When changing the length of a VARCHAR column, we don't need to copy the table

h2/src/main/org/h2/command/ddl/AlterTableAlterColumn.java

@@ -32,6 +32,7 @@ import org.h2.table.Column;
 import org.h2.table.Table;
 import org.h2.table.TableView;
 import org.h2.util.New;
+import org.h2.value.Value;
 
 /**
  * This class represents the statements
@@ -112,16 +113,31 @@ public class AlterTableAlterColumn extends SchemaCommand {
             break;
         }
         case CommandInterface.ALTER_TABLE_ALTER_COLUMN_CHANGE_TYPE: {
-            oldColumn.setSequence(null);
-            oldColumn.setDefaultExpression(session, null);
-            oldColumn.setConvertNullToDefault(false);
-            if (oldColumn.isNullable() && !newColumn.isNullable()) {
-                checkNoNullValues();
-            } else if (!oldColumn.isNullable() && newColumn.isNullable()) {
-                checkNullable();
+            // If the change is only increasing the length of a VARCHAR type, then we don't need to copy the table
+            // because the length is only a constraint, and does not affect the storage structure.
+            if (oldColumn.getType() == Value.STRING
+                && oldColumn.getPrecision() <= newColumn.getPrecision()
+                && oldColumn.isNullable() == newColumn.isNullable()
+                && oldColumn.isAutoIncrement() == newColumn.isAutoIncrement()
+                && oldColumn.isPrimaryKey() == newColumn.isPrimaryKey()
+                && oldColumn.getType() == newColumn.getType())
+            {
+                oldColumn.setPrecision(newColumn.getPrecision());
+                db.update(session, table);
+            }
+            else
+            {
+                oldColumn.setSequence(null);
+                oldColumn.setDefaultExpression(session, null);
+                oldColumn.setConvertNullToDefault(false);
+                if (oldColumn.isNullable() && !newColumn.isNullable()) {
+                    checkNoNullValues();
+                } else if (!oldColumn.isNullable() && newColumn.isNullable()) {
+                    checkNullable();
+                }
+                convertAutoIncrementColumn(newColumn);
+                copyData();
             }
-            convertAutoIncrementColumn(newColumn);
-            copyData();
             break;
         }
         case CommandInterface.ALTER_TABLE_ADD_COLUMN: {

h2/src/main/org/h2/table/Column.java

@@ -62,7 +62,7 @@ public class Column {
     public static final int NULLABLE_UNKNOWN = ResultSetMetaData.columnNullableUnknown;
 
     private final int type;
-    private final long precision;
+    private long precision;
     private final int scale;
     private final int displaySize;
     private Table table;
@@ -242,6 +242,10 @@ public class Column {
         return precision;
     }
 
+    public void setPrecision(long p) {
+        precision = p;
+    }
+    
     public int getDisplaySize() {
         return displaySize;
     }

h2/src/test/org/h2/test/db/TestAlter.java

@@ -40,6 +40,7 @@ public class TestAlter extends TestBase {
         testAlterTableAlterColumn();
         testAlterTableDropIdentityColumn();
         testAlterTableAddColumnIfNotExists();
+        testAlterTableAlterColumn2();
         conn.close();
         deleteDb("alter");
     }
@@ -135,4 +136,11 @@ public class TestAlter extends TestBase {
         stat.execute("drop table t");
     }
 
+    private void testAlterTableAlterColumn2() throws SQLException {
+        // ensure that increasing a VARCHAR columns length takes effect because we optimise this case
+        stat.execute("create table t(x varchar(2)) as select 'x'");
+        stat.execute("alter table t alter column x varchar(20)");
+        stat.execute("insert into t values('Hello')");
+        stat.execute("drop table t");
+    }
 }