Mprove does it the way the Looker did it before:

   CREATE TEMPORARY FUNCTION mprove_array_sum(ar ARRAY<STRING>) AS
  ((SELECT SUM(CAST(REGEXP_EXTRACT(val, '\\|\\|(\\-?\\d+(?:.\\d+)?)$') AS FLOAT64)) FROM UNNEST(ar) as val));

...

   SELECT COALESCE(mprove_array_sum(ARRAY_AGG(DISTINCT CONCAT(CONCAT(CAST(a.id AS STRING), '||'), CAST(a.population AS STRING)))), 0) as a_cohort_size

Recently, Looker began to do it differently, most likely to improve bigquery performance:

  COALESCE(ROUND(COALESCE(CAST( ( SUM(DISTINCT (CAST(ROUND(COALESCE(lesson_5_cohorts.population ,0)*(1/1000*1.0), 9) AS NUMERIC) + (cast(cast(concat('0x', substr(to_hex(md5(CAST(lesson_5_cohorts.id  AS STRING))), 1, 15)) as int64) as numeric) * 4294967296 + cast(cast(concat('0x', substr(to_hex(md5(CAST(lesson_5_cohorts.id  AS STRING))), 16, 8)) as int64) as numeric)) * 0.000000001 )) - SUM(DISTINCT (cast(cast(concat('0x', substr(to_hex(md5(CAST(lesson_5_cohorts.id  AS STRING))), 1, 15)) as int64) as numeric) * 4294967296 + cast(cast(concat('0x', substr(to_hex(md5(CAST(lesson_5_cohorts.id  AS STRING))), 16, 8)) as int64) as numeric)) * 0.000000001) )  / (1/1000*1.0) AS FLOAT64), 0), 6), 0) AS lesson_5_cohorts_m_sum_distinct