Re: How many columns can I have in an ORDER BY clause?

On 26 February 2010 03:11, Mary Anderson <maryfran@demog.berkeley.edu> wrote:
> Hi,
>
>    I have a query that has an ORDER BY clause that appears to break.
> It has 12 or 13 columns.  Have I exceeded postgresql's sorting capacity?
>
> Here is my query:
>  BIG_SELECT_cmd = SELECT
>
> value,x.d15_dv_id,x.d5_dv_id,x.dm1_dv_id,x.dm2_dv_id,x.d8_dv_id,x.d12_dv_id,v.data_batch_id
>
>  FROM display.perm_tmp_xprod_table_550 x LEFT JOIN
>  display.perm_tmp_data_values_550 v
>  ON x.d15_dv_id = v.d15_dv_id
>  AND x.d5_dv_id = v.d5_dv_id
>  AND x.dm1_dv_id = v.dm1_dv_id
> AND x.dm2_dv_id v.dm2_dv_id
> AND x.d8_dv_id = v.d8_dv_id
> AND x.d12_dv_id =v.d12_dv_id
>
> ORDER BY
> x.d15_group_order,x.d15_value_rank,

This will order by x.d15_group_order.  If any of the values in that
column are the same, then those rows will be sorted by
x.d15_value_rank.

> x.d5_group_order,x.d5_value_rank,

If any rows have the same x.d15_group_order and x.d15_group_rank then
those matching rows will be sorted by x.d5_group_order.  If there are
any that have the same d15_* and d5_group_order then those will be
sorted by x.d5_group_rank.

> x.dm1_group_order,x.dm1_value_rank,
> x.dm2_group_order,x.dm2_value_rank,
> x.d8_group_order,x.d8_value_rank,
> x.d12_group_order,x.d12_value_rank,
> v.DMEM_status,

etc.

So by the time you get to x.dm1_group* and x.dm2_group* you probably
don't have any rows with all the other columns the same and therefore
the order of the values in dm1* and dm2* will not appear to be sorted
(when you look at the whole column).

> '2050-01-01'::DATE - v.data_batch_created_date

This is a strange ORDER BY clause.  I think you want
"v.data_batch_created_date DESC".

> I checked the underlying tables.  They seem to be reasonable -- no nulls
> But the dm1 values and the dm2 values do not sort at all.

e.g. if you have the following values:

a,b,c,d
=======
0,0,9,5
0,0,1,3
0,0,7,9
0,0,2,8
0,1,2,6
1,3,5,7

then SELECT * FROM table_name ORDER BY a, b, c, d; would give you:

a,b,c,d
=======
0,0,1,3
0,0,2,8
0,0,7,9
0,0,9,5
0,1,2,6
1,3,5,7

If you look at columns c and d without considering a and b, they look
unsorted, only because the ordering depends on all columns together
with a being the most important, b next, etc.

If this does not explain what you're seeing, please provide some
example output and what you were expecting to see instead.

--
Michael Wood <esiotrot@gmail.com>