db: Add indices on tx_out and pool_update

schema/migration-3-0004-20200810.sql

@@ -0,0 +1,28 @@
+CREATE FUNCTION migrate() RETURNS void AS $$
+DECLARE
+  next_version int ;
+BEGIN
+  SELECT stage_three + 1 INTO next_version FROM schema_version ;
+  IF next_version <= 4 THEN
+    -- -------------------------------------------------------------------------
+    -- More hand crafted indices for performance
+
+    -- helpful in case anyone is looking for a utxo by a payment credential.
+
+    CREATE INDEX idx_tx_out_payment_cred
+    ON tx_out(payment_cred);
+
+    CREATE INDEX idx_pool_update_hash_id
+    ON pool_update(hash_id);
+
+    -- -------------------------------------------------------------------------
+
+    UPDATE schema_version SET stage_three = 4 ;
+    RAISE NOTICE 'DB has been migrated to stage_three version %', next_version ;
+  END IF ;
+END ;
+$$ LANGUAGE plpgsql ;
+
+SELECT migrate() ;
+
+DROP FUNCTION migrate() ;