When oracle invalidates result_cache function results without any changes in objects on which depends

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On our production servers we have simple function with result_cache, like this:

create or replace function f_rc(p_id number) return number result_cache
is
  ret number;
begin
  select t.val into ret from rc_table t where t.id=p_id;
  return ret;
exception 
  when no_data_found then 
     return null;
end;
/

And oracle invalidates its results very frequently without any changes in table or function. I found only 2 cases when oracle invalidates result_cache results without any changes in table:
1. “select for update” from this table with commit; (strictly speaking, select-for-update is DML and it changes blocks, so it’s ok)
2. deleting rows from parent table that has no child records if there is unindexed foreign key with “on delete cascade”.
I test it on 11.2.0.1, 11.2.0.3, on solaris x64 and windows. Test cases for this i will show below.
But none of them can be the cause of our situation: we have no unindexed fk, and even if i lock all rows with “select for update”, it still does not stop invalidating.
In what other cases this happens? Am I right that the oracle does not track any changes, but the captures of the locks and “commits”?
Continue reading→

An interesting question from Valentin Nikotin

Posted on by Posted in Uncategorized 1,689 Page views Leave a comment

Test table:

create table tclob(c clob);

What is this code going to put out from the following blocks with and without “rollback”:

Task 1

[sourcecode language=”sql”]
declare
cl1 clob;
cl2 clob;
cl3 clob;
cl4 clob;
begin
cl1:=’1′;
insert into tclob values(cl1) returning c into cl2;
cl3:=cl2;
dbms_lob.append(cl3,’2′);
select c into cl4 from tclob;
— rollback;

dbms_output.put_line(cl1);
dbms_output.put_line(cl2);
dbms_output.put_line(cl3);
dbms_output.put_line(cl4);
end;
/
[/sourcecode]

[collapse]

Task 2

[sourcecode language=”sql”]
declare
cl1 clob;
cl2 clob;
cl3 clob;
cl4 clob;
begin
cl1 := ‘1’;
insert into tclob values (cl1) returning c into cl2;
cl3 := cl2;
dbms_lob.append(cl2, ‘2’);
select c into cl4 from tclob;
— rollback;

dbms_output.put_line(cl1);
dbms_output.put_line(cl2);
dbms_output.put_line(cl3);
dbms_output.put_line(cl4);
end;
/
[/sourcecode]

[collapse]
Task 3

[sourcecode language=”sql”]
declare
cl1 clob;
cl2 clob;
cl3 clob;
cl4 clob;
begin
cl1 := ‘1’;
insert into tclob values (cl1) returning c into cl2;
cl3 := cl2;
dbms_lob.append(cl2, ‘2’);
dbms_lob.append(cl3, ‘3’);
select c into cl4 from tclob;
— rollback;

dbms_output.put_line(cl1);
dbms_output.put_line(cl2);
dbms_output.put_line(cl3);
dbms_output.put_line(cl4);
end;
/
[/sourcecode]

[collapse]
Task 4

[sourcecode language=”sql”]
declare
cl1 clob;
cl2 clob;
cl3 clob;
cl4 clob;
begin
cl1 := ‘1’;
insert into tclob values (cl1) returning c into cl2;
cl3 := cl2;
dbms_lob.append(cl2, ’22’);
dbms_lob.append(cl3, ‘3’);
dbms_lob.append(cl2, ’44’);
select c into cl4 from tclob;
— rollback;
dbms_output.put_line(cl1);
dbms_output.put_line(cl2);
dbms_output.put_line(cl3);
dbms_output.put_line(cl4);
end;
/
[/sourcecode]

[collapse]

Check it both in Windows and Solaris/Linux 🙂

Explanation
It is clear that this bug is platform-depending and that the matter is in specifics of working with memory. The answer lies in the fact that cl3 and cl2 have unsynchronized lengths, which means that Oracle “forgets” to change the lengths of all remaining variables, that point to this “clob”, and as every operation of changing cl2/cl3 in fact changes the same thing, it turns out that “superfluous” becomes overwritten.

A lot of “latch free dml allocation latch” in concurrent queries to v$lock

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This problem is old actually, but it became possible to find out what is the real matter here and to deal with it only now thanks to the “Oracle Core” book by Jonathan Lewis.

Here is a quote from the chapter “Latches for lock”:

If the enqueue resource is in place already, then pick a row from the relevant enqueue structure (x$ksqeq, et al.), but to do this you have to get the associated enqueue latch to stop other people from picking the same enqueue row at the same time. The latch you need to acquire depends on the specific type of enqueue you are using; for example, if you want a row from x$ksqeq you need to get the enqueue latch but for a row from x$ktadm you need to get the dml allocation latch. Drop this latch as soon as you have made the enqueue row safe.

And this “fixed” table itself is in “v$lock”, which I had repeatedly seen in the plans for it:

Plan

[sourcecode language=”sql”]
DB11G/XTENDER> explain plan for select * from v$lock;

Explained.

