Parse DUAL, SYS.DUAL, SYSDUMMY1, and SYSIBM.SYSDUMMY1 in better way

h2/src/main/org/h2/command/Parser.java

@@ -684,10 +684,6 @@ public class Parser {
             if (equalsToken("SESSION", schemaName)) {
                 // for local temporary tables
                 schema = database.getSchema(session.getCurrentSchemaName());
-            } else if (database.getMode().sysDummy1 &&
-                    "SYSIBM".equals(schemaName)) {
-                // IBM DB2 and Apache Derby compatibility: SYSIBM.SYSDUMMY1
-                schema = database.getSchema(session.getCurrentSchemaName());
             }
         }
         return schema;
@@ -1372,7 +1368,7 @@ public class Parser {
     private TableFilter readTableFilter() {
         Table table;
         String alias = null;
-        if (readIf("(")) {
+        label: if (readIf("(")) {
             if (isSelect()) {
                 Query query = parseSelectUnion();
                 read(")");
@@ -1406,7 +1402,19 @@ public class Parser {
             table = parseValuesTable(0).getTable();
         } else {
             String tableName = readIdentifierWithSchema(null);
-            Schema schema = getSchema();
+            Schema schema;
+            if (schemaName == null) {
+                schema = null;
+            } else {
+                schema = findSchema(schemaName);
+                if (schema == null) {
+                    if (isDualTable(tableName)) {
+                        table = getDualTable(false);
+                        break label;
+                    }
+                    throw DbException.get(ErrorCode.SCHEMA_NOT_FOUND_1, schemaName);
+                }
+            }
             boolean foundLeftBracket = readIf("(");
             if (foundLeftBracket && readIf("INDEX")) {
                 // Sybase compatibility with
@@ -1442,13 +1450,8 @@ public class Parser {
                     }
                     table = new FunctionTable(mainSchema, session, expr, call);
                 }
-            } else if (equalsToken("DUAL", tableName)) {
-                table = getDualTable(false);
-            } else if (database.getMode().sysDummy1 &&
-                    equalsToken("SYSDUMMY1", tableName)) {
-                table = getDualTable(false);
             } else {
-                table = readTableOrView(tableName);
+                table = readTableOrView(tableName, true);
             }
         }
         ArrayList<String> derivedColumnNames = null;
@@ -5933,15 +5936,23 @@ public class Parser {
         return command;
     }
 
+    boolean isDualTable(String tableName) {
+        return ((schemaName == null || equalsToken(schemaName, "SYS")) && equalsToken("DUAL", tableName))
+                || (database.getMode().sysDummy1 && (schemaName == null || equalsToken(schemaName, "SYSIBM")))
+                        && equalsToken("SYSDUMMY1", tableName);
+    }
+
     private Table readTableOrView() {
-        return readTableOrView(readIdentifierWithSchema(null));
+        return readTableOrView(readIdentifierWithSchema(null), false);
     }
 
-    private Table readTableOrView(String tableName) {
-        // same algorithm than readSequence
+    private Table readTableOrView(String tableName, boolean allowDual) {
         if (schemaName != null) {
-            return getSchema().getTableOrView(session, tableName);
+            Table table = getSchema().resolveTableOrView(session, tableName);
+            if (table != null) {
+                return table;
             }
+        } else {
             Table table = database.getSchema(session.getCurrentSchemaName())
                     .resolveTableOrView(session, tableName);
             if (table != null) {
@@ -5957,6 +5968,10 @@ public class Parser {
                     }
                 }
             }
+        }
+        if (allowDual && isDualTable(tableName)) {
+            return getDualTable(false);
+        }
         throw DbException.get(ErrorCode.TABLE_OR_VIEW_NOT_FOUND_1, tableName);
     }
 

h2/src/test/org/h2/test/scripts/TestScript.java

@@ -87,6 +87,7 @@ public class TestScript extends TestBase {
 
         testScript("testScript.sql");
         testScript("derived-column-names.sql");
+        testScript("dual.sql");
         testScript("indexes.sql");
         testScript("information_schema.sql");
         testScript("joins.sql");

h2/src/test/org/h2/test/scripts/dual.sql

+-- Copyright 2004-2018 H2 Group. Multiple-Licensed under the MPL 2.0,
+-- and the EPL 1.0 (http://h2database.com/html/license.html).
+-- Initial Developer: H2 Group
+--
+
+SELECT * FROM DUAL;
+>> 1
+
+CREATE TABLE DUAL(A INT);
+> ok
+
+INSERT INTO DUAL VALUES (2);
+> update count: 1
+
+SELECT A FROM DUAL;
+>> 2
+
+SELECT * FROM SYS.DUAL;
+>> 1
+
+DROP TABLE DUAL;
+> ok
+
+SET MODE DB2;
+> ok
+
+SELECT * FROM SYSDUMMY1;
+>> 1
+
+CREATE TABLE SYSDUMMY1(A INT);
+> ok
+
+INSERT INTO SYSDUMMY1 VALUES (2);
+> update count: 1
+
+SELECT A FROM SYSDUMMY1;
+>> 2
+
+SELECT * FROM SYSIBM.SYSDUMMY1;
+>> 1
+
+DROP TABLE SYSDUMMY1;
+> ok
+
+SET MODE Regular;
+> ok