SQL Plan Baseline Manual Evolution

I’ve never really spent time on evolution because most of the time I use baselines for emergency sql plan management and not with the added value of controlled evolution with which the feature was conceived.

But some observations on sql plan baseline evolution originating from the questions:

  • What does evolution actually do?
  • What information does it use?
  • What happens with DML?

Starting point – one baselined plan

1. FTS plan in memory from SQL which (more...)

Heuristic Temp Table Transformation – 2

Heuristic Temp Table Transformation - 2 Some time ago I've demonstrated the non-cost based decision for applying the temp table transformation when using CTEs (Common Table/Subquery Expressions). In this note I want to highlight another aspect of this behaviour.Consider the following data creating a table with delibrately wide columns:

create table a
as
select
rownum as id
, rownum as id2
, rpad('x', 4000) as large_vc1
, rpad('x', 4000) as large_vc2
, rpad('x', 4000) (more...)

Strategies for Minimising SQL Execution Plan Instability

Execution Plan Instability – What is the problem?

The Oracle Optimizer is a complex piece of software and with every release it becomes more complex.

RBO

In the beginning, the Optimizer was rule-based.

The Optimizer had a ranked list of heuristics used to optimize a query, picking the lowest ranked rule available.

This rule-based mode, whilst still in use with some internal Oracle dictionary queries, has been unsupported since version 10.1.

This means that (more...)

Enabling Edition Based Redefinition On A Schema With Object-Relational Tables

This is just a heads-up for those thinking about using Edition Based Redefinition (EBR) and enabling it on an existing schema with objects. Although EBR isn't exactly a new feature its current adoption level is probably not that high (which probably changes in future as Oracle E-Business Suite uses EBR now as part of their default upgrade procedure as far as I understood).I was recently contacted by someone who enabled EBR on an existing (more...)

Function-Based Indexes And CURSOR_SHARING = FORCE

In general it is known that Function-Based Indexes (FBIs) can no longer be used by the optimizer if the expression contains literals and CURSOR_SHARING = FORCE / SIMILAR (deprecated) turns those literals into bind variables. Jonathan Lewis described the issue quite a while ago here in detail.In a recent OTN thread this issue was raised again, but to my surprise when I played around with a test case that mimicked the OP's problem query (more...)

ORA-00353: log corruption near block change time – 11.2.0.3

Recently I came across “ORA-00353: log corruption near block change time” error.

Errors in file /oracle/app/diag/rdbms/test/test/trace/test_ms00_54284.trc  (incident=1490189):
ORA-00353: log corruption near block  change  time
ORA-00353: log corruption near block 4631374 change 263470473806052 time 04/16/2015 06:53:16
ORA-00312: online log 43 thread 3: '+REDO02/test/onlinelog/group_43.258.875922665'
ORA-00312: online log 43 thread 3: '+REDO01/test/onlinelog/group_43.258.875922633'
Incident details in: /oracle/app/diag/rdbms/test/test/incident/incdir_1490189/test_ms00_54284_i1490189.trc
Thu Apr 16 08:14:42 2015
Sweep [inc][1490188]: completed
Thu Apr 16 08:14:55 2015
LOGMINER: WARNING: error  (more...)

Combined ACCESS And FILTER Predicates – Excessive Throw-Away

Catchy title... Let's assume the following data setup:

create table t1
as
select
rownum as id
, 1 as id2
, rpad('x', 100) as filler
from
dual
connect by
level < = 1e4
;

create table t2
as
select
rownum as id
, 1 as id2
, rpad('x', 100) as filler
from
dual
connect by
level < = 1e4
;

create table t3
as
select
rownum as id
, 1 as id2
, rpad('x', 100) as filler
from
dual
connect by
(more...)

ORA-01628: max # extents (32765) reached for rollback segment _SYSSMUxxx

Recently I came across “ORA-01628: max # extents (32765)” error. The database had resumable_timeout parameter set I received page for a session in resumable state. Looking into the alert log showed

statement in resumable session ‘User USER_E3(375), Session 9028, Instance 1′ was suspended due to
ORA-01628: max # extents (32765) reached for rollback segment _SYSSMU375_247595031$
Wed Apr 08 02:41:42 2015

Looking at the sid# 9028 details, INSERT sql was running and the session was on (more...)

ORA-17629 ORA-20079

Yesterday I received page for ” ORA-17629: Cannot connect to the remote database server”  reported in Primary Database alert log.

Fri Apr 10 06:47:41 2015
Errors in file /oracle/app/diag/rdbms/testdb/testdb/trace/testdb_ora_161991.trc:
ORA-17627:
ORA-17629: Cannot connect to the remote database server
Errors in file /oracle/app/diag/rdbms/testdb/testdb/trace/testdb_ora_161991.trc:
ORA-17629: Cannot connect to the remote database server
ORA-17627:
ORA-17629: Cannot connect to the remote database server
Fri Apr 10 06:48:22 2015

The error trace file shows

*** 2015-04-10 (more...)

SQLDeveloper VAR NUMBER

Just been tearing my hair out on some unexpected SQL behaviour in SQL Developer.
Eventually I managed to get to the root cause of the problem as illustrated below.