Elapsed: 00:00:00.28
DB11G/XTENDER> @xplan

PLAN_TABLE_OUTPUT
——————————————————————————————–

Plan hash value: 3074737110

——————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
——————————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 131 | 1 (100)| 00:00:01 |
| 1 | NESTED LOOPS | | 1 | 131 | 1 (100)| 00:00:01 |
|* 2 | HASH JOIN | | 1 | 98 | 1 (100)| 00:00:01 |
|* 3 | FIXED TABLE FULL | X$KSUSE | 1 | 30 | 0 (0)| 00:00:01 |
| 4 | VIEW | GV$_LOCK | 10 | 680 | 0 (0)| 00:00:01 |
| 5 | UNION-ALL | | | | | |
|* 6 | FILTER | | | | | |
| 7 | VIEW | GV$_LOCK1 | 2 | 136 | 0 (0)| 00:00:01 |
| 8 | UNION-ALL | | | | | |
|* 9 | FIXED TABLE FULL | X$KDNSSF | 1 | 94 | 0 (0)| 00:00:01 |
|* 10 | FIXED TABLE FULL | X$KSQEQ | 1 | 94 | 0 (0)| 00:00:01 |
|* 11 | FIXED TABLE FULL | X$KTADM | 1 | 94 | 0 (0)| 00:00:01 |
|* 12 | FIXED TABLE FULL | X$KTATRFIL | 1 | 94 | 0 (0)| 00:00:01 |
|* 13 | FIXED TABLE FULL | X$KTATRFSL | 1 | 94 | 0 (0)| 00:00:01 |
|* 14 | FIXED TABLE FULL | X$KTATL | 1 | 94 | 0 (0)| 00:00:01 |
|* 15 | FIXED TABLE FULL | X$KTSTUSC | 1 | 94 | 0 (0)| 00:00:01 |
|* 16 | FIXED TABLE FULL | X$KTSTUSS | 1 | 94 | 0 (0)| 00:00:01 |
|* 17 | FIXED TABLE FULL | X$KTSTUSG | 1 | 94 | 0 (0)| 00:00:01 |
|* 18 | FIXED TABLE FULL | X$KTCXB | 1 | 94 | 0 (0)| 00:00:01 |
|* 19 | FIXED TABLE FIXED INDEX| X$KSQRS (ind:1) | 1 | 33 | 0 (0)| 00:00:01 |
——————————————————————————————–
[/sourcecode]

[collapse]

Now as we know the root of the problem, what’s left is to find out if we can escape unnecessary requests to X$KTADM, which seems possible considering that it’s a part of “union all”.
So we had to “parse” this v$lock, to get the full text of the query. To do this I began with getting the query text by tracing 10053:

