Add index on (project, time)

This allows for efficient queries for all ref-updates in a given project
before/after some time.

Change-Id: If725ffa945d265167f8b3caaf8a0742264f5aa65

src/main/java/com/googlesource/gerrit/plugins/spannerrefdb/DatabaseSchemaCreator.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/spannerrefdb/DatabaseSchemaCreator.java b/src/main/java/com/googlesource/gerrit/plugins/spannerrefdb/DatabaseSchemaCreator.java
index 0bf042e..aa8c4ac 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/spannerrefdb/DatabaseSchemaCreator.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/spannerrefdb/DatabaseSchemaCreator.java
@@ -62,7 +62,8 @@
                       + ")"
                       + "PRIMARY KEY (project, ref)",
                   "ALTER TABLE refs ADD COLUMN IF NOT EXISTS "
-                      + "time TIMESTAMP OPTIONS (allow_commit_timestamp=true)"),
+                      + "time TIMESTAMP OPTIONS (allow_commit_timestamp=true)",
+                  "CREATE INDEX IF NOT EXISTS project_time ON refs(project, time)"),
               null)
           .get();
     } catch (Exception e) {