Re: Slow SELECT

On Tue, 26 May 2020 at 19:23, Frank Millman <frank(at)chagford(dot)com> wrote:
> The table sizes are -
> my_table : 167 rows
> table_1 : 21 rows
> table_2 : 11 rows
> table_3 : 3 rows
> table_4 : 16 rows
>
> Therefore for each tran_date in my_table there are potentially
> 21x11x3x16 = 11088 rows. Most will be null.
>
> I want to select the row_id for the last tran_date for each of those
> potential groups. This is my select -
>
> SELECT (
> SELECT a.row_id FROM my_table a
> WHERE 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.deleted_id = 0
> ORDER BY a.tran_date DESC LIMIT 1
> )
> FROM table_1 b, table_2 c, table_3 d, table_4 e
>
> Out of 11088 rows selected, 103 are not null.

Perhaps SQL Server is doing something to rewrite the subquery in the
target list to a LEFT JOIN. PostgreSQL currently does not do that.

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.

David