Spoiler

[sourcecode language=”sql”]
SELECT "S"."INST_ID" "INST_ID",
"L"."LADDR" "ADDR",
"L"."KADDR" "KADDR",
"S"."KSUSENUM" "SID",
"R"."KSQRSIDT" "TYPE",
"R"."KSQRSID1" "ID1",
"R"."KSQRSID2" "ID2",
"L"."LMODE" "LMODE",
"L"."REQUEST" "REQUEST",
"L"."CTIME" "CTIME",
DECODE("L"."LMODE", 0, 0, "L"."BLOCK") "BLOCK"
FROM (SELECT "GV$_LOCK"."LADDR" "LADDR",
"GV$_LOCK"."KADDR" "KADDR",
"GV$_LOCK"."SADDR" "SADDR",
"GV$_LOCK"."RADDR" "RADDR",
"GV$_LOCK"."LMODE" "LMODE",
"GV$_LOCK"."REQUEST" "REQUEST",
"GV$_LOCK"."CTIME" "CTIME",
"GV$_LOCK"."BLOCK" "BLOCK"
FROM ( (SELECT USERENV(‘INSTANCE’) "INST_ID",
"V$_LOCK1"."LADDR" "LADDR",
"V$_LOCK1"."KADDR" "KADDR",
"V$_LOCK1"."SADDR" "SADDR",
"V$_LOCK1"."RADDR" "RADDR",
"V$_LOCK1"."LMODE" "LMODE",
"V$_LOCK1"."REQUEST" "REQUEST",
"V$_LOCK1"."CTIME" "CTIME",
"V$_LOCK1"."BLOCK" "BLOCK"
FROM (SELECT "GV$_LOCK1"."LADDR" "LADDR",
"GV$_LOCK1"."KADDR" "KADDR",
"GV$_LOCK1"."SADDR" "SADDR",
"GV$_LOCK1"."RADDR" "RADDR",
"GV$_LOCK1"."LMODE" "LMODE",
"GV$_LOCK1"."REQUEST" "REQUEST",
"GV$_LOCK1"."CTIME" "CTIME",
"GV$_LOCK1"."BLOCK" "BLOCK"
FROM ((SELECT "X$KDNSSF"."INST_ID" "INST_ID",
"X$KDNSSF"."ADDR" "LADDR",
"X$KDNSSF"."KSQLKADR" "KADDR",
"X$KDNSSF"."KSQLKSES" "SADDR",
"X$KDNSSF"."KSQLKRES" "RADDR",
"X$KDNSSF"."KSQLKMOD" "LMODE",
"X$KDNSSF"."KSQLKREQ" "REQUEST",
"X$KDNSSF"."KSQLKCTIM" "CTIME",
"X$KDNSSF"."KSQLKLBLK" "BLOCK"
FROM SYS."X$KDNSSF" "X$KDNSSF"
WHERE BITAND("X$KDNSSF"."KSSOBFLG", 1) <> 0
AND ("X$KDNSSF"."KSQLKMOD" <> 0 OR
"X$KDNSSF"."KSQLKREQ" <> 0))
UNION ALL
(SELECT "X$KSQEQ"."INST_ID" "INST_ID",
"X$KSQEQ"."ADDR" "LADDR",
"X$KSQEQ"."KSQLKADR" "KADDR",
"X$KSQEQ"."KSQLKSES" "SADDR",
"X$KSQEQ"."KSQLKRES" "RADDR",
"X$KSQEQ"."KSQLKMOD" "LMODE",
"X$KSQEQ"."KSQLKREQ" "REQUEST",
"X$KSQEQ"."KSQLKCTIM" "CTIME",
"X$KSQEQ"."KSQLKLBLK" "BLOCK"
FROM SYS."X$KSQEQ" "X$KSQEQ"
WHERE BITAND("X$KSQEQ"."KSSOBFLG", 1) <> 0
AND ("X$KSQEQ"."KSQLKMOD" <> 0 OR
"X$KSQEQ"."KSQLKREQ" <> 0))
) "GV$_LOCK1"
WHERE "GV$_LOCK1"."INST_ID" = USERENV(‘INSTANCE’)
) "V$_LOCK1"
)
UNION ALL
(SELECT "X$KTADM"."INST_ID" "INST_ID",
"X$KTADM"."ADDR" "LADDR",
"X$KTADM"."KSQLKADR" "KADDR",
"X$KTADM"."KSQLKSES" "SADDR",
"X$KTADM"."KSQLKRES" "RADDR",
"X$KTADM"."KSQLKMOD" "LMODE",
"X$KTADM"."KSQLKREQ" "REQUEST",
"X$KTADM"."KSQLKCTIM" "CTIME",
"X$KTADM"."KSQLKLBLK" "BLOCK"
FROM SYS."X$KTADM" "X$KTADM" /*** 1 ***/
WHERE BITAND("X$KTADM"."KSSOBFLG", 1) <> 0
AND ("X$KTADM"."KSQLKMOD" <> 0 OR
"X$KTADM"."KSQLKREQ" <> 0)
)
UNION ALL
(SELECT "X$KTATRFIL"."INST_ID" "INST_ID",
"X$KTATRFIL"."ADDR" "LADDR",
"X$KTATRFIL"."KSQLKADR" "KADDR",
"X$KTATRFIL"."KSQLKSES" "SADDR",
"X$KTATRFIL"."KSQLKRES" "RADDR",
"X$KTATRFIL"."KSQLKMOD" "LMODE",
"X$KTATRFIL"."KSQLKREQ" "REQUEST",
"X$KTATRFIL"."KSQLKCTIM" "CTIME",
"X$KTATRFIL"."KSQLKLBLK" "BLOCK"
FROM SYS."X$KTATRFIL" "X$KTATRFIL"
WHERE BITAND("X$KTATRFIL"."KSSOBFLG", 1) <> 0
AND ("X$KTATRFIL"."KSQLKMOD" <> 0 OR
"X$KTATRFIL"."KSQLKREQ" <> 0)
)
UNION ALL
(SELECT "X$KTATRFSL"."INST_ID" "INST_ID",
"X$KTATRFSL"."ADDR" "LADDR",
"X$KTATRFSL"."KSQLKADR" "KADDR",
"X$KTATRFSL"."KSQLKSES" "SADDR",
"X$KTATRFSL"."KSQLKRES" "RADDR",
"X$KTATRFSL"."KSQLKMOD" "LMODE",
"X$KTATRFSL"."KSQLKREQ" "REQUEST",
"X$KTATRFSL"."KSQLKCTIM" "CTIME",
"X$KTATRFSL"."KSQLKLBLK" "BLOCK"
FROM SYS."X$KTATRFSL" "X$KTATRFSL"
WHERE BITAND("X$KTATRFSL"."KSSOBFLG", 1) <> 0
AND ("X$KTATRFSL"."KSQLKMOD" <> 0 OR
"X$KTATRFSL"."KSQLKREQ" <> 0))
UNION ALL
(SELECT "X$KTATL"."INST_ID" "INST_ID",
"X$KTATL"."ADDR" "LADDR",
"X$KTATL"."KSQLKADR" "KADDR",
"X$KTATL"."KSQLKSES" "SADDR",
"X$KTATL"."KSQLKRES" "RADDR",
"X$KTATL"."KSQLKMOD" "LMODE",
"X$KTATL"."KSQLKREQ" "REQUEST",
"X$KTATL"."KSQLKCTIM" "CTIME",
"X$KTATL"."KSQLKLBLK" "BLOCK"
FROM SYS."X$KTATL" "X$KTATL"
WHERE BITAND("X$KTATL"."KSSOBFLG", 1) <> 0
AND ("X$KTATL"."KSQLKMOD" <> 0 OR
"X$KTATL"."KSQLKREQ" <> 0))
UNION ALL
(SELECT "X$KTSTUSC"."INST_ID" "INST_ID",
"X$KTSTUSC"."ADDR" "LADDR",
"X$KTSTUSC"."KSQLKADR" "KADDR",
"X$KTSTUSC"."KSQLKSES" "SADDR",
"X$KTSTUSC"."KSQLKRES" "RADDR",
"X$KTSTUSC"."KSQLKMOD" "LMODE",
"X$KTSTUSC"."KSQLKREQ" "REQUEST",
"X$KTSTUSC"."KSQLKCTIM" "CTIME",
"X$KTSTUSC"."KSQLKLBLK" "BLOCK"
FROM SYS."X$KTSTUSC" "X$KTSTUSC"
WHERE BITAND("X$KTSTUSC"."KSSOBFLG", 1) <> 0
AND ("X$KTSTUSC"."KSQLKMOD" <> 0 OR
"X$KTSTUSC"."KSQLKREQ" <> 0))
UNION ALL
(SELECT "X$KTSTUSS"."INST_ID" "INST_ID",
"X$KTSTUSS"."ADDR" "LADDR",
"X$KTSTUSS"."KSQLKADR" "KADDR",
"X$KTSTUSS"."KSQLKSES" "SADDR",
"X$KTSTUSS"."KSQLKRES" "RADDR",
"X$KTSTUSS"."KSQLKMOD" "LMODE",
"X$KTSTUSS"."KSQLKREQ" "REQUEST",
"X$KTSTUSS"."KSQLKCTIM" "CTIME",
"X$KTSTUSS"."KSQLKLBLK" "BLOCK"
FROM SYS."X$KTSTUSS" "X$KTSTUSS"
WHERE BITAND("X$KTSTUSS"."KSSOBFLG", 1) <> 0
AND ("X$KTSTUSS"."KSQLKMOD" <> 0 OR
"X$KTSTUSS"."KSQLKREQ" <> 0))
UNION ALL
(SELECT "X$KTSTUSG"."INST_ID" "INST_ID",
"X$KTSTUSG"."ADDR" "LADDR",
"X$KTSTUSG"."KSQLKADR" "KADDR",
"X$KTSTUSG"."KSQLKSES" "SADDR",
"X$KTSTUSG"."KSQLKRES" "RADDR",
"X$KTSTUSG"."KSQLKMOD" "LMODE",
"X$KTSTUSG"."KSQLKREQ" "REQUEST",
"X$KTSTUSG"."KSQLKCTIM" "CTIME",
"X$KTSTUSG"."KSQLKLBLK" "BLOCK"
FROM SYS."X$KTSTUSG" "X$KTSTUSG"
WHERE BITAND("X$KTSTUSG"."KSSOBFLG", 1) <> 0
AND ("X$KTSTUSG"."KSQLKMOD" <> 0 OR
"X$KTSTUSG"."KSQLKREQ" <> 0))
UNION ALL
(SELECT "X$KTCXB"."INST_ID" "INST_ID",
"X$KTCXB"."KTCXBXBA" "LADDR",
"X$KTCXB"."KTCXBLKP" "KADDR",
"X$KTCXB"."KSQLKSES" "SADDR",
"X$KTCXB"."KSQLKRES" "RADDR",
"X$KTCXB"."KSQLKMOD" "LMODE",
"X$KTCXB"."KSQLKREQ" "REQUEST",
"X$KTCXB"."KSQLKCTIM" "CTIME",
"X$KTCXB"."KSQLKLBLK" "BLOCK"
FROM SYS."X$KTCXB" "X$KTCXB"
WHERE BITAND("X$KTCXB"."KSSPAFLG", 1) <> 0
AND ("X$KTCXB"."KSQLKMOD" <> 0 OR
"X$KTCXB"."KSQLKREQ" <> 0))) "GV$_LOCK"
WHERE "GV$_LOCK"."INST_ID" = USERENV(‘INSTANCE’)
) "L",
SYS."X$KSUSE" "S",
SYS."X$KSQRS" "R"
WHERE "L"."SADDR" = "S"."ADDR"
AND "L"."RADDR" = "R"."ADDR"
[/sourcecode]

