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SELECT 
  product_id, 
  group_id 
FROM 
  cscart_product_variation_group_products 
WHERE 
  group_id IN (120, 25, 87, 84, 31, 82, 86, 89)

Query time 0.00040

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "40.21"
    },
    "table": {
      "table_name": "cscart_product_variation_group_products",
      "access_type": "range",
      "possible_keys": [
        "idx_group_id"
      ],
      "key": "idx_group_id",
      "used_key_parts": [
        "group_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 23,
      "rows_produced_per_join": 23,
      "filtered": "100.00",
      "index_condition": "(`cscart`.`cscart_product_variation_group_products`.`group_id` in (120,25,87,84,31,82,86,89))",
      "cost_info": {
        "read_cost": "35.61",
        "eval_cost": "4.60",
        "prefix_cost": "40.21",
        "data_read_per_join": "368"
      },
      "used_columns": [
        "product_id",
        "group_id"
      ]
    }
  }
}

Result

product_id	group_id
565	25
564	25
567	25
566	25
592	31
593	31
831	82
830	82
829	82
828	82
827	82
835	84
834	84
846	86
845	86
844	86
843	86
842	86
851	87
850	87
849	87
848	87
847	87
858	89
857	89
856	89
855	89
854	89
972	120
973	120
974	120
975	120

product_id	group_id
565	25
564	25
567	25
566	25
592	31
593	31
831	82
830	82
829	82
828	82
827	82
835	84
834	84
846	86
845	86
844	86
843	86
842	86
851	87
850	87
849	87
848	87
847	87
858	89
857	89
856	89
855	89
854	89
972	120
973	120
974	120
975	120

product_id	group_id
565	25
564	25
567	25
566	25
592	31
593	31
831	82
830	82
829	82
828	82
827	82
835	84
834	84
846	86
845	86
844	86
843	86
842	86
851	87
850	87
849	87
848	87
847	87
858	89
857	89
856	89
855	89
854	89
972	120
973	120
974	120
975	120

product_id	group_id
565	25
564	25
567	25
566	25
592	31
593	31
831	82
830	82
829	82
828	82
827	82
835	84
834	84
846	86
845	86
844	86
843	86
842	86
851	87
850	87
849	87
848	87
847	87
858	89
857	89
856	89
855	89
854	89
972	120
973	120
974	120
975	120