Re: [HACKERS] SELECT DISTINCT question

Hannu Krosing <hannu(at)trust(dot)ee> writes:
>> "DISTINCT will eliminate all duplicate rows from the selection.
>> DISTINCT ON column will eliminate all duplicates in the specified column;
>> this is equivalent to using GROUP BY column."

> If it is equivalent to GROUP BY then it should allow only aggregates
> in non-distinct columns, like:
> select distinct on date date, sum(bytes) from access_log;
> If it does not, then it should be files as a bug imho.

It does not. Whether that is a bug is hard to say, since there is no
standard I know of that says what it *is* supposed to do.

If you look at the select_distinct_on regress test outputs, I bet you
will be even less happy:

QUERY: SELECT DISTINCT ON string4 two, string4, ten
FROM tmp
ORDER BY two using <, string4 using <, ten using <;
two|string4|ten
---+-------+---
0|AAAAxx | 0
0|HHHHxx | 0
0|OOOOxx | 0
0|VVVVxx | 0
1|AAAAxx | 1
1|HHHHxx | 1
1|OOOOxx | 1
1|VVVVxx | 1
(8 rows)

That's not exactly my idea of "distinct" values of string4 ---
but apparently whoever made up the regress test thought it was OK!

Can anyone defend this feature or provide a coherent definition
of what it's supposed to be doing? My urge to rip it out is
growing stronger and stronger...

regards, tom lane