Please take a look at the following query:

SELECT cc.number_card
FROM customers_cards cc
JOIN customers_card_psp ccp
  ON cc.id = ccp.card_id AND ccp.psp_id = '12' AND ccp.status = 'old_ofogh_koorosh'
JOIN xxx
  ON xxx.card_number = cc.number_card

Note, xxx is a temp table:

CREATE TEMPORARY TABLE xxx ( card_number varchar(191) COLLATE utf8mb4_unicode_ci )

Here is the result of EXPLAIN when there is no index for xxx table:

And here is the result of EXPLAIN when there is a index xxx(card_number):

As you can see, in the no-index situation, less rows get scanned. So, what’s preferred? Using an index or not?