SQL*Plus:

SQL> var ps number
SQL> exec :ps := 4001644945;

PL/SQL procedure successfully completed.

SQL> select :ps from dual;

       :PS
----------
4001644945

SQL>

SQL Developer v3.2 and v4.0 via “Run As Script(F5)”:

var ps number
exec :ps := 4001644945;
select :ps from dual;

anonymous  (more...)

SQL Developer: Viewing Trace Files

Just a quick plug for looking at raw sql trace files via SQL Developer.

There is a nice Tree View:
sqldev_trace

Which can be expanded:
sqldev_trace_expand

Also summary view of statistics, filterable:
sqldev_trace_stats

And a list view, filterable and orderable:

sqldev_trace_list

Some sort of right click summary for binds/waits might be a nice addition.


Real time sql monitoring – bulk bound statement

Moving through some observations of an ongoing investigation tackling the easiest ones first.

Previously I observed the slow execution of a recursive delete of statistics history but coming not from MMON but from a number of application processes which should have only been calculating stats for their own partitions.

Statement was sql id 9v9n97qj8z1dg:

	
delete /*+ dynamic_sampling(4) */
from sys.wri$_optstat_histhead_history
where savtime < :1
and rownum <= NVL(:2, rownum);

From the real time sql (more...)

COUNT STOPKEY – NVL

Yesterday I mentioned issues with a recursive delete operation on statistics history.

This is a quick illustration of the last points I made on that post regarding the lack of a COUNT STOPKEY optimisation because of the use of NVL.

COUNT STOPKEY is an optimisation which allows processing to stop once the target number of rows has been reached.

For example:

create table t1 as select * from dba_objects;

alter session set statistics_level = all;

 (more...)

Understanding vs Resolution – Statistics History Cleanup

Today I helped resolve a problem quickly but to the detriment of my understanding of exactly what was going on and why.

And as part of the quicker resolution, I had to destroy the evidence which would have helped understand better.

So… now need to go back and figure it out if I have time to model it properly, etc.

Here’s what little I know so far.

What happened was that there were a number (more...)

DBMS_STATS.PURGE_STATS

Prior to 11.2.0.4, DBMS_STATS.PURGE_STATS does a slow delete of stats before the parameterised input timestamp.

Why might you be purging? Here’s one such illustration:

https://jhdba.wordpress.com/tag/dbms_stats-purge_stats/

This delete can be slow if these tables are large and there are a number of reasons why they might be so, notably if MMON cannot complete the purge within its permitted timeframe.

But note that if you’re happy to purge all history, there (more...)

Call vs Exec

Just a reference to a really simple difference between CALL and EXEC.
I thought I had mentioned this before but couldn’t find it so…

EXEC/EXECUTE is a SQL*Plus command which wraps the proc call in an anonymous BEGIN … END; block.

CALL is a SQL command hence it is limited to SQL data types and there are other restrictions which the documentation sums up pretty well.

Because CALL is SQL, there is on key behavioural (more...)

New Version Of XPLAN_ASH Utility – In-Memory Support

A new version 4.21 of the XPLAN_ASH utility is available for download. I publish this version because it will be used in the recent video tutorials explaining the Active Session History functionality of the script.

As usual the latest version can be downloaded here.

This is mainly a maintenance release that fixes some incompatibilities of the 4.2 version with less recent versions (10.2 and 11.2.0.1).

As an extra however, (more...)

Primary on FileSystem and Standby on ASM

For one of the client, standby server went down. We had another standby server which was kept down for more than a month. Decision was taken to start the server and apply incremental SCN based backup on the standby database.

The standby was on ASM and the Primary on filesystem.Incremental backup was started from the SCN reported by below query

select min(fhscn) from x$kcvfh;

Once the backup completed, it was transferred to standby, standby (more...)

Video Tutorial: XPLAN_ASH Active Session History – Introduction

I finally got around preparing another part of the XPLAN_ASH video tutorial.

This part is about the main funcationality of XPLAN_ASH: SQL statement execution analysis using Active Session History and Real-Time SQL Monitoring.

In this video tutorial I'll explain what the output of XPLAN_ASH is supposed to mean when using the Active Session History functionality of the script. Before diving into the details of the script output using sample reports I provide some overview and (more...)

"SELECT * FROM TABLE" Runs Out Of TEMP Space

Now that I've shown in the previous post in general that sometimes Parallel Execution plans might end up with unnecessary BUFFER SORT operations, let's have a look what particular side effects are possible due to this.

What would you say if someone tells you that (s)he just did a simple, straightforward "SELECT * FROM TABLE" that took several minutes to execute without returning, only to then error out with "ORA-01652 unable to extend temp segment", (more...)