Fix migration that drops unique index (#384)

packages/backend/src/peripherals/database/migrations/032_drop_uniqueness_from_preprocessed.ts

@@ -14,27 +14,38 @@ should create a new migration file that fixes the issue.
 import { Knex } from 'knex'
 
 export async function up(knex: Knex) {
+  const uniqueIndexName =
+    'preprocessed_asset_history_stark_key_asset_hash_or_id_unique'
+  const indexExists = await pgCheckIfIndexExists(knex, uniqueIndexName)
+  if (!indexExists) {
+    // This migration was already run, so we can safely
+    // return without doing anything. This is because the down()
+    // migration is not possible to run - see the comment there.
+    return
+  }
+
   await knex.schema.table('preprocessed_asset_history', (table) => {
-    table.dropIndex(
-      [],
-      'preprocessed_asset_history_stark_key_asset_hash_or_id_unique'
-    )
+    table.dropIndex([], uniqueIndexName)
 
     table.index(['stark_key', 'asset_hash_or_id'], undefined, {
       predicate: knex.whereRaw('is_current = true'),
     })
   })
 }
 
-export async function down(knex: Knex) {
-  await knex.schema.table('preprocessed_asset_history', (table) => {
-    table.dropIndex(
-      [],
-      'preprocessed_asset_history_stark_key_asset_hash_or_id_index'
-    )
+export async function down() {
+  // Not possible to re-add unique index (duplicates in the table),
+}
 
-    table.unique(['stark_key', 'asset_hash_or_id'], {
-      predicate: knex.whereRaw('is_current = true'),
-    })
-  })
+async function pgCheckIfIndexExists(
+  knex: Knex,
+  indexName: string
+): Promise<boolean> {
+  // eslint-disable-next-line @typescript-eslint/no-unsafe-assignment
+  const result = await knex.raw(
+    "SELECT * FROM pg_indexes WHERE indexname = ? AND schemaname = 'public'",
+    [indexName]
+  )
+  // eslint-disable-next-line @typescript-eslint/no-unsafe-member-access
+  return result.rows.length > 0
 }