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SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = 'ea978e7cd367e812ea8db69290cdbbb5fafa5cb1886ed75dd9f166a4cc5bdb5e' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00015

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "no matching row in const table"
  }
}