add index on wire_targets

- better performance on local db
- many queries use wire_target_serial_id as join condition -> discussion use this as shard key?

src/exchangedb/exchange-0001.sql

@@ -88,6 +88,14 @@ CREATE TABLE IF NOT EXISTS wire_targets_default
   PARTITION OF wire_targets
   FOR VALUES WITH (MODULUS 1, REMAINDER 0);
 
+-- FIXME partition by serial_id rather than h_payto, 
+-- it is used more in join conditions - crucial for sharding to select this.
+-- Author: (Boss Marco)
+CREATE INDEX IF NOT EXISTS wire_targets_serial_id_index
+  ON wire_targets
+  (wire_target_serial_id
+  );
+
 CREATE TABLE IF NOT EXISTS reserves
   (reserve_uuid BIGINT GENERATED BY DEFAULT AS IDENTITY
   ,reserve_pub BYTEA PRIMARY KEY CHECK(LENGTH(reserve_pub)=32)