Strange periodic group by problems (ORA-00979)

We are using the following SQL query to produce monthly averages. The statement is heavy used and works pretty well, but it fails with a 'ORA-00979: not a GROUP BY expression' every month or two and we have no idea why.

First about the process:

• we have raw data every few minutes,
• => the raw data is getting averaged to hourly, daily, monthly and yearly values

raw -> hourly

• creates entries in the averages table with average_type 2
• never problems

hourly => daily / daily => monthly / monthly => yearly

• Statements are pretty similar
• averages of a 'lower' type is being averaged to a higher type

• Average types are: 2 hourly, 3 daily, (4 weekly not used) 5 monthly and 6 yearly

• The bug only appears with the step "daily => monthly".

Query:

• We can't reproduce the bug, the next run of the aggregation job usually works without problems.
• The errors occurs every 50-60 days, without a real pattern
• Environment: Oracle 10g

Does anybody have an idea what the problem could be?

INSERT INTO averages
SELECT averages_seq.NEXTVAL,
       avg.*
FROM (
  SELECT
      m.city_id,            m.city_name,
      m.state_id,           m.state_name,
      m.district_id,        m.district_name,
      m.country_id,         m.country_name,
      m.currency_id,        m.currency_name,
      m.category_id,        m.category_name,
      5 average_type, -- average_type 5 ==> monthly average
      0 analysis_type,
      TRUNC(m.average_date, 'MM')  average_date,
      AVG(m.value) value,
      SUM(m.sum) sum,
      NULL uncertainty,
      NULL uncertainty_type,
      MIN(m.value_min) value_min,
      MAX(m.value_max) value_max,
      SUM(number_of_measurements) number_of_measurements,
      -- 6 * 24 => measurements per day
      -- (ADD_MONTHS(...)) => days per month 
      100 * SUM(number_of_measurements) / 
           (6 * 24 *
           (ADD_MONTHS(TRUNC(average_date, 'MM'), 1)  - TRUNC(average_date, 'MM'))) coverage_percent,
      SUM(customers) customers,
      NULL dummy_field,
      CURRENT_TIMESTAMP calculation_date,
      CURRENT_TIMESTAMP creation_date,
      'AGGREGATION' creation_user,
      CURRENT_TIMESTAMP modification_date,
      'AGGREGATION' modification_user,
      'n' constant_1,
      3   constant_2,
      -1 average_state
  FROM averages m
  WHERE   m.average_type = 3 -- average type 3 ==> daily average
  AND     m.average_date
      BETWEEN
        TO_TIMESTAMP('2011-06-01T00:00:00Z', 'YYYY-MM-DD"T"HH24:MI:SS"Z"')
        AND
        TO_TIMESTAMP('2011-06-30T23:59:59Z', 'YYYY-MM-DD"T"HH24:MI:SS"Z"')
  AND     m.analysis_type = 0
  GROUP BY
        m.city_id,            m.city_name,
        m.state_id,           m.state_name,
        m.district_id,        m.district_name,
        m.country_id,         m.country_name,
        m.currency_id,        m.currency_name,
        m.category_id,        m.category_name,
        TRUNC(m.average_date, 'MM')
  ) avg

I would add a group by:

(ADD_MONTHS(TRUNC(average_date, 'MM'), 1)  - TRUNC(average_date, 'MM'))

I know this can't change without TRUNC(m.average_date, 'MM') changing, but it seems to be the only non-aggregate column not in your GROUP BY.

In addition, you could remove all the non-aggregated, constant, columns outside the inner sql, and explicitly name the columns you're inserting and select these constants at the same time:

eg.

INSERT INTO averages(city_id, city_name, ...average_type, analysis_type, ...)
SELECT averages_seq.NEXTVAL,
avg.city_id, avg.city_name, ...
5, 0, ...

...can't say this part will fix the issue, but it would certainly remove them from suspicion of not being aggregated.