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Going Backend of new feature of 11g related to Sequences

December 15th, 2007 by Rajender Singh · 4 Comments

On 12th july 2007, I wrote a post about new feature of 11g related to sequence.

In this post I mention that in 11g we don’t need to use dual table to fetch next value of a sequence and using simple PL/SQL expression the next value can be fetched.

Any way thats the old story and lot of people have wrote about it.

But recently I got to know from Asif blog, what exactly is happening in background.

I follows what he did and confirms that he 100% right!

Created the new sequence

SQL> create sequence new11g_seq;

Sequence created.


Give a name to the trace file that will be generated

SQL> alter session set tracefile_identifier=’new11g_seq’;

Session altered.

Set the Event No and Level till I want to trace

SQL> alter session set events ’10046 trace name context forever,level 12′;

Session altered.

Run one PL/SQL involving the new sequence as a PL/SQL expression

SQL> declare
2 l_no number;
3 begin
4 l_no := new11g_seq.nextval;
5 end;
6 /

PL/SQL procedure successfully completed.

Set the tracing off:

SQL> alter session set events ’10046 trace name context off’;

Session altered.

Checked the destination of the trace file:

SQL> select value from v$diag_info where name = ‘Default Trace File’;

VALUE
————————————————————————-

d:\oracle\diag\rdbms\dcs\dcs\trace\dcs_ora_4084_new11g_seq.trc

Now I opened the given trace file and find some interesting fact in following lines:

=====================
PARSING IN CURSOR #3 len=59 dep=0 uid=5 oct=47 lid=5 tim=12331650243 hv=2968413633 ad=’1e1874a8′ sqlid=’b5v90tqsfwtf1′
declare
l_no number;
begin
l_no := new11g_seq.nextval;
end;
END OF STMT
PARSE #3:c=31250,e=81603,p=0,cr=2,cu=0,mis=1,r=0,dep=0,og=1,tim=12331650236
BINDS #3:
=====================
PARSING IN CURSOR #5 len=35 dep=1 uid=5 oct=3 lid=5 tim=12331651486 hv=3823849384 ad=’1e18717c’ sqlid=’9fhs7ymjyqmx8′
Select NEW11G_SEQ.NEXTVAL from dual
END OF STMT
PARSE #5:c=0,e=989,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=1,tim=12331651479
BINDS #5:
EXEC #5:c=0,e=81,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=1,tim=12331651654
=====================

Wow!

It means even though oracle has made life easier for programmers as we don’t need to write whole SQL to fetch new sequence number, but from performance point of view and internal process of fetching the sequence nothing has changed

It still converts the PL/SQL expression to SQL and fetch the next value!

Tags: Oracle 11g New Features · SQL and PL/SQL

4 responses so far ↓

  • 1 Gary // Dec 17, 2007 at 3:29 am

    It does the same for SYSDATE (and I think USER too). The only way to get database information is through SQL.
    Of course, since 10g, dual doesn’t need any consistent gets, so the real performance benefit came back in that version.

  • 2 Rajender Singh // Dec 17, 2007 at 9:56 am

    Thanks Gary!

  • 3 Rakesh Kumar Gupta // Dec 17, 2007 at 12:58 pm

    I would like to know the feasibility of using next value of a sequence via 11g method over convensitional way like SELECT sqe.NEXTVAL FROM DUAL.

  • 4 Rajender Singh // Dec 17, 2007 at 10:22 pm

    Hi Rakesh,

    If we do simple performance comparison between using 11g method and conventional way there is no performance improvement at all.

    Only thing which as a developer, we have gain with new feature is that now we don’t need to write sql to use it.

    However from knowledge sharing point of view as Gary has mention “Of course, since 10g, dual doesn’t need any consistent gets, so the real performance benefit came back in that version.”

    He is taking about Oracle’s “Fast dual” feature which was introduced in 10g.

    Before 10g, any query from dual tables results in the FULL TABLE SCAN (even though there is only one row in dual). However starting from 10g, Oracle has made changes in optimization process and introduced a feature called FAST DUAL, now Oracle does not do a full table scan on DUAL. This reduces the consistent reads which result in significant performance improvement.

    Check out very good explanation through example by Eddie Awad here.

    Regards

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