Optimize indexes (#65)

The existing `blocks_size_created_status` index did not work well with
packing queries, so it is replaced by `blocks_pack_status_index` which
proved to work better after some experiments.

In addition we are adding `blocks_deleted_false_index` to optimize on
some queries that select blocks that have not been deleted yet.

db/db.go

@@ -106,6 +106,16 @@ func (db *DB) Migration() *migrate.Migration {
 					`CREATE INDEX IF NOT EXISTS blocks_size_created_status_index ON blocks ( size, created, pack_status )`,
 				},
 			},
+			{
+				DB:          &db.DB,
+				Description: "Improve indexes",
+				Version:     2,
+				Action: migrate.SQL{
+					`DROP INDEX IF EXISTS blocks_size_created_status_index`,
+					`CREATE INDEX IF NOT EXISTS blocks_pack_status_index ON blocks ( pack_status ) INCLUDE ( size, created )`,
+					`CREATE INDEX IF NOT EXISTS blocks_deleted_false_index ON blocks ( deleted ) WHERE deleted = false`,
+				},
+			},
 		},
 	}
 }