[collapse]

But I was too lazy to format it after beautifier processing, so I decided to parse it progressively with v$fixed_view_definition. This led to a much better result:

10.2.0.4-11.2.0.1

[sourcecode language=”sql”]
/————————– V$LOCK
select ADDR , KADDR , SID , TYPE , ID1 , ID2 , LMODE , REQUEST , CTIME , BLOCK
from GV$LOCK
where inst_id = USERENV(‘Instance’)
/————————– GV$LOCK
select s.inst_id INST_ID
,l.laddr ADDR
,l.kaddr KADDR
,s.ksusenum SID
,r.ksqrsidt TYPE
,r.ksqrsid1 ID1
,r.ksqrsid2 ID2
,l.lmode LMODE
,l.request REQUEST
,l.ctime CTIME
,decode(l.lmode, 0, 0, l.block) BLOCK
from v$_lock l, x$ksuse s, x$ksqrs r
where l.saddr=s.addr and l.raddr=r.addr
/——————– V$_LOCK —————————————–
select LADDR , KADDR , SADDR , RADDR , LMODE , REQUEST , CTIME , BLOCK
from GV$_LOCK
where inst_id = USERENV(‘Instance’)
/——————– GV$_LOCK —————————————–
select USERENV(‘Instance’) inst_id,laddr,kaddr,saddr,raddr,lmode,request,ctime, block
from v$_lock1
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktadm /**** 1 *****/
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktatrfil
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktatrfsl
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktatl
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktstusc
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktstuss
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktstusg
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,ktcxbxba,ktcxblkp,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktcxb
where bitand(ksspaflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
/—————— V$_LOCK1
select LADDR , KADDR , SADDR , RADDR , LMODE , REQUEST , CTIME , BLOCK
from GV$_LOCK1
where inst_id = USERENV(‘Instance’)
/—————— GV$_LOCK1
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim, ksqlklblk
from x$kdnssf
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim, ksqlklblk
from x$ksqeq
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
[/sourcecode]

