Query takes long when using UNION

Question

Hi ,
I habe a query as follows;
SELECT
                    '9999' site_id,
                    m.ghi_prov_num provnum,
                       SUBSTR (l.seq_num, 1, 3)provloc,
                    t.dea_number dea,
                        t.license_number statelicensenumber ,
                        n.npi_num npi,
                        m.prefix prefixname,
                        m.lastname lastname,
                        m.firstname firstname,
                        t.middle_name middleinitial,
                        m.suffix suffixname,
                        null clinicname,
                        l.street1 addressline1,
                        l.street2 addressline2,
                        l.city city,
                        l.state state,
                        l.zip5 zip,
                        l.phone phoneprimary,
                        null ext,
                        null fax,
                        null email,
                        null alt_phone,
                        null alt_phone_ext
                 FROM provider m, LOCATION l, npi n,TEMP_VITAL_CACTUS t,test_provider_pin pin
                  WHERE m.ghi_prov_num=l.ghi_prov_num
                      and m.ghi_prov_num=n.ghi_prov_num(+)
                      and m.ghi_prov_num=t.ghi_prov_num(+)
                      and m.tax_id=pin.tax_id
     UNION
      SELECT   
                    '9999', m.ghi_prov_num ,
                        m.location provloc,
                       null  ,
                       null  ,
                        n.npi_num  ,
                        null ,
                        m.lastname ,
                        m.firstname  ,
                        null,
                        null  ,
                        null  ,
                        m.street1  ,
                        m.street2  ,
                        m.city ,
                        m.state  ,
                        m.zip5 ,
                        m.phone  ,
                        null  ,
                        null  ,
                        null  ,
                        null  ,
                        null
                       FROM dental_provider m, npi n,test_provider_pin pin
                   WHERE m.ghi_prov_num=n.tax_id(+)
                       and m.location=n.location(+)
                        and pin.tax_id=m.ghi_prov_num;The query takes for ever;
But Individual query takes less than a sec to execute.Is there any way can i rewrite the query?
Please help
Hena.

Answer 1

user11253970 wrote:
But Individual query takes less than a sec to execute.Is there any way can i rewrite the query?Have a feeling you are using Toad/SQL Navigator or similar tool which returns data one screen at a time. If so, then it does not "takes less than a sec to execute" but rather to fetch first screen of rows. When you use UNION Oracle has to return distinct rows from both queries. Therefore it must fetch not just first screen but all rows. To verify, issue first query and in yiour GUI tool click on get last screen. Then you'll know how long whole select takes. Do the same for second query. Also, do you need distinct rows or akll rows? IF all rows, change UNION to UNION ALL.
SY.