Цикл while же выполняется пропорционально количеству цифр в числе..Да. А количество цифр в числе C это ⌊log10C⌋ + 1.
UPDATE не выйдет.ON DUPLICATE KEY UPDATE, кидать в таблицу каждый раз новую строку, а потом в фоне её периодически пылесосить от мусора. 
i = 0
while i < len(arr):
x = arr[i]
j = i+1
while j < len(arr):
if ShouldDelete(arr[j], arr[i]):
del a[j]arr[i+1:] = y for y in arr[i+1:] if not ShouldDelete(x, arr[i])A phantom power technique is used to allow the powered pairs to also carry data. This permits its use not only with 10BASE-T and 100BASE-TX, which use only two of the four pairs in the cable, but also with 1000BASE-T (gigabit Ethernet), 2.5GBASE-T, 5GBASE-T, and 10GBASE-T which use all four pairs for data transmission. This is possible because all versions of Ethernet over twisted pair cable specify differential data transmission over each pair with transformer coupling; the DC supply and load connections can be made to the transformer center-taps at each end. Each pair thus operates in common mode as one side of the DC supply, so two pairs are required to complete the circuit. The polarity of the DC supply may be inverted by crossover cables; the powered device must operate with either pair: spare pairs 4–5 and 7–8 or data pairs 1–2 and 3–6. Polarity is defined by the standards on spare pairs, and ambiguously implemented for data pairs, with the use of a diode bridge.
SELECT
ch.id_element,
ch.`table`,
ch.param,
GROUP_CONCAT(cu.name) AS value_new
FROM
c_history ch
LEFT JOIN
c_object co ON co.id = ch.id_element
LEFT JOIN
c_user cu ON FIND_IN_SET(cu.id, ch.value_old)
WHERE
ch.`table` = 'c_object'
AND ch.param = 'ids_user_merge'
GROUP BY
ch.id_element, ch.`table`, ch.param;