Performance ... normal selects and the same selects in functions

Question

Hi there,
hope you guys can help me a little.
We use a bunch of dictionary tables to map certain values to IDs. In the "real" data tables we save the IDs instead of the values themselves.
Those tables are usually pretty small.
Normally we would join these tables into our selects, such as this:
select ...
from data_table d,
lookup_table l
where d.l_id=l.id
But we also got some functions we use to lookup these values to have simple selects:
select ..., my_function(l_id)
from data_table
Or for reverse lookup:
select ...
from data_table
where l_id=my_function(l_id)
Do you know, if smaller selects capsuled in functions are more or less performant then simply joining these tables into the big selects?
Sometimes we also use small statements on large tables (4.000.000 records) ... sometimes we use them in functions as well ...
select bla
from large_table
where id=...
Thanks for your hints,
Steff

Answer 1

How you did your tests?
A short conclusion of test below:
I've created 2 tables one base table and one lookup table.
Then created function to get lookup value from lookup table.
then run select to get all lookup values from base table using join and using function.
Join needed 0.01 sec, function needed at least 2.08 secs.
Then I've created an anonymous script to loop through all base values and get aproprite lookup values using function and using join and compared results using runstats.
As a result join performed at more than 2 times faster and required more than 2 times less latches.
and here is the listing
SQL> create table lookup_table (
  2  lkp_id number not null,
  3  lkp_val varchar2(10) not null);
Table created.
Elapsed: 00:00:00.00
SQL> alter table lookup_table add constraint lkp_pk primary key (lkp_id);
Table altered.
Elapsed: 00:00:00.00
SQL>
SQL> insert into lookup_table select rownum, substr(object_name, 1, 10) from dba_objects;
42221 rows created.
Elapsed: 00:00:00.05
SQL> create or replace function get_lkp_value (in_id number) return varchar2
  2  is
  3    ret lookup_table.lkp_val%TYPE;
  4  begin
  5    select lkp_val into ret from lookup_table where lkp_id = in_id;
  6    return ret;
  7  end;
  8  /
Function created.
Elapsed: 00:00:00.01
SQL> create table base_table (
  2  bas_id number not null,
  3  bas_lkp_id number not null,
  4  bas_data varchar2(1000) not null);
Table created.
Elapsed: 00:00:00.00
SQL>
SQL> insert into base_table select rownum, rownum, object_name
  2  from dba_objects where rownum <= 42221;
42221 rows created.
Elapsed: 00:00:00.05
SQL> alter table base_table add constraint bas_pk primary key (bas_id);
Table altered.
Elapsed: 00:00:00.02
SQL> alter table base_table add constraint bas_lkp_fk foreign key (bas_lkp_id)
  2  references lookup_table (lkp_id);
Table altered.
Elapsed: 00:00:00.01
SQL> create index bas_lkp_idx on base_table (bas_lkp_id);
Index created.
Elapsed: 00:00:00.02
SQL> exec dbms_stats.gather_table_stats(user, 'lookup_table');
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.01
SQL> exec dbms_stats.gather_table_stats(user, 'base_table');
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.02
SQL> select count(distinct lkp_val) from (
  2    select bas_id, bas_data, get_lkp_value(bas_lkp_id) lkp_val
  3    from base_table);
COUNT(DISTINCTLKP_VAL)                                                         
                 13622                                                         
Elapsed: 00:00:03.01
SQL> /
COUNT(DISTINCTLKP_VAL)                                                         
                 13622                                                         
Elapsed: 00:00:02.08
SQL> /
COUNT(DISTINCTLKP_VAL)                                                         
                 13622                                                         
Elapsed: 00:00:03.01
SQL> select count(distinct lkp_val) from (
  2    select bas_id, bas_data, lkp_val
  3    from base_table, lookup_table
  4    where bas_lkp_id = lkp_id);
COUNT(DISTINCTLKP_VAL)                                                         
                 13622                                                         
Elapsed: 00:00:00.01
SQL> /
COUNT(DISTINCTLKP_VAL)                                                         
                 13622                                                         
Elapsed: 00:00:00.01
SQL> ed
Wrote file afiedt.buf
  1  declare
  2    cursor c1 (in_id in number) is
  3    select bas_id, bas_data, lkp_val
  4    from base_table, lookup_table
  5    where bas_lkp_id = lkp_id
  6      and bas_id = in_id;
  7    val1 c1%ROWTYPE;
  8    cursor c2 (in_id in number) is
  9    select bas_id, bas_data, get_lkp_value(bas_lkp_id)
10    from base_table
11    where bas_id = in_id;
12    val2 c2%ROWTYPE;
13  begin
14  runstats_pkg.rs_start;
15  for i in 1..42220 loop
16    open c1(i);
17    fetch c1 into val1;
18    close c1;
19  end loop;
20  runstats_pkg.rs_middle;
21  for i in 1..42220 loop
22    open c2(i);
23    fetch c2 into val1;
24    close c2;
25  end loop;
26  runstats_pkg.rs_stop;
27* end;
SQL> /
Run1 ran in 390 hsecs                                                          
Run2 ran in 910 hsecs                                                          
