JDBC automatical query turned to be very slow

Question

I am maintaining an application creating an Oracle DB via JDBC. Starting from today this query:

SELECT  NULL                                                   AS pktable_cat  ,
        p.owner                                                AS pktable_schem,
        p.table_name                                           AS pktable_name ,
        pc.column_name                                         AS pkcolumn_name,
        NULL                                                   AS fktable_cat  ,
        f.owner                                                AS fktable_schem,
        f.table_name                                           AS fktable_name ,
        fc.column_name                                         AS fkcolumn_name,
        fc.position                                            AS key_seq      ,
        NULL                                                   AS update_rule  ,
        DECODE (f.delete_rule, 'CASCADE', 0, 'SET NULL', 2, 1) AS delete_rule  ,
        f.constraint_name                                      AS fk_name      ,
        p.constraint_name                                      AS pk_name      ,
        DECODE(f.deferrable, 'DEFERRABLE',5 ,'NOT DEFERRABLE',7 , 'DEFERRED', 6 ) deferrability
FROM    all_cons_columns pc,
        all_constraints p  ,
        all_cons_columns fc,
        all_constraints f
WHERE   1                      = 1
        AND p.table_name       = :1
        AND p.owner            = :3
        AND f.constraint_type  = 'R'
        AND p.owner            = f.r_owner
        AND p.constraint_name  = f.r_constraint_name
        AND p.constraint_type  = 'P'
        AND pc.owner           = p.owner
        AND pc.constraint_name = p.constraint_name
        AND pc.table_name      = p.table_name
        AND fc.owner           = f.owner
        AND fc.constraint_name = f.constraint_name
        AND fc.table_name      = f.table_name
        AND fc.position        = pc.position
ORDER BY fktable_schem,
        fktable_name  ,
        key_seq

started becoming really slow due to some oracle internals as it seems to be the same for all my branches.

Does somebody know one possible reason and how to face this?

Regards, Nunzio

Answer 1

Data dictionary or fixed object statistics might be old, try re-gathering them:

exec dbms_stats.gather_dictionary_stats;
exec dbms_stats.gather_fixed_objects_stats;
alter system flush shared_pool;

Even that does not necessarily gather statistics for all system objects. Some objects, like X$KFTBUE, must be gathered manually. Although that's a rare data dictionary problem that may not be relevant here.

If that doesn't work some next possible steps are looking at tools like SQL Tuning Advisor to create a profile, or using SQL Plan Management to force the optimizer to use a specific plan that has worked before. Tuning a data dictionary query can be very difficult since you don't have much control.

Answer 2

here is another more graceful solution.. I found that forcing the rule base optimizer with a sql patch also works.. 2 patches are necessary because sometimes the jdbc driver uses :1 and :3 as bind variables and sometimes it uses :2 & :4.. The SQL must match exactly for the patch to work.

run this in your database as sysdba..

    begin 
        dbms_sqldiag_internal.i_create_patch ( 
      sql_text =>'SELECT NULL AS pktable_cat,
        p.owner as pktable_schem, p.table_name as pktable_name, 
        pc.column_name as pkcolumn_name, NULL as fktable_cat, f.owner as       
        fktable_schem, f.table_name as fktable_name, 
        fc.column_name as fkcolumn_name, fc.position as key_seq, NULL as 
        update_rule, decode 
        (f.delete_rule, ''CASCADE'', 0, ''SET NULL'', 2, 1) as delete_rule, 
        f.constraint_name as fk_name, p.constraint_name as pk_name, 
        decode(f.deferrable, ''DEFERRABLE'',5 ,''NOT DEFERRABLE'',7 , ''DEFERRED'', 6)  
        deferrability
        FROM all_cons_columns pc, all_constraints p, all_cons_columns fc, 
        all_constraints f
        WHERE 1 = 1 AND p.table_name = :1  AND p.owner = :3 AND 
         f.constraint_type = ''R'' AND p.owner = f.r_owner AND 
         p.constraint_name = f.r_constraint_name AND p.constraint_type = ''P'' 
         AND pc.owner = p.owner AND pc.constraint_name = p.constraint_name AND
         pc.table_name = p.table_name AND fc.owner = f.owner AND 
         fc.constraint_name = f.constraint_name AND 
         fc.table_name = f.table_name AND fc.position = pc.position 
        ORDER BY fktable_schem, fktable_name, key_seq' ,
      hint_text => 'RULE', 
      name => 'jdbcpatch');
    end;
    /

