Sandbox

SELECT 
  cscart_cp_popups.* 
FROM 
  cscart_cp_popups 
  LEFT JOIN cscart_companies ON cscart_companies.company_id = cscart_cp_popups.company_id 
WHERE 
  cscart_cp_popups.section IN ('A', 'C') 
  AND cscart_cp_popups.status = 'A' 
  AND (
    cscart_cp_popups.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_cp_popups.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_cp_popups.usergroup_ids
    )
  ) 
  AND cscart_companies.company_id = 1 
ORDER BY 
  cscart_cp_popups.priority DESC

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "Impossible WHERE noticed after reading const tables"
  }
}