PL/SQL context switch, part 2

This is the second blogpost on using PL/SQL inside SQL. If you landed on this page and have not read the first part, click this link and read that first. I gotten some reactions on the first article, of which one was: how does this look like with ‘pragma udf’ in the function?

Pragma udf is a way to speed up using PL/SQL functions in (user defined function), starting from version 12. If you want to know more about the use of pragma udf, and when it does help, and when it doesn’t, please google for it.

create or replace function add_one( value number ) return number is
        pragma udf;
        l_value number(10):= value;
begin
        return l_value+1;
end;
/

select sum(add_one(id)) from t2;

As you can see, really the only thing you have to do is add ‘pragma udf’ in the declaration section of PL/SQL.

Here is how the flamegraph looks like:

What is visible, is that the functions between the plsql interpreter (pfrrun) and the function that makes the operand evaluation switch to PL/SQL (evapls) now is only one function, peidxrex. However, inside the evapls function there are two additional functions called (kkxmsagof, kkxmsagif, not readable) which take noticeable time. Conclusion at this point is pragma udf is doing it in yet another way than a native PL/SQL function and the subquery factoring.

Profiling this using the systemtap script:

global evapls_time, pfrrun_time, evapls_tot=0, pfrrun_tot=0

probe begin {
	printf("Begin.\n")
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("evapls") {
	if ( pid() == target() )
		evapls_time=local_clock_us()
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("evapls").return {
	if ( pid() == target() )
		evapls_tot+=local_clock_us()-evapls_time
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("pfrrun") {
	if ( pid() == target() )
		pfrrun_time=local_clock_us()
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("pfrrun").return {
	if ( pid() == target() )
		pfrrun_tot+=local_clock_us()-pfrrun_time
}

probe end {
	printf("\nevapls time: %12d\npfrrun time: %12d\n", evapls_tot, pfrrun_tot)
}

Shows:

# stap -x 92509 plsql.stap
Begin.
^C
evapls time:      2211412
pfrrun time:       804178

So, that’s very close to using this function using subquery factoring, a bit longer (2192685). This is very strictly depending on what is actually done, so milage may vary for your own use.

While we are at it, let’s have a look how this looks like when no PL/SQL is used, so:

select sum(id+1) from t2;

Here it is:

The function used for adding is now evaaddrset. From the size of the kdst_fetch function can be seen that it takes way lesser time. Let’s measure it with a changed version of the systemtap script:

global evaaddrset_time, evaaddrset_tot=0

probe begin {
	printf("Begin.\n")
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("evaaddrset") {
	if ( pid() == target() )
		evaaddrset_time=local_clock_us()
}
probe process("/u01/app/oracle/product/12.1.0.2/dbhome_1/bin/oracle").function("evaaddrset").return {
	if ( pid() == target() )
		evaaddrset_tot+=local_clock_us()-evaaddrset_time
}

probe end {
	printf("\nevaaddrset time: %12d\n", evaaddrset_tot)
}

This is how the output looks like:

# stap -x 92509 plsql.stap
Begin.
^C
evaaddrset time:        43389

A simple calculation shows that doing the addition native in SQL only takes 43389/2211412*100=2% of the runtime of PL/SQL with pragma udf.

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2 comments
  1. sqlmdx said:

    Great research!
    I’ve also measured overhead introduced by context switches but using documented tools:
    dbms_hprof and dbms_trace (event 10938, level 511).
    Your article helped to understand some details though.
    Thanks.

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