Unusable unique constraint

Another annoying thing about unusable indexes

I’m surprised that I can’t remember coming across this before before.

I want to archive some data from a bunch of partitions.

Unfortunately, I can’t follow the virtuous circle of data lifecycling and partition the table in such a way that I can archive all the data from a particular partition (or subpartition) via exchange partition.

Without going into too much detail and skipping a bunch of other steps, (more...)

Remote Surprise

Here is an example of surprising behaviour from a remote DB from an OTN forum thread

Setup a link to a remote DB (I’ve used an actual remote DB and not tested a loopback)

Remote DB:

create table t1
(col1  varchar2(1));

Local DB:

create or replace view v1 
as 
select count(*) c1 from t1@l1;

Then alternate variations on this sequence of events:

1. On local DB execute SELECT:

SELECT * FROM v1;

2. On remote (more...)

An example of where the optimizer should ignore a hint

Here’s an example of where the optimizer really should ignore a hint.

I’m testing an implementation of deliberately unusable index partitions – i.e. some empty index partitions, some usable.

This is 11.2.0.3 so whilst there are enhancements in 12c, they’re no use here.

SQL> create table t1
  2  (col1 number)
  3  partition by list(col1)
  4  (partition p0 values(0),
  5   partition pdef values(default));

Table created.

SQL> create index i1 on t1 (col1)  (more...)

Oracle 12c VirtualBox VM

Just a quick note to highlight that the Oracle pre-built Developer VMs have been updated for Oracle 12c.

Oracle pre-built Developer VMs


AWR: Was a baselined plan used?

Sometimes a simple question turns out to be harder than expected.

“Can we see if a particular SQL execution in AWR used a baselined plan?”

Initial thoughts might be:

Q: Does DBMS_XPLAN.DISPLAY_AWR tell us this?
A: Apparently not. See below. This question could also be rephrased as two other possible questions:

Q:Isn’t there a column on DBA_HIST_SQLSTAT which tell us this?
A: No. You’d think there should be. There is a SQL_PROFILE column. (more...)

SQL utils using XML

You may have previously seen a short post I did on a SQL statement to identify which statements are using dynamic sampling.

If not, quick recap:

SELECT p.sql_id, t.val
FROM   v$sql_plan p
,      xmltable('for $i in /other_xml/info
                 where $i/@type eq "dynamic_sampling"
                 return $i'
                passing xmltype(p.other_xml)
                columns attr  (more...)

Failed Logins can cause row cache lock on dc_users

One “feature” to be aware of in old-fashioned Oracle 11g – took me a while to figure it out, so just in case anyone else gets hit by it…

I changed the password of a schema in a test database to stop anything connecting whilst that schema was being logically (more...)

ORs, IN lists and LNNVL

I’ve previously written about manually rewriting an OR condition into a UNION ALL using LNNVL.

This is a description of a performance issue observed in the real world from the optimizer coming up with a CONCATENATION operation against many child operations including an INLIST operator and other children which then (more...)

Which of my sql statements are using dynamic sampling?

Uncategorized
| Feb 8, 2013

From a reply I gave on an OTN forum thread, how to see all queries currently in memory which use dynamic sampling?

Dynamic sampling is an attribute in V$SQL_PLAN.OTHER_XML.

In 11gR1, it says “yes”.
In 11gR2, it gives the level.

Using EXTRACTVALUE which is deprecated in 11.2:

select p.sql_id,  extractvalue(h.column_value,'/info') lvl
from   v$sql_plan p
,      table(xmlsequence(extract(xmltype(p.other_xml),'/other_xml/info'))) h
where  p.other_xml is not null
and    extractvalue(h.column_value,'/info/@type') = 'dynamic_sampling';

Another approach using one of the prefered alternatives – XMLTABLE:

select p.sql_id, t.val
from   v$sql_plan p
,      xmltable('for $i in /other_xml/info
                 where $i/@type eq  (more...)

Fun with Distributed Transactions II

Uncategorized
| Jan 23, 2013

Short on examples, long on words…

Last week I did a quick post about a couple of the more obscure implications of using distributed transactions, in particular:

  • The current impact on subquery materialisations
  • The incompatibility with MVs
  • The unusual situation of transactions with locks but no sessions

Anyway, earlier today I was having a closer look at some “DFS lock handle” wait events in an 11gR1 RAC database (not because there was an obvious problem but because I saw a few of them an wanted to have a closer look).

On a DFS lock handle wait, you have to (more...)