SELECT 
  a.modifier, 
  a.modifier_type, 
  IF(
    shared_option_variants.variant_id IS NOT NULL, 
    shared_option_variants.modifier, 
    a.modifier
  ) as modifier, 
  IF(
    shared_option_variants.variant_id IS NOT NULL, 
    shared_option_variants.modifier_type, 
    a.modifier_type
  ) as modifier_type, 
  a.variant_id 
FROM 
  cscart_product_option_variants a 
  LEFT JOIN cscart_ult_product_option_variants shared_option_variants ON shared_option_variants.variant_id = a.variant_id 
  AND shared_option_variants.company_id = 1 
WHERE 
  1 
  AND a.variant_id IN (6)

Query time 0.00030

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "nested_loop": [
      {
        "table": {
          "table_name": "a",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "shared_option_variants",
          "access_type": "const",
          "possible_keys": ["PRIMARY", "company_id"],
          "key": "PRIMARY",
          "key_length": "7",
          "used_key_parts": ["variant_id", "company_id"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100,
          "unique_row_not_found": true
        }
      }
    ]
  }
}