OF-3277: Introduce helper method to determine database 'limit' keyword

xmppserver/src/main/java/org/jivesoftware/database/DbConnectionManager.java

@@ -1143,6 +1143,33 @@ public static String getIdentifierQuoteString() {
         return identifierQuoteString;
     }
 
+    /**
+     * Returns the keyword or keyword phrase used by the active database to limit result sets.
+     *
+     * Depending on the database, this can be {@link ResultSetLimitKeyword#TOP},
+     * {@link ResultSetLimitKeyword#LIMIT}, or {@link ResultSetLimitKeyword#FETCH_FIRST}.
+     *
+     * @return the result-set limit keyword used by the current database.
+     * @see <a href="https://igniterealtime.atlassian.net/browse/OF-3277">OF-3277</a>
+     */
+    public static ResultSetLimitKeyword getResultSetLimitKeyword()
+    {
+        return databaseType.getResultSetLimitKeyword();
+    }
+
+    /**
+     * Indicates if the result-set limit keyword is used as a prefix before the selected columns.
+     *
+     * This is true for databases such as SQL Server that use {@code SELECT TOP (...) ...}.
+     *
+     * @return {@code true} if the keyword is prefix-style, otherwise {@code false}.
+     * @see <a href="https://igniterealtime.atlassian.net/browse/OF-3277">OF-3277</a>
+     */
+    public static boolean isResultSetLimitKeywordPrefix()
+    {
+        return databaseType.isResultSetLimitKeywordPrefix();
+    }
+
     public static String getTestSQL(String driver) {
         if (driver == null) {
             return "select 1";
@@ -1196,5 +1223,58 @@ public String escapeIdentifier(final String keyword) {
                 return keyword;
             }
         }
+
+        /**
+         * Returns the keyword or keyword phrase that should be used to limit result sets for this database type.
+         *
+         * @return the result-set limit keyword, or a sensible default when the database type is unknown.
+         * @see <a href="https://igniterealtime.atlassian.net/browse/OF-3277">OF-3277</a>
+         */
+        public ResultSetLimitKeyword getResultSetLimitKeyword()
+        {
+            return switch (this) {
+                case sqlserver -> ResultSetLimitKeyword.TOP;
+                case oracle, db2 -> ResultSetLimitKeyword.FETCH_FIRST;
+                default -> ResultSetLimitKeyword.LIMIT;
+            };
+        }
+
+        /**
+         * Indicates if the result-set limit keyword is used before the column list in a SELECT statement.
+         *
+         * @return {@code true} when the keyword is prefix-style (for example {@code TOP}), otherwise {@code false}.
+         * @see <a href="https://igniterealtime.atlassian.net/browse/OF-3277">OF-3277</a>
+         */
+        public boolean isResultSetLimitKeywordPrefix()
+        {
+            return getResultSetLimitKeyword().isPrefix();
+        }
+    }
+
+    /**
+     * Identifies how a database limits result sets.
+     *
+     * @see <a href="https://igniterealtime.atlassian.net/browse/OF-3277">OF-3277</a>
+     */
+    public enum ResultSetLimitKeyword
+    {
+        TOP(true),
+        FETCH_FIRST(false),
+        LIMIT(false);
+
+        private final boolean prefix;
+
+        ResultSetLimitKeyword(final boolean prefix) {
+            this.prefix = prefix;
+        }
+
+        /**
+         * Indicates if the keyword is used before the selected columns in a {@code SELECT} statement.
+         *
+         * @return {@code true} for prefix-style keywords such as {@code TOP}, otherwise {@code false}.
+         */
+        public boolean isPrefix() {
+            return prefix;
+        }
     }
 }

xmppserver/src/main/java/org/jivesoftware/openfire/pubsub/DefaultPubSubPersistenceProvider.java

@@ -1631,21 +1631,26 @@ else if (maxPublished != -1)
 		{
             con = DbConnectionManager.getConnection();
             // Get published items of the specified node
-            if (DbConnectionManager.getDatabaseType().equals(DbConnectionManager.DatabaseType.sqlserver)) {
-                pstmt = con.prepareStatement(LOAD_LAST_ITEMS_TOP);
-            } else if (DbConnectionManager.getDatabaseType().equals(DbConnectionManager.DatabaseType.oracle)) {
-                pstmt = con.prepareStatement(LOAD_LAST_ITEMS_FETCHFIRST);
-            } else {
-                pstmt = con.prepareStatement(LOAD_LAST_ITEMS_LIMIT);
+            switch (DbConnectionManager.getDatabaseType().getResultSetLimitKeyword()) {
+                case TOP:
+                    pstmt = con.prepareStatement(LOAD_LAST_ITEMS_TOP);
+                    break;
+                case FETCH_FIRST:
+                    pstmt = con.prepareStatement(LOAD_LAST_ITEMS_FETCHFIRST);
+                    break;
+                case LIMIT: // Intended fall-through
+                default:
+                    pstmt = con.prepareStatement(LOAD_LAST_ITEMS_LIMIT);
+                    break;
             }
             pstmt.setMaxRows(max);
             int paramIndex = 0;
-            if (DbConnectionManager.getDatabaseType().equals(DbConnectionManager.DatabaseType.sqlserver)){
+            if (DbConnectionManager.getDatabaseType().isResultSetLimitKeywordPrefix()) {
                 pstmt.setLong(++paramIndex, max);
             }
             pstmt.setString(++paramIndex, node.getUniqueIdentifier().getServiceIdentifier().getServiceId());
             pstmt.setString(++paramIndex, encodeNodeID(node.getNodeID()));
-            if (!DbConnectionManager.getDatabaseType().equals(DbConnectionManager.DatabaseType.sqlserver)){
+            if (!DbConnectionManager.getDatabaseType().isResultSetLimitKeywordPrefix()) {
                 pstmt.setLong(++paramIndex, max);
             }
             rs = pstmt.executeQuery();

