Re: Slow SELECT

On 2020-05-26 12:04 PM, David Rowley wrote:
> 
> Since "my_table" is small, you'd likely be much better doing a manual
> rewrite of the query to join a subquery containing the required
> details from "my_table".  It looks like you want the row_id from the
> latest tran_date for each fld_N column. So something like:
> 
> SELECT a.row_id
> FROM table_1 b
> CROSS JOIN table_2 c
> CROSS JOIN table_3 d
> CROSS JOIN table_4 e
> LEFT OUTER JOIN (
> SELECT fld_1,fld_2,fld_3,fld_4,row_id,tran_date,
> ROW_NUMBER() OVER (PARTITION BY fld_1,fld_2,fld_3,fld_4 ORDER BY
> tran_date DESC) row_num
> FROM my_table
> WHERE deleted_id = 0
> ) a ON a.fld_1 = b.row_id AND a.fld_2 = c.row_id AND a.fld_3 =
> d.row_id AND a.fld_4 = e.row_id AND a.row_num = 1;
> 
> Should do it. You could also perhaps do something with DISTINCT ON
> instead of using ROW_NUMBER(). That might be a bit more efficient, but
> it's unlikely to matter too much since there are only 167 rows in that
> table.
> 

I have studied the above SELECT, and I now more or less understand it. I 
would not have come up with that unaided, so many thanks.

I tried DISTINCT ON, and it was very efficient, but unfortunately that 
is not supported by SQL Server or sqlite3.

Then I came up with this alternative, which works on all three platforms 
and seems a bit faster than the above -

SELECT a.row_id FROM (
     SELECT row_id,
         ROW_NUMBER() OVER (PARTITION BY fld_1, fld_2, fld_3, fld_4
         ORDER BY tran_date DESC) row_num
     FROM my_table
     WHERE deleted_id = 0
    ) as a
WHERE a.row_num = 1

Do you see any problem with this?

Thanks

Frank