Very slow Sub Select Query

Question

Oracle Version 9.2.0.7.0
Sorry to ask what is probably a stupid question that only serves to highlight how useless my SQL is!
I have this SQL which counts total requisition lines, and then counts how many of those lines have converted to Sales Orders:
SELECT prha.segment1
     , (SELECT COUNT(*)
          FROM po.po_requisition_lines_all prla
         WHERE prla.requisition_header_id = prha.requisition_header_id)
                                                                  req_line_ct
     , (SELECT COUNT(*)
          FROM po.po_requisition_lines_all prla
             , ont.oe_order_lines_all sol
         WHERE prla.requisition_line_id = sol.source_document_line_id
           AND prla.requisition_header_id = prha.requisition_header_id)
                                                                   so_line_ct
  FROM po.po_requisition_headers_all prha
WHERE  prha.creation_date >= '03-JUL-2008';The SQL works, but it is really, really slow. Is that just because it is running through lots and lots of data?
If I comment out the 2nd Count SELECT, it runs immediately. Perhaps it is so slow because the join between the po_requisition_lines_all and the oe_order_lines_all table is not via primary keys. Also, there is not an index on the oe_order_lines_all table related to the source_document_line_id field.
I'm sorry I haven't provided the full table designs for the tables involved - but the primary keys joining the requisition tables = via the requisition_headers_all table, and the join via the po_requisition_lines_all and the oe_order_lines_all table is not via a primary key or an index.
Sorry once again, for probably talking nonsense.

Answer 1

Please note this is a untested code as you didn't provide any of the required information like
a) Test Data
b) Relationship between the tables
You could try something like this. But as already @someoneelse suggested it is definitely worth reading the link.
select prha.segment1, count(case when prla.requisition_header_id is not null then 1 end) req_line_ct,
count(case when sol.source_document_line_id is not null then 1 end) so_line_ct
from
po.po_requisition_header_all prha,
po.po_requisition_lines_all prla,
ont.oe_order_lines_all sol
where
prha.creation_date >= to_date('03.07.2008','dd.mm.yyyy')
and
prha.requisition_header_id = prla.requisition_header_id (+)
and
prla.requsition_line_id = sol.source_document_line_id (+)
group by prha.segment1;Regards
Raj