[collapse]

11.2.0.3

[sourcecode language=”sql”]
/————————– V$LOCK
select ADDR , KADDR , SID , TYPE , ID1 , ID2 , LMODE , REQUEST , CTIME , BLOCK
from GV$LOCK
where inst_id = USERENV(‘Instance’)
/————————– GV$LOCK
select s.inst_id INST_ID
,l.laddr ADDR
,l.kaddr KADDR
,s.ksusenum SID
,r.ksqrsidt TYPE
,r.ksqrsid1 ID1
,r.ksqrsid2 ID2
,l.lmode LMODE
,l.request REQUEST
,l.ctime CTIME
,decode(l.lmode, 0, 0, l.block) BLOCK
from v$_lock l, x$ksuse s, x$ksqrs r
where l.saddr=s.addr and concat(USERENV(‘Instance’),l.raddr)=concat(r.inst_id,r.addr)
/——————– V$_LOCK —————————————–
select LADDR , KADDR , SADDR , RADDR , LMODE , REQUEST , CTIME , BLOCK
from GV$_LOCK
where inst_id = USERENV(‘Instance’)
/——————– GV$_LOCK —————————————–
select USERENV(‘Instance’) inst_id,laddr,kaddr,saddr,raddr,lmode,request,ctime, block
from v$_lock1
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktadm
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktatrfil
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktatrfsl
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktatl
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktstusc
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktstuss
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktstusg
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,ktcxbxba,ktcxblkp,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktcxb
where bitand(ksspaflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
/—————— V$_LOCK1
select LADDR , KADDR , SADDR , RADDR , LMODE , REQUEST , CTIME , BLOCK
from GV$_LOCK1
where inst_id = USERENV(‘Instance’)
/—————— GV$_LOCK1
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim, ksqlklblk
from x$kdnssf
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim, ksqlklblk
from x$ksqeq
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
[/sourcecode]

[collapse]

By the way, as you can see gv$lock varies in different versions, which has to be taken into account (I haven’t tried it on 11.2.0.2 so I will update it later):
in 11.2.0.3 predicate “l.raddr=r.addr” was changed to concat(USERENV(‘Instance’),l.raddr)=concat(r.inst_id,r.addr).
Only particular blockings were queried from v$lock in my case, where most important of them were “user-locks”, which means they were of ‘UL’ type. That’s why after parsing the code, I had to get what locks are returned by each particular “union all” block. For this purpose I have created a modified GV$LOCK:
xt_gv$_lock for 11.2.0.3

[sourcecode language=”sql”]
create or replace view xt_gv$_lock as
with XT_GV$_LOCK as (
select 1 sq
,USERENV(‘Instance’) inst_id,laddr,kaddr,saddr,raddr,lmode,request,ctime, block
from v$_lock1
union all
select 2
,inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktadm
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select 3
,inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktatrfil
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select 4
,inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktatrfsl
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select 5
,inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktatl
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select 6
,inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktstusc
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select 7
,inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktstuss
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select 8
,inst_id,addr,ksqlkadr,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktstusg
where bitand(kssobflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
union all
select 9
,inst_id,ktcxbxba,ktcxblkp,ksqlkses,ksqlkres,ksqlkmod,ksqlkreq, ksqlkctim,ksqlklblk
from x$ktcxb
where bitand(ksspaflg,1)!=0 and (ksqlkmod!=0 or ksqlkreq!=0)
)
select l.sq
,s.inst_id INST_ID
,l.laddr ADDR
,l.kaddr KADDR
,s.ksusenum SID
,r.ksqrsidt TYPE
,r.ksqrsid1 ID1
,r.ksqrsid2 ID2
,l.lmode LMODE
,l.request REQUEST
,l.ctime CTIME
,decode(l.lmode,0,0,l.block) BLOCK
from XT_GV$_LOCK l, x$ksuse s, x$ksqrs r
where l.saddr=s.addr and concat(USERENV(‘Instance’),l.raddr)=concat(r.inst_id,r.addr)
/
create or replace public synonym xt_gv$lock for xt_gv$_lock
/
grant select on xt_gv$lock to public
/
[/sourcecode]

[collapse]

Now we can get relevance of block types ~ particular block:

with t as (select distinct sq,type from xt_gv$lock l)
select sq
      ,listagg(t.TYPE,',') within group(order by t.type)
from t
group by sq

Where “sq” is a number of “union all” block.
Or just get the block number using filter by block type. So, for example ‘UL’ will now be in the first block, and if we need to select them we can just make queries from this new view adding “sq=1” predicate to avoid torturing other blocks.

select * 
from xt_gv$lock l
where l.type='UL'
and l.sq=1 -- ul in first block only

During a load testing in a concurrency my modified query with “user locks” not only completely solved the problem with latches, but also increased the speed of query processing by 200 times and lowered the CPU usage.

