SELECT 
  SQL_CALC_FOUND_ROWS products.product_id, 
  products.product_type, 
  products.parent_product_id 
FROM 
  cscart_products as products 
  LEFT JOIN cscart_product_descriptions as descr1 ON descr1.product_id = products.product_id 
  AND descr1.lang_code = 'de' 
  INNER JOIN cscart_products_categories as products_categories ON products_categories.product_id = products.product_id 
  INNER JOIN cscart_categories ON cscart_categories.category_id = products_categories.category_id 
  AND (
    cscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_categories.usergroup_ids
    )
  ) 
WHERE 
  1 
  AND pos_availabilty IN ('S', 'N') 
  AND cscart_categories.category_id IN (9) 
  AND (
    products.usergroup_ids = '' 
    OR FIND_IN_SET(0, products.usergroup_ids) 
    OR FIND_IN_SET(1, products.usergroup_ids)
  ) 
  AND products.status = 'A' 
  AND products.parent_product_id = 0 
GROUP BY 
  products.product_id 
ORDER BY 
  product asc 
LIMIT 
  36, 12

Query time 0.00656

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "328.45"
    },
    "ordering_operation": {
      "using_filesort": true,
      "grouping_operation": {
        "using_temporary_table": true,
        "using_filesort": false,
        "nested_loop": [
          {
            "table": {
              "table_name": "cscart_categories",
              "access_type": "const",
              "possible_keys": [
                "PRIMARY",
                "c_status",
                "p_category_id"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "category_id"
              ],
              "key_length": "3",
              "ref": [
                "const"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 1,
              "filtered": "100.00",
              "cost_info": {
                "read_cost": "0.00",
                "eval_cost": "0.10",
                "prefix_cost": "0.00",
                "data_read_per_join": "4K"
              },
              "used_columns": [
                "category_id",
                "usergroup_ids"
              ]
            }
          },
          {
            "table": {
              "table_name": "products_categories",
              "access_type": "ref",
              "possible_keys": [
                "PRIMARY",
                "pt"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "category_id"
              ],
              "key_length": "3",
              "ref": [
                "const"
              ],
              "rows_examined_per_scan": 664,
              "rows_produced_per_join": 664,
              "filtered": "100.00",
              "using_index": true,
              "cost_info": {
                "read_cost": "4.39",
                "eval_cost": "66.40",
                "prefix_cost": "70.79",
                "data_read_per_join": "10K"
              },
              "used_columns": [
                "product_id",
                "category_id"
              ]
            }
          },
          {
            "table": {
              "table_name": "products",
              "access_type": "eq_ref",
              "possible_keys": [
                "PRIMARY",
                "status",
                "idx_parent_product_id"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "product_id"
              ],
              "key_length": "3",
              "ref": [
                "dbggbern.products_categories.product_id"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 72,
              "filtered": "10.87",
              "cost_info": {
                "read_cost": "166.00",
                "eval_cost": "7.22",
                "prefix_cost": "303.19",
                "data_read_per_join": "368K"
              },
              "used_columns": [
                "product_id",
                "product_type",
                "status",
                "usergroup_ids",
                "parent_product_id",
                "pos_availabilty"
              ],
              "attached_condition": "((`dbggbern`.`products`.`parent_product_id` = 0) and (`dbggbern`.`products`.`pos_availabilty` in ('S','N')) and ((`dbggbern`.`products`.`usergroup_ids` = '') or (0 <> find_in_set(0,`dbggbern`.`products`.`usergroup_ids`)) or (0 <> find_in_set(1,`dbggbern`.`products`.`usergroup_ids`))) and (`dbggbern`.`products`.`status` = 'A'))"
            }
          },
          {
            "table": {
              "table_name": "descr1",
              "access_type": "eq_ref",
              "possible_keys": [
                "PRIMARY",
                "product_id"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "product_id",
                "lang_code"
              ],
              "key_length": "9",
              "ref": [
                "dbggbern.products_categories.product_id",
                "const"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 72,
              "filtered": "100.00",
              "cost_info": {
                "read_cost": "18.04",
                "eval_cost": "7.22",
                "prefix_cost": "328.45",
                "data_read_per_join": "275K"
              },
              "used_columns": [
                "product_id",
                "lang_code",
                "product"
              ]
            }
          }
        ]
      }
    }
  }
}