext.getSessionVariable().getSqlMode()));
+
+        String selectStr = "explain select @@sql_mode;";
+        connectContext.getState().reset();
+        stmtExecutor = new StmtExecutor(connectContext, selectStr);
+        stmtExecutor.execute();
+        Expr expr = stmtExecutor.getParsedStmt().getResultExprs().get(0);
+        Assert.assertTrue(expr instanceof SlotRef);
+        Assert.assertTrue(expr.getType() == Type.BIGINT);
     }
 
     @Test

fe/fe-core/src/test/java/org/apache/doris/utframe/UtFrameUtils.java

@@ -220,5 +220,16 @@ public static String getSQLPlanOrErrorMsg(ConnectContext ctx, String queryStr) t
             return ctx.getState().getErrorMessage();
         }
     }
+
+    public static Planner getSQLPlanner(ConnectContext ctx, String queryStr) throws Exception {
+        ctx.getState().reset();
+        StmtExecutor stmtExecutor = new StmtExecutor(ctx, queryStr);
+        stmtExecutor.execute();
+        if (ctx.getState().getStateType() != QueryState.MysqlStateType.ERR) {
+            return stmtExecutor.planner();
+        } else {
+            return null;
+        }
+    }
 }