ChangeMessageAccess: Add index for byPathSet method.

When using a sql datastore, ChangeMessageAccess#byPathSet ends
up creating a request filtering results with columns patchset_change_id
and patchset_patch_set_id, none of which is indexed. This results
in a full table scan.

Since change I6bc6391f456d9faeab7f1212b9c0463e7514cc05, change messages
are retrieved by patch set for highlighting purpose in ListChanges.

This can have a significant impact on dashboard. On a gerrit instance
I manage, which has about 70,000 lines in change_messages, one user
dashboard with 137 displayed changes went from an average display
time of 2.4s to 0.7s by adding this index.

Change-Id: If92f5b4f924617c69a1318f31a5ca01f320af792

gerrit-reviewdb/src/main/resources/com/google/gerrit/reviewdb/server/index_generic.sql

@@ -122,6 +122,9 @@ ON patch_set_approvals (change_open, account_id, change_sort_key);
 -- ChangeMessageAccess
 --    @PrimaryKey covers: byChange
 
+--    covers:             byPatchSet
+CREATE INDEX change_messages_byPatchset
+ON change_messages (patchset_change_id, patchset_patch_set_id);
 
 -- *********************************************************************
 -- PatchLineCommentAccess

gerrit-reviewdb/src/main/resources/com/google/gerrit/reviewdb/server/index_postgres.sql

@@ -203,6 +203,9 @@ WHERE change_open = 'N';
 -- ChangeMessageAccess
 --    @PrimaryKey covers: byChange
 
+--    covers:             byPatchSet
+CREATE INDEX change_messages_byPatchset
+ON change_messages (patchset_change_id, patchset_patch_set_id);
 
 -- *********************************************************************
 -- PatchLineCommentAccess

gerrit-server/src/main/java/com/google/gerrit/server/schema/SchemaVersion.java

@@ -32,7 +32,7 @@ import java.util.List;
 /** A version of the database schema. */
 public abstract class SchemaVersion {
   /** The current schema version. */
-  public static final Class<Schema_72> C = Schema_72.class;
+  public static final Class<Schema_73> C = Schema_73.class;
 
   public static class Module extends AbstractModule {
     @Override

gerrit-server/src/main/java/com/google/gerrit/server/schema/Schema_73.java

@@ -0,0 +1,43 @@
+// Copyright (C) 2012 The Android Open Source Project
+//
+// 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 com.google.gerrit.server.schema;
+
+import com.google.gerrit.reviewdb.server.ReviewDb;
+import com.google.gwtorm.jdbc.JdbcSchema;
+import com.google.inject.Inject;
+import com.google.inject.Provider;
+
+import java.sql.SQLException;
+import java.sql.Statement;
+
+
+public class Schema_73 extends SchemaVersion {
+  @Inject
+  Schema_73(Provider<Schema_72> prior) {
+    super(prior);
+  }
+
+  @Override
+  protected void migrateData(final ReviewDb db, final UpdateUI ui)
+      throws SQLException {
+    final Statement stmt = ((JdbcSchema) db).getConnection().createStatement();
+    try {
+      stmt.executeUpdate("CREATE INDEX change_messages_byPatchset ON change_messages (patchset_change_id, patchset_patch_set_id )");
+    }
+    finally {
+      stmt.close();
+    }
+  }
+}