Script files:
11.2.0.1: xt_gv$_lock_11_2_0_1.sql
11.2.0.3: xt_gv$_lock_11_2_0_3.sql

dbms_random in parallel

Posted on by Posted in documentation, oracle, parallel 1,812 Page views Leave a comment

The documentation for dbms_random states:

It will automatically initialize with the date, user ID, and process ID if no explicit initialization is performed.

This phrase does not answer the question, which “process id” is going to be used for initialization in case of parallel execution. That’s why I decided to give a vivid example that shows independence of “dbms_random” generator from “process id” of slave, that is generating identical values in parallel:

with
 t  as ( select/*+ materialize */ level n from dual connect by level<=4000)
,t1 as (
         select--+ materialize parallel(t 4)
            dbms_random.string('x',4)
            ||';'
            ||(select sid||';'||process||';'||pid
               from v$session, v$process
               where sid=sys_context('USERENV','SID')
                 and PADDR=ADDR
                 and n>0
              ) f
         from t
)
,t2 as (
         select
            t1.f
           ,count(*) over(partition by regexp_substr(f,'^[^;]+')) cnt
         from t1
)
select f
      ,regexp_substr(f,'[^;]+') rnd
      ,regexp_substr(f,'[^;]+',1,2) sid
      ,regexp_substr(f,'[^;]+',1,3) process
      ,regexp_substr(f,'[^;]+',1,4) pid
from t2 
where cnt>1
order by f

Result:


F RND SID PROCESS PID
AARV;130;5472;30 AARV 130 5472 30
AARV;68;2228;29 AARV 68 2228 29
AC2R;130;5472;30 AC2R 130 5472 30
AC2R;68;2228;29 AC2R 68 2228 29
AC8O;130;5472;30 AC8O 130 5472 30
AC8O;68;2228;29 AC8O 68 2228 29
AKVZ;130;5472;30 AKVZ 130 5472 30
AKVZ;68;2228;29 AKVZ 68 2228 29
ALTQ;130;5472;30 ALTQ 130 5472 30
ALTQ;68;2228;29 ALTQ 68 2228 29
… … … … …

About the performance of exception handling

Posted on by Posted in oracle, query optimizing 1,878 Page views 1 Comment

This article is about a well-known fact about the poor performance of exception handling.

Yes, the exception handling is rather slow, however, it is not necessary to try to avoid exceptions whenever possible, and by any means. For example, I often see that people are trying to avoid them even in cases of search by primary key where probability of receiving “no_data_found” is minimal.
In general, we should analyze the possible frequency of exceptions and “overhead”, which is added by the chosen way with exception handlers.

Let me explain this with an example, which I mentioned earlier: suppose we have a code that returns a field from the table by “pk” and it returns “null” in case there is no such entry.
Test table:

create table t_test(a primary key, b)
as
select level,level from dual connect by level<=1e5;

Lets create a standard function for tests:

create or replace function f1(p in number) return number
as
  res number;
begin
  select/*+ F1 */ b into res
  from t_test t
  where t.a=p;
  return res;
exception when no_data_found then
  return null;
end;

The most common options to avoid the exceptions mechanism in these cases are the followings:

Variant 1

[sourcecode language=”sql”]
create or replace function f2(p in number) return number
as
begin
for rec in (select/*+ F2 */ b from t_test t where t.a=p) loop
return rec.b;
end loop;
return null;
end;
[/sourcecode]

[collapse]

By the way, don’t use this in case your cursor can not have more than one entry:
Spoiler

[sourcecode language=”sql”]
create or replace function f2(p in number) return number
as
res number;
begin
for rec in (select/*+ F2 */ b from t_test t where t.a=p) loop
res:=rec.b;
end loop;
return res;
end;
[/sourcecode]

[collapse]

Otherwise, there will be attempts of the second iteration, which you can observe in the profiler.
Variant 2

[sourcecode language=”sql”]
create or replace function f3(p in number) return number
as
res number;
begin
select/*+ F3 */ min(b) into res
from t_test t
where t.a=p;
return res;
end;
[/sourcecode]

[collapse]

Here I would like to propose my own option for this:
Spoiler

[sourcecode language=”sql”]
create or replace function f4(p in number) return number
as
res number;
begin
select/*+ F4 */
(select b from t_test t where t.a=p)
into res
from dual;
return res;
end;
[/sourcecode]

[collapse]

And now lets carry out a basic test by executing these functions for the test table:

declare
  v       integer;
  v_start integer:= 1;
  v_end   integer:= 100000;
    l_timer integer := dbms_utility.get_time;
    procedure print(msg varchar2) is
    begin
      dbms_output.put_line(to_char((dbms_utility.get_time-l_timer)/100,'9990.00')||' '||msg);
      l_timer:=dbms_utility.get_time;
    end;
     
begin
  print('start');
  for i in v_start..v_end loop
    v:=f1(i);
  end loop;
  print('1');
  for i in v_start..v_end loop
    v:=f2(i);
  end loop;
  print('2');
  for i in v_start..v_end loop
    v:=f3(i);
  end loop;
  print('3');
  for i in v_start..v_end loop
    v:=f4(i);
  end loop;
  print('4');
end;

As a result, we get the following ratio:

