add PRIMARY KEY

Author: Ghost-chu

src/main/java/org/maxgamer/quickshop/database/DatabaseHelper.java

@@ -110,7 +110,7 @@ private void createLogsTable() {
 
     private void createExternalCacheTable() {
         String createTable = "CREATE TABLE " + plugin.getDbPrefix()
-                + "external_cache ( x  INTEGER(32) NOT NULL, y  INTEGER(32) NOT NULL, z  INTEGER(32) NOT NULL, world VARCHAR(32) NOT NULL );";
+                + "external_cache ( x  INTEGER(32) NOT NULL, y  INTEGER(32) NOT NULL, z  INTEGER(32) NOT NULL, world VARCHAR(32) NOT NULL ) PRIMARY KEY (x, y, z, world);";
         manager.runInstantTask(new DatabaseTask(createTable));
         createColumn("external_cache", "space", new DataType(DataTypeMapping.INT, null));
         createColumn("external_cache", "stock", new DataType(DataTypeMapping.INT, null));