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Category Archives: troubleshooting

CBO and Partial indexing

Posted on November 2, 2022 by Sayan Malakshinov Posted in bug, CBO, oracle, query optimizing, SQL, trace, troubleshooting Leave a comment

Oracle 12c introduced Partial indexing, which works well for simple partitioned tables with literals. However, it has several significant issues:

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cbo oracle partial indexes partial indexing query optimization troubleshooting undocumented oracle

Slow index access “COL=:N” where :N is NULL

Posted on October 31, 2022 by Sayan Malakshinov Posted in CBO, curious, Funny, oracle, query optimizing, SQL, troubleshooting Leave a comment

All Oracle specialists know that a predicate X=NULL can never be true and we should use “X is NULL” in such cases. The Oracle optimizer knows about that, so if we create a table like this:

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cbo oracle query optimization troubleshooting

Where does the commit or rollback happen in PL/SQL code?

Posted on September 12, 2021 by Sayan Malakshinov Posted in diagnostic event 10046, oracle, PL/SQL, trace, troubleshooting, undocumented 1 Comment

One of the easiest ways is to use diagnostic events:

alter session set events 'sql_trace {callstack: fname xctend} errorstack(1)';
Image
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oracle pl/sql troubleshooting undocumented oracle

ORA exceptions that can’t be caught by exception handler

Posted on August 12, 2021 by Sayan Malakshinov Posted in curious, Funny, oracle, PL/SQL, SQL, troubleshooting Leave a comment

I know 2 “special” exceptions that can’t be processed in exception handler:

  • “ORA-01013: user requested cancel of current operation”
  • “ORA-03113: end-of-file on communication channel”
  • and + “ORA-00028: your session has been killed” from Matthias Rogel

Tanel Poder described the first one (ORA-01013) in details here: https://tanelpoder.com/2010/02/17/how-to-cancel-a-query-running-in-another-session/ where Tanel shows that this error is based on SIGURG signal (kill -URG):

-- 1013 will not be caught:
declare
 e exception;
 pragma exception_init(e,-1013);
begin
  raise e;
exception when others then dbms_output.put_line('caught');
end;
/

declare
*
ERROR at line 1:
ORA-01013: user requested cancel of current operation
ORA-06512: at line 5
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exceptions ora-errors oracle oracle undocumented behaviour pl/sql troubleshooting undocumented oracle

Another interesting troubleshooting case

Posted on May 29, 2021 by Sayan Malakshinov Posted in oracle, PL/SQL, PL/SQL optimization, trace, troubleshooting Leave a comment

Got an interesting question today in RuOUG:

Some very simple PL/SQL procedures usually are completed within ~50ms, but sometimes sporadically longer than a second. For example, the easiest one from these procedures:

create or replace PROCEDURE XXXX (
  P_ORG_NUM IN number,
  p_result OUT varchar2, 
  p_seq OUT number 
) AS 
BEGIN
  p_seq := P_ORG_NUM; p_result:='';
END;

sql_trace shows that it was executed for 1.001sec and all the time was “ON CPU”:

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dtrace nanosleep native compilation pl/sql troubleshooting truss

SQL*Plus tips #9: Reading traces and incident files

Posted on May 28, 2021 by Sayan Malakshinov Posted in diagnostic event 10046, oracle, SQL*Plus, SQL*PLus tips, trace, troubleshooting Leave a comment

@tracefile_read_last_by_mask filemask [regexp] [ignore_regexp]
– finds last trace by filemask and filters rows by regexp and filters out rows by ignore_regexp:

@ tracefile_by_mask.sql [mask] – finds and shows last 10 trace files by mask

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SQL*Plus sqlplus traces

Oracle diagnostic events — Cheat sheet

Posted on May 20, 2021 by Sayan Malakshinov Posted in diagnostic event 10046, oracle, statistics, trace, troubleshooting, undocumented Leave a comment

Oracle diagnostic events is a great feature, but unfortunately poorly documented and nonintuitive, so it’s difficult to remember all events/actions/parameters and even read its internal documentation using oradebug. So I decided to compile its internal doc as a more convenient html-version (https://orasql.org/files/events/) and make a cheat sheet of some unknown or little-known use cases.

