Conditional uniqueness

A quick fly through the options for conditional uniqueness.

Requirement #1: I want uniqueness on a column but only under certain conditions.

For example, I have an active flag and I want to make sure there is only one active record for a particular attribute but there can be many inactive rows.

Initial setup:

create table t1
(col1      number       not null
,col2      varchar2(24) not null
,is_active number(1)    not null
,constraint pk_t1 primary key (col1)
,constraint  (more...)

Audit space threat – a perfect storm

According to http://en.wikipedia.org/wiki/Perfect_storm:

A “perfect storm” is an expression that describes an event where a rare combination of circumstances will aggravate a situation drastically.

The other day we had a bit of a panic when AUD$ space grew by 45GB in 15 minutes and set off some alerts.

The audit_trail parameter was set to DB, EXTENDED.

which means:

Behaves the same as AUDIT_TRAIL=DB, but also populates the SQL bind and SQL text (more...)

Oracle Support – Grrr

Is this an appropriate response?

Others have raised this before – in fact I think I’ve raised the question before – but my initial search on Oracle Support did not turn up an answer so I thought I would raise a question again.

Problem Summary
—————————————————
Question only: Null predicates in DBA_HIST_SQL_PLAN

Problem Description
—————————————————
Everyone knows that the FILTER_PREDICATES and ACCESS_PREDICATES columns in DBA_HIST_SQL_PLAN are not populated.
If I remember correctly, this was originally (more...)

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...)