On 2020-05-26 12:04 PM, David Rowley wrote:
On Tue, 26 May 2020 at 19:23, Frank Millman <frank@xxxxxxxxxxxx> 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.
Thank you David. I tried that and it produced the correct result in 53ms, which is what I am looking for.
It will take me some time to understand it fully, so I have some homework to do!
Much appreciated. Frank
