db: Fix uniqueness constraint on CollateralTxIn table

The uniqueness constraints used to be on `txOutId` and `txOutIndex` but
that is not correct because collateral inputs that are not consumed can
be reused. Instead we put the uniqueness constraint on the `txInId`.

Closes: #664

cardano-db/src/Cardano/Db/Schema.hs

@@ -136,7 +136,7 @@ share
     txInId              TxId                OnDeleteCascade     -- The transaction where this is used as an input.
     txOutId             TxId                OnDeleteCascade     -- The transaction where this was created as an output.
     txOutIndex          Word16              sqltype=txindex
-    UniqueColTxin       txOutId txOutIndex
+    UniqueColTxin       txInId
 
   -- A table containing metadata about the chain. There will probably only ever be one
   -- row in this table.

schema/migration-2-0008-20210623.sql

@@ -0,0 +1,20 @@
+-- Persistent generated migration.
+
+CREATE FUNCTION migrate() RETURNS void AS $$
+DECLARE
+  next_version int ;
+BEGIN
+  SELECT stage_two + 1 INTO next_version FROM schema_version ;
+  IF next_version = 8 THEN
+    EXECUTE 'ALTER TABLE "collateral_tx_in" DROP CONSTRAINT "unique_col_txin"' ;
+    EXECUTE 'ALTER TABLE "collateral_tx_in" ADD CONSTRAINT "unique_col_txin" UNIQUE("tx_in_id")' ;
+    -- Hand written SQL statements can be added here.
+    UPDATE schema_version SET stage_two = next_version ;
+    RAISE NOTICE 'DB has been migrated to stage_two version %', next_version ;
+  END IF ;
+END ;
+$$ LANGUAGE plpgsql ;
+
+SELECT migrate() ;
+
+DROP FUNCTION migrate() ;