xmppserver/src/test/java/org/jivesoftware/database/DbConnectionManagerTest.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2026 Ignite Realtime Foundation. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.jivesoftware.database;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class DbConnectionManagerTest {
+
+    /**
+     * Verifies that SQL Server uses TOP as a prefix-style row limiting keyword.
+     */
+    @Test
+    public void sqlServerUsesTopAsAPrefixKeyword() {
+        assertEquals(DbConnectionManager.ResultSetLimitKeyword.TOP, DbConnectionManager.DatabaseType.sqlserver.getResultSetLimitKeyword(), "SQL Server should map to the TOP limit keyword.");
+        assertTrue(DbConnectionManager.DatabaseType.sqlserver.isResultSetLimitKeywordPrefix(), "TOP should be marked as a prefix-style keyword.");
+    }
+
+    /**
+     * Verifies that Oracle uses FETCH FIRST as a suffix-style row limiting keyword.
+     */
+    @Test
+    public void oracleUsesFetchFirstAsASuffixKeyword() {
+        assertEquals(DbConnectionManager.ResultSetLimitKeyword.FETCH_FIRST, DbConnectionManager.DatabaseType.oracle.getResultSetLimitKeyword(), "Oracle should map to the FETCH_FIRST limit keyword.");
+        assertFalse(DbConnectionManager.DatabaseType.oracle.isResultSetLimitKeywordPrefix(), "FETCH_FIRST should be marked as a suffix-style keyword.");
+    }
+
+    /**
+     * Verifies that DB2 uses FETCH FIRST as a suffix-style row limiting keyword.
+     */
+    @Test
+    public void db2UsesFetchFirstAsASuffixKeyword() {
+        assertEquals(DbConnectionManager.ResultSetLimitKeyword.FETCH_FIRST, DbConnectionManager.DatabaseType.db2.getResultSetLimitKeyword(), "DB2 should map to the FETCH_FIRST limit keyword.");
+        assertFalse(DbConnectionManager.DatabaseType.db2.isResultSetLimitKeywordPrefix(), "DB2 should use a suffix-style limit keyword.");
+    }
+
+    /**
+     * Verifies that common ANSI-style databases map to LIMIT.
+     */
+    @Test
+    public void commonAnsiStyleDatabasesUseLimit() {
+        assertEquals(DbConnectionManager.ResultSetLimitKeyword.LIMIT, DbConnectionManager.DatabaseType.mysql.getResultSetLimitKeyword(), "MySQL should map to the LIMIT keyword.");
+        assertEquals(DbConnectionManager.ResultSetLimitKeyword.LIMIT, DbConnectionManager.DatabaseType.postgresql.getResultSetLimitKeyword(), "PostgreSQL should map to the LIMIT keyword.");
+        assertEquals(DbConnectionManager.ResultSetLimitKeyword.LIMIT, DbConnectionManager.DatabaseType.hsqldb.getResultSetLimitKeyword(), "HSQLDB should map to the LIMIT keyword.");
+        assertEquals(DbConnectionManager.ResultSetLimitKeyword.LIMIT, DbConnectionManager.DatabaseType.unknown.getResultSetLimitKeyword(), "Unknown database types should default to LIMIT.");
+    }
+
+    /**
+     * Verifies prefix-position metadata for all supported row limiting keywords.
+     */
+    @Test
+    public void enumKnowsWhetherItIsPrefixStyle() {
+        assertTrue(DbConnectionManager.ResultSetLimitKeyword.TOP.isPrefix(), "TOP should be treated as a prefix-style keyword.");
+        assertFalse(DbConnectionManager.ResultSetLimitKeyword.FETCH_FIRST.isPrefix(), "FETCH_FIRST should be treated as a suffix-style keyword.");
+        assertFalse(DbConnectionManager.ResultSetLimitKeyword.LIMIT.isPrefix(), "LIMIT should be treated as a suffix-style keyword.");
+    }
+}
+
+