Performance issue about using JDBC?

Question

Since no one reply me, I post again. :(
I just got a big performance problem lately, and I tried all the possible ways, still can't fix it. Could you help me out or give me more suggestions?
Oracle 8i for Solaris 2.6
A web application with back end is Oracle database, developed by Java, use JDBC driver. It also uses Servelet. Report generated in browser is using dynamic SQL.
When I click some link to generate report in browser, it will run the corresponding SQL script, then return the result to browser. The problem is it takes long long time to get the result. For simple query, it takes around 2-3 minutes. But if I run the same SQL script in
SQL*Plus, it only takes 4-5 seconds, or even less. So I think the index for this query is fine. (I also rebuild all indices, some result.) And all the hit ratios in SGA are also OK. When browser generate reports, I didn't see high CPU usage or I/O activity.
I really have no idea why this happens. But I think the Oracle DB is fine, 'cause query is run normally in SQL*Plus. The problem may related to the JDBC driver or JDBC connection. The developers also have no clue about this. When the Java app run the query, does it use the same way to access the tables and indexes as used in SQL*Plus?
Any idea or suggestions?
Thanks a lot and have a good day!
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Answer 1

Thanks for all.
So do you guys has any suggestion on the following code?
DESCRIBE TABLE gt_vbeln LINES l_lines.
  IF l_lines = 0.
***>>Links20060411
*  ELSEIF l_lines GT c_1000.
*    SELECT vbelv posnv vbeln posnn vbtyp_n rfmng
*        APPENDING TABLE gt_vbfa_all PACKAGE SIZE c_1000
*        FROM vbfa
*        FOR   ALL ENTRIES IN gt_vbeln
*        WHERE vbelv   EQ  gt_vbeln-vbelv
*          AND posnv   EQ gt_vbeln-posnv
*          AND vbtyp_n IN ('T', 'J', 'R', 'h').
*    ENDSELECT.
*  ELSE.
*    SELECT vbelv posnv vbeln posnn vbtyp_n rfmng
*        INTO TABLE gt_vbfa_all FROM vbfa
*        FOR   ALL ENTRIES IN gt_vbeln
*        WHERE vbelv   EQ  gt_vbeln-vbelv
*          AND posnv   EQ  gt_vbeln-posnv
*          AND vbtyp_n IN ('T', 'J', 'R', 'h').
  ELSEIF l_lines > c_1000.
    SELECT vbelv posnv vbeln posnn vbtyp_n rfmng
        APPENDING TABLE gt_vbfa PACKAGE SIZE c_1000
        FROM vbfa
        FOR   ALL ENTRIES IN gt_vbeln
        WHERE vbelv   =  gt_vbeln-vbelv
          AND posnv   = gt_vbeln-posnv
          AND vbtyp_n IN ('T', 'J').
    ENDSELECT.
  ELSE.
    SELECT vbelv posnv vbeln posnn vbtyp_n rfmng
        INTO TABLE gt_vbfa FROM vbfa
        FOR   ALL ENTRIES IN gt_vbeln
        WHERE vbelv   =  gt_vbeln-vbelv
          AND posnv   =  gt_vbeln-posnv
          AND vbtyp_n IN ('T', 'J').
  ENDIF.
Currently it runs timeout ,as the l_lines is very very large.
I think maybe we  can change the package size. But what's the best package size for performance?
Thanks..