Variant Time(sec)
Variant 1(with exception) 3.03
Variant 2(with cycle) 3.62
Variant 3(with min) 3.34
Variant 4(scalar subquery) 3.10

As you can see, the original query is the fastest in case the exceptions are not called! Lets now check it with different percents of exceptions: exceptions will be for queries с i<=0, the total number of calls will be 100001, I will change v_start and v_end in pairs: (-5000, 95000), (10000, 90000), (-50000, 50000), (-90000, 10000): 

declare
  v       integer;
  v_start integer:=-50000;
  v_end   integer:= 50000;
    l_timer integer := dbms_utility.get_time;
    procedure print(msg varchar2) is
    begin
      dbms_output.put_line(to_char((dbms_utility.get_time-l_timer)/100,'9990.00')||' '||msg);
      l_timer:=dbms_utility.get_time;
    end;
     
begin
  print('start');
  for i in v_start..v_end loop
    v:=f1(i);
  end loop;
  print('1');
  for i in v_start..v_end loop
    v:=f2(i);
  end loop;
  print('2');
  for i in v_start..v_end loop
    v:=f3(i);
  end loop;
  print('3');
  for i in v_start..v_end loop
    v:=f4(i);
  end loop;
  print('4');
end;
/

Summary table of multiple comparisons:

Variant 0% ~5% ~10% ~50% ~90%
Variant 1(with exception) 3.04 3.12 3.16 3.82 4.51
Variant 2(with cycle) 3.18 3.21 3.20 3.51 3.85
Variant 3(with min) 3.37 3.34 3.29 3.25 3.18
Variant 4(scalar subquery) 3.12 3.06 3.03 2.98 2.94

What conclusions can be drawn from this:

  • As you can see, 5% of exceptions is a kind of turning point for this table, when the standard option with exception becomes less effective than the option with subquery (by about ~4.5% to be precise), and about ~10% than the remaining two.
  • Options with “min” and a cycle as a whole are worse than the option with a subquery.
  • Options with subquery and “min” becomes faster when the number of “empty” queries increases.

A funny fact about collect

Posted on by Posted in collect, oracle, parallel, query optimizing 2,291 Page views 1 Comment

Many people know that oracle creates domain types on its own when necessary, for example when using a type declared in a package (before 11g they could be observed in dba_objects with the name like ‘PLSQL%’).

Fact #1

It acts in the same way when calling an aggregate function “collect”.

-- Firstly we check if there are such types
DB11G/XTENDER> select t.type_name,t.type_name,t.typecode 
 2 from dba_types t 
 3 where t.type_name like 'SYSTP%';
 
no rows selected
 
 
-- Executing a query with collect
DB11G/XTENDER> select collect(level) from dual connect by level<=10;
 
COLLECT(LEVEL)
-------------------------------------------------------------------------
 
SYSTPZvGjVQTySRSjYVlHXyEE2Q==(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
 
1 row selected.
 
 
-- We check it again and observe that a new type SYSTP% has appeared
DB11G/XTENDER> select t.type_name,t.type_name,t.typecode 
 2 from dba_types t 
 3 where t.type_name like 'SYSTP%';
 
TYPE_NAME                      TYPE_NAME                      TYPECODE
------------------------------ ------------------------------ ------------
SYSTPZvGjVQTySRSjYVlHXyEE2Q==  SYSTPZvGjVQTySRSjYVlHXyEE2Q==  COLLECTION

And also we select data on it from sys.obj$. We will need it later:

DB11G/XTENDER> select obj#,type#,ctime,mtime,stime,status
  2  from sys.obj$ o$
  3  where o$.name = 'SYSTPZvGjVQTySRSjYVlHXyEE2Q==';
 
  OBJ#  TYPE# CTIME               MTIME               STIME              
------ ------ ------------------- ------------------- -------------------
103600     10 28.04.2012 01:02:35 28.04.2012 01:02:35 28.04.2012 01:02:35

This is a natural result as you can not return information to the client without having described it.

Fact #2

Now as we know that the type is created, it is interesting to know what will happen to this type: whether it will be removed either after fetch or after disconnecting the client? For example, the domain type was automatically dropped when the packet type was removed. Are we going to have a similar automatic remove here?

According to Bug 4033868: COLLECT FUNCTION LEAVES TEMPORARY SYS TYPES BEHIND this bug is fixed in “11.0”, but I am testing it on 11.2.0.1 and a basic check after disconnection showed, that this type exists until instance restart. In fact, it stays there even after that, but it is not displayed in dba_objects.

I will drop it manually, so I won’t have to restart the instance once again. This is absolutely similar to how oracle “removes” this type in 11.2:

DB11G/XTENDER> drop type "SYSTPZvGjVQTySRSjYVlHXyEE2Q==";
 
Type dropped.