    begin 
        dbms_sqldiag_internal.i_create_patch ( 
      sql_text =>'SELECT NULL AS pktable_cat,
        p.owner as pktable_schem, p.table_name as pktable_name, 
        pc.column_name as pkcolumn_name, NULL as fktable_cat, f.owner as       
        fktable_schem, f.table_name as fktable_name, 
        fc.column_name as fkcolumn_name, fc.position as key_seq, NULL as 
        update_rule, decode 
        (f.delete_rule, ''CASCADE'', 0, ''SET NULL'', 2, 1) as delete_rule, 
        f.constraint_name as fk_name, p.constraint_name as pk_name, 
        decode(f.deferrable, ''DEFERRABLE'',5 ,''NOT DEFERRABLE'',7 , ''DEFERRED'', 6)  
        deferrability
        FROM all_cons_columns pc, all_constraints p, all_cons_columns fc, 
        all_constraints f
        WHERE 1 = 1 AND p.table_name = :2  AND p.owner = :4 AND 
         f.constraint_type = ''R'' AND p.owner = f.r_owner AND 
         p.constraint_name = f.r_constraint_name AND p.constraint_type = ''P'' 
         AND pc.owner = p.owner AND pc.constraint_name = p.constraint_name AND
         pc.table_name = p.table_name AND fc.owner = f.owner AND 
         fc.constraint_name = f.constraint_name AND 
         fc.table_name = f.table_name AND fc.position = pc.position 
        ORDER BY fktable_schem, fktable_name, key_seq' ,
      hint_text => 'RULE', 
      name => 'jdbcpatch2');
    end;
    /

Answer 3

The query in the question is generated by a call to java.sql.DatabaseMetaData.getExportedKeys() which delegates to oracle.jdbc.OracleDatabaseMetaData.getExportedKeys() to enumerate foreign keys referencing given table.

As stated in @Jon's answer, Oracle sometimes uses suboptimal plan for this query that may or may not be avoided by gathering statistics.

Other alternatives if the code can be changed:

• look at Easiest way to obtain database metadata in Java? for alternative implementation
• rewrite the query and call it directly

The second option was chosen by Liquibase project that used to call DatabaseMetaData in older versions. New versions use optimized query with proper joins from CORE-1844:

SELECT NULL AS pktable_cat, p.owner as pktable_schem, 
    p.table_name as pktable_name, pc.column_name as pkcolumn_name,    
    NULL as fktable_cat, f.owner as fktable_schem, f.table_name as fktable_name,    
    fc.column_name as fkcolumn_name, fc.position as key_seq, NULL as update_rule,    
    decode (f.delete_rule, 'CASCADE', 0, 'SET NULL', 2, 1) as delete_rule,    
    f.constraint_name as fk_name, p.constraint_name as pk_name,    
    decode(f.deferrable, 'DEFERRABLE', 5, 'NOT DEFERRABLE', 7, 'DEFERRED', 6) deferrability  
FROM    all_constraints p
INNER JOIN  all_cons_columns pc ON pc.owner = p.owner    
    AND pc.constraint_name = p.constraint_name    
    AND pc.table_name = p.table_name    
INNER JOIN all_constraints f ON p.owner = f.r_owner    
    AND p.constraint_name = f.r_constraint_name
INNER JOIN all_cons_columns fc ON fc.owner = f.owner    
    AND fc.constraint_name = f.constraint_name
    AND fc.table_name = f.table_name
    AND fc.position = pc.position
WHERE p.owner = :jdbcSchemaName
    AND p.constraint_type in ('P', 'U')    
    AND f.constraint_type = 'R'    
ORDER BY fktable_schem, fktable_name, key_seq