run 1 ran in 42,86% of the time                                                
Name                                  Run1        Run2        Diff             
LATCH.kwqit: protect wakeup ti           0           1           1             
LATCH.spilled msgs queues list           0           1           1             
STAT...change write time                 0           1           1             
LATCH.active checkpoint queue            1           3           2             
LATCH.cache buffer handles              24          26           2             
LATCH.process allocation                 0           2           2             
STAT...cleanout - number of kt           3           5           2             
STAT...consistent gets - exami     253,323     253,325           2             
STAT...consistent gets             253,323     253,325           2             
STAT...calls to kcmgcs                   3           5           2             
STAT...active txn count during           3           5           2             
LATCH.session timer                      1           3           2             
LATCH.event group latch                  0           2           2             
LATCH.channel handle pool latc           0           4           4             
LATCH.transaction allocation             0           4           4             
STAT...db block gets                   520         524           4             
LATCH.list of block allocation           0           4           4             
LATCH.process group creation             0           4           4             
LATCH.dummy allocation                   0           4           4             
LATCH.post/wait queue                   11          16           5             
LATCH.Consistent RBA                    18          24           6             
LATCH.mostly latch-free SCN             18          24           6             
STAT...consistent changes              510         516           6             
STAT...session logical reads       253,843     253,849           6             
STAT...db block changes              1,020       1,026           6             
LATCH.lgwr LWN SCN                      18          24           6             
LATCH.channel operations paren           4          12           8             
LATCH.user lock                          0           8           8             
LATCH.simulator lru latch               13           2         -11             
LATCH.sequence cache                    56          71          15             
LATCH.redo writing                      59          82          23             
LATCH.enqueues                          30          60          30             
STAT...hot buffers moved to he          31           0         -31             
LATCH.messages                         100         149          49             
LATCH.cache buffers lru chain           91          33         -58             
LATCH.session idle bit                 230         290          60             
LATCH.SQL memory manager worka         136         223          87             
STAT...free buffer requested            91           4         -87             
STAT...shared hash latch upgra          87           0         -87             
STAT...physical reads                   87           0         -87             
LATCH.checkpoint queue latch            69         181         112             
LATCH.session allocation                44         178         134             
LATCH.redo allocation                  622         819         197             
LATCH.undo global data                  61         263         202             
LATCH.dml lock allocation               92         312         220             
LATCH.row cache enqueue latch          356         580         224             
LATCH.simulator hash latch          12,430      12,204        -226             
LATCH.row cache objects                360         626         266             
LATCH.enqueue hash chains              141         477         336             
STAT...recursive cpu usage             259         596         337             
STAT...Elapsed Time                    392         913         521             
LATCH.library cache pin alloca         402       1,100         698             
LATCH.cache buffers chains         257,991     260,038       2,047             
STAT...calls to get snapshot s      42,221      84,441      42,220             
STAT...execute count                42,221      84,441      42,220             
STAT...recursive calls             126,662     168,882      42,220             
STAT...session pga memory                0      65,536      65,536             
LATCH.shared pool                   44,133     213,801     169,668             
LATCH.library cache pin             86,473     256,185     169,712             
LATCH.library cache                 87,511     342,530     255,019             
Run1 latches total versus runs -- difference and pct                           
Run1        Run2        Diff       Pct                                         
491,506   1,090,381     598,875     45.08%                                     
PL/SQL procedure successfully completed.
Elapsed: 00:00:13.06Gints Plivna
http://www.gplivna.eu