And now lets check it:

DB11G/XTENDER> select * from dba_types
   where type_name='SYSTPZvGjVQTySRSjYVlHXyEE2Q==';
 
no rows selected

It seems like we have removed it, but what if we look in the sys.obj$:

DB11G/XTENDER> select obj#,type#,ctime,mtime,stime,status
  2  from sys.obj$ o$
  3  where o$.name = 'SYSTPZvGjVQTySRSjYVlHXyEE2Q==';
 
  OBJ#  TYPE# CTIME               MTIME               STIME              
------ ------ ------------------- ------------------- -------------------
103600     10 28.04.2012 01:02:35 28.04.2012 01:40:37 31.12.4712 23:59:59

As you can see, the object is still there, but with type#=10 and with “stime” equal to the last date of year 4712, and before that it was type#=13 and stime=mtime=ctime, and in 10.2 after manual drop of this type no entries were left. I will explain the correspondence of the fields from “sys.obj$” and “dba_objects” to clarify this: obj# – object_id, type# ~ type code, ctime,mtime,stime – created, last_ddl_time, timestamp respectively. By the “dba_objects” view code we will see that type# = 10 is supposedly “NON-EXISTENT” and displaying it is not necessary.

and (o.type# not in (1  /* INDEX - handled below */,
                      10 /* NON-EXISTENT */)

And the date being set to 31.12.4712 23:59:59 indicates its irrelevance – this future is too distant 🙂

Fact #3

“Collect” is a pretty buggy thing in general, as I have repeatedly noticed, and Metalink has information about lots of bugs related to “collect” (for example, Bug 11906197 “Parallel query with COLLECT function fails with ORA-7445/ORA-600.”,”Bug 8912282: COLLECT+UNIQUE+ORDER DOES NOT REMOVE DUPLICATES”, “Bug 6145841: ORA-600[KOLOGSF2] ON CAST(COLLECT(..)) CALL”,”Bug 11802848: CAST/COLLECT DOES NOT WORK IN VERSION 11.2.0.2 WITH TYPE SYS.DBMS_DEBUG_VC2COLL”, “Bug 6996176: SELECT COLLECT DISTINCT GROUP BY STATEMENT RETURNS DUPLICATE VALUES”)

Test table

[sourcecode language=”sql”]
create table test_parallel parallel 8 as
select mod(level,8) a, level b
from dual
connect by level<=1000;
create index IX_TEST_PARALLEL on TEST_PARALLEL (A);
[/sourcecode]

[collapse]

Errors

[sourcecode language=”sql”]
DB11G/XTENDER> select/*+ PARALLEL(2)*/ cast(collect(a) as number_table) from test_parallel ;
select/*+ PARALLEL(2)*/ cast(collect(a) as number_table) from test_parallel
*
ERROR at line 1:
ORA-12801: error signaled in parallel query server P000
ORA-21710: argument is expecting a valid memory address of an object
 
Elapsed: 00:00:00.12
DB11G/XTENDER> select cast(collect(b) as number_table) from test_parallel group by a;
select cast(collect(b) as number_table) from test_parallel group by a
*
ERROR at line 1:
ORA-12805: parallel query server died unexpectedly
 
Elapsed: 00:00:17.57
[/sourcecode]

[collapse]

I don’t experience such errors while using my slow aggregate. Here is a common dilemma: whether to use an unstable but speedy “collect”, or a slow aggregate of my own…

An example of an aggregate

[sourcecode language=”sql”]
create or replace type ncollect_type as object
(

data sys.ku$_objnumset,

static function ODCIAggregateInitialize
( sctx in out ncollect_type )
return number ,

member function ODCIAggregateIterate
( self in out ncollect_type ,
val in number
) return number ,

member function ODCIAggregateDelete
( self in out ncollect_type,
val in number
) return number ,
member function ODCIAggregateTerminate
( self in ncollect_type,
returnval out sys.ku$_objnumset,
flags in number
) return number ,

member function ODCIAggregateMerge
( self in out ncollect_type,
ctx2 in ncollect_type
) return number
)
/
create or replace type body ncollect_type is

static function ODCIAggregateInitialize
( sctx in out ncollect_type )
return number
is
begin
sctx := ncollect_type( sys.ku$_objnumset()) ;
return ODCIConst.Success ;
end;

member function ODCIAggregateIterate
( self in out ncollect_type ,
val in number
) return number
is
begin
self.data:=self.data multiset union sys.ku$_objnumset(val);
return ODCIConst.Success;
end;

member function ODCIAggregateDelete
( self in out ncollect_type,
val in number
) return number
is
begin
self.data:=self.data multiset except sys.ku$_objnumset(val);
return ODCIConst.Success;
end;

member function ODCIAggregateTerminate
( self in ncollect_type ,
returnval out sys.ku$_objnumset ,
flags in number
) return number
is
begin
returnval:=self.data;
return ODCIConst.Success;
end;

member function ODCIAggregateMerge
( self in out ncollect_type ,
ctx2 in ncollect_type
) return number
is
begin
self.data := self.data multiset union ctx2.data;
return ODCIConst.Success;
end;
end;
/
[/sourcecode]

[collapse]

And the results:

Variant Time(sec)
select/*+ NO_PARALLEL*/ cast(collect(b) as number_table) from test_parallel group by a; 0.03
select/*+ NO_PARALLEL*/ ncollect(b) from test_parallel group by a 0.08
select ncollect(b) from test_parallel group by a; 0.07
select/*+ NO_PARALLEL*/ collect(a) from test_parallel; 0.02
select/*+ NO_PARALLEL*/ ncollect(a) from test_parallel 0.18
select ncollect(a) from test_parallel; 0.19
select/*+ NO_PARALLEL*/ collect(b) from test_parallel; 0.02
select/*+ NO_PARALLEL*/ ncollect(b) from test_parallel 0.18
select ncollect(b) from test_parallel; 0.06