Example 1:

alter system set events 
   'kg_event[1476]
        {occurence: start_after 1, end_after 3}
            trace("stack is: %\n", shortstack())
            errorstack(2)
    ';
  1. kg_event[errno] – Kernel Generic event in library Generic for error number events, which instructs to trace ORA-errno errors;
  2. {occurence: start_after X, end_after Y} – is a filter, which instructs to skip X event checks and trace just Y times;
  3. trace(format, str1, str2, …, str15) – is a function from ACTIONS for printing into a trace file;
  4. shortstack() – is a function from ACTIONS , which returns a short call stack as a string;
  5. errorstack(level) – is a function from ACTIONS, which prints extended info (level: 0 – errorstack only, 1 – errorstack + call stack, 2 – as level 1 + processtate, 3 – as level 2 + context area). You can get more details with  PROCESSSTATE or SYSTEMSTATE. If you need just a call stack, you can use CALLSTACK(level) , with function arguments in case of level>1.

Example 2:

alter system set events 
    'trace[SQL_Compiler.* | SQL_Execution.*]
        [SQL: ...]
            {process: ospid = ...}
            {occurence:end_after 3}
                controlc_signal()';
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diagnostic events oracle oradebug

Simple function returning Parallel slave info

Posted on September 15, 2020 by Sayan Malakshinov Posted in oracle, parallel, query optimizing, SQL, statistics, troubleshooting Leave a comment

You can add also any information from v$rtsm_sql_plan_monitor if needed

create or replace function px_session_info return varchar2 parallel_enable as
   vSID int;
   res varchar2(30);
begin
   vSID:=userenv('sid');
   select 
           to_char(s.server_group,'fm000')
    ||'-'||to_char(s.server_set,'fm0000')
    ||'-'||to_char(s.server#,'fm0000')
    ||'('||s.sid||','||s.degree||'/'||s.req_degree||')'
    into res
   from v$px_session s 
   where s.sid=vSID;
   return res;
exception when no_data_found then
   return 'no_parallel';
end;
/

Simple example:

select--+ parallel
  px_session_info, count(*)
from sys.obj$
group by px_session_info
/
PX_SESSION_INFO           COUNT(*)
------------------------  --------
001-0002-0001(630,2/2)     38298
001-0002-0002(743,2/2)     34706
oracle parallel presentations troubleshooting

Smart Scan and Recursive queries

Posted on March 5, 2020 by Roger MacNicol Posted in adaptive serial direct path reads, CBO, hints, oracle, parallel, SmartScan, trace, troubleshooting Leave a comment

Since Christmas I have been asked to investigate two different “failures to use Smart Scan”. It turns out they both fell into the same little known restriction on the use of Direct Read. Smart Scan critically depends on Direct Read in order to read the synthetic output blocks into private buffers in PGA so with Direct Read disabled Smart Scan is also disabled. In these two cases the restriction is on using Direct Read on Serial Recursive queries.

Case 1: Materialized View Refresh

A customer asked me to investigate why his MView refresh was running slowly and was failing to use Smart Scan. He had used 'trace[NSMTIO] disk=highest' which showed the cause as:

Direct Read for serial qry: disabled(::recursive_call::kctfsage:::)
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direct path reads hints oracle Roger MacNicol SmartScan troubleshooting

Just short note for myself: OJPPD limitations

Posted on December 2, 2019 by Sayan Malakshinov Posted in CBO, collect, oracle, SQL, troubleshooting Leave a comment

As of Oracle 19c OJPPD doesn’t support connect-by and TABLE():

OJPPD: OJPPD bypassed: query block contains START WITH/CONNECT BY.
OJPPD: OJPPD bypassed: View contains TABLE expression.
cbo JPPD ojppd
photo Sayan Malakshinov

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Recent Posts

  • CBO and Partial indexing
  • Slow index access “COL=:N” where :N is NULL
  • Where does the commit or rollback happen in PL/SQL code?
  • :1 and SP2-0553: Illegal variable name “1”.
  • ORA exceptions that can’t be caught by exception handler

Recent Comments

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  • Oracle SQL | Oracle diagnostic events — Cheat sheet on Where does the commit or rollback happen in PL/SQL code?
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