add alter database doc for flink and spark and support comment when c…

…reate database in flink

docs/content/flink/sql-alter.md

@@ -227,3 +227,11 @@ The following SQL modifies the watermark strategy to `ts - INTERVAL '2' HOUR`.
 ```sql
 ALTER TABLE my_table MODIFY WATERMARK FOR ts AS ts - INTERVAL '2' HOUR
 ```
+
+# ALTER DATABASE
+
+The following SQL sets one or more properties in the specified database. If a particular property is already set in the database, override the old value with the new one.
+
+```sql
+ALTER DATABASE [catalog_name.]db_name SET (key1=val1, key2=val2, ...)
+```

docs/content/spark/sql-alter.md

@@ -240,3 +240,13 @@ The following SQL changes the type of a nested column `f2` to `BIGINT` in a stru
 -- column v previously has type MAP<INT, STRUCT<f1: STRING, f2: INT>>
 ALTER TABLE my_table ALTER COLUMN v.value.f2 TYPE BIGINT;
 ```
+
+
+# ALTER DATABASE
+
+The following SQL sets one or more properties in the specified database. If a particular property is already set in the database, override the old value with the new one.
+
+```sql
+ALTER { DATABASE | SCHEMA | NAMESPACE } my_database
+    SET { DBPROPERTIES | PROPERTIES } ( property_name = property_value [ , ... ] )
+```

paimon-flink/paimon-flink-common/src/main/java/org/apache/paimon/flink/FlinkCatalog.java

@@ -239,19 +239,19 @@ public CatalogDatabase getDatabase(String databaseName)
     @Override
     public void createDatabase(String name, CatalogDatabase database, boolean ignoreIfExists)
             throws DatabaseAlreadyExistException, CatalogException {
+        Map<String, String> properties;
         if (database != null) {
+            properties = new HashMap<>(database.getProperties());
             if (database.getDescription().isPresent()
                     && !database.getDescription().get().equals("")) {
-                throw new UnsupportedOperationException(
-                        "Create database with description is unsupported.");
+                properties.put(Catalog.COMMENT_PROP, database.getDescription().get());
             }
+        } else {
+            properties = Collections.emptyMap();
         }
 
         try {
-            catalog.createDatabase(
-                    name,
-                    ignoreIfExists,
-                    database == null ? Collections.emptyMap() : database.getProperties());
+            catalog.createDatabase(name, ignoreIfExists, properties);
         } catch (Catalog.DatabaseAlreadyExistException e) {
             throw new DatabaseAlreadyExistException(getName(), e.database());
         }

paimon-flink/paimon-flink-common/src/test/java/org/apache/paimon/flink/FlinkCatalogTest.java

@@ -99,6 +99,7 @@
 import static org.assertj.core.api.Assertions.assertThatCode;
 import static org.assertj.core.api.Assertions.assertThatCollection;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.mockito.Mockito.doNothing;
 import static org.mockito.Mockito.spy;
@@ -581,11 +582,9 @@ public void testCreateDb_DatabaseWithProperties() throws Exception {
     }
 
     @Test
-    public void testCreateDb_DatabaseWithCommentException() {
+    public void testCreateDb_DatabaseWithCommentSuccessful() throws DatabaseAlreadyExistException {
         CatalogDatabaseImpl database = new CatalogDatabaseImpl(Collections.emptyMap(), "haha");
-        assertThatThrownBy(() -> catalog.createDatabase(path1.getDatabaseName(), database, false))
-                .isInstanceOf(UnsupportedOperationException.class)
-                .hasMessage("Create database with description is unsupported.");
+        assertDoesNotThrow(() -> catalog.createDatabase(path1.getDatabaseName(), database, false));
     }
 
     @ParameterizedTest
@@ -612,6 +611,8 @@ public void testAlterDb() throws DatabaseAlreadyExistException, DatabaseNotExist
         catalog.createDatabase(path1.getDatabaseName(), database, false);
         Map<String, String> properties = Collections.singletonMap("haa", "ccc");
         CatalogDatabaseImpl newDatabase = new CatalogDatabaseImpl(properties, "haha");
+        // as file system catalog don't support alter database, so we have to use mock to overview
+        // this method to test
         Catalog mockCatalog = spy(catalog);
         doNothing().when(mockCatalog).alterDatabase(path1.getDatabaseName(), newDatabase, false);
         when(mockCatalog.getDatabase(path1.getDatabaseName())).thenReturn(database);