Change SQL statement InsertNewAsset into an upsert.

tapdb/sqlc/migrations/000013_assets_unique_constraint.up.sql

@@ -0,0 +1,31 @@
+-- We will apply a new unique constraint on the assets table. This constraint
+-- will be on the columns (anchor_utxo_id, genesis_id, script_key_id).
+-- Rows in the existing table may violate this constraint, so we need to delete
+-- all but one row in each violating set of rows before applying the constraint.
+WITH duplicate_rows AS (
+    SELECT
+        asset_id,
+        anchor_utxo_id,
+        genesis_id,
+        script_key_id,
+
+        -- This is the row number of the row within the set of rows that violate
+        -- the constraint. We will delete all rows with a row number greater
+        -- than 1.
+        ROW_NUMBER() OVER (PARTITION BY anchor_utxo_id, genesis_id, script_key_id ORDER BY asset_id) AS row_num
+    FROM assets
+)
+DELETE FROM assets
+WHERE (anchor_utxo_id, genesis_id, script_key_id) IN (
+    SELECT anchor_utxo_id, genesis_id, script_key_id
+    FROM duplicate_rows
+    -- Delete all rows with a row number greater than 1. This should leave
+    -- exactly one row (row number 0) for each set of rows that violate the
+    -- constraint.
+    WHERE row_num > 1
+);
+
+-- Create a unique index on the new table
+CREATE UNIQUE INDEX assets_uniqueness_index_anchor_utxo_id_genesis_id_script_key_id
+ON assets (anchor_utxo_id, genesis_id, script_key_id);
+