11gR2

Chris Antognini's picture

Foreign Keys and Library Cache Locks

In this post I would like to describe a behavior of Oracle Database that, at least for me, isn’t obvious at all. Actually, it’s something that I can’t explain why it works in that way.

Let’s start by setting the scene by describing the schema I’m using for the following tests. As you can see from the image, there are three tables: one table (PARENT) that is referenced by two other tables (CHILD1 and CHILD2). Schema used for the tests In my case every table is owned by a different schema (P, C1 and C2 respectively). But, the behavior I describe is independent from that fact (i.e. it works in the same way if all tables are owned by the same schema). If you are interested, here is the SQL*Plus script I used to create them.

oraclebase's picture

Fedora 22/23 and Oracle 11gR2/12cR1

linux-tuxAs always, installations of Oracle server products on Fedora are not a great idea, as explained here.

I was reading some stuff about the Fedora 23 Alpha and realised Fedora 22 had passed me by. Not sure how I missed that. :)

Anyway, I did a run through of the usual play stuff.

randolf.geist's picture

Parallel Projection

A recent case at a client reminded me of something that isn't really new but not so well known - Oracle by default performs evaluation at the latest possible point in the execution plan.So if you happen to have expressions in the projection of a simple SQL statement that runs parallel it might be counter-intuitive that by default Oracle won't evaluate the projection in the Parallel Slaves but in the Query Coordinator - even if it was technically possible - because the latest possible point is the SELECT operation with the ID = 0 of the plan, which is always performed by the Query Coordinator.Of course, if you make use of expressions that can't be evaluated in parallel or aren't implemented for parallel evaluation, then there is no other choice than doing this in the Query Coordinator.The specific case in question was a generic expo

randolf.geist's picture

Temp Table Transformation Cardinality Estimates - 2

Continuing from the previous part - which was about the Temp Table Transformation and join cardinality estimates - using the same simple table setup here is a slight variation of the previously used query to demonstrate the potential impact on single table cardinality estimates:


explain plan for
with
cte as (
select /* inline */ id from t1 t
where 1 = 1
)
select /*+
no_merge(a) no_merge(b)
*/ * from cte a, cte b
where a.id = b.id
and a.id > 990 and b.id > 990
;

-- 11.2.0.x Plan with TEMP transformation
--------------------------------------------------------------------------------
randolf.geist's picture

Temp Table Transformation Cardinality Estimates - 1

Having published recently two notes about the Temp Table Transformation highlighting the heuristics based decision and other weaknesses, for example regarding the projection of columns, it's time to publish some more notes about it.The transformation can also have significant impact on cardinality estimates, both join and single table cardinality.Looking at the difference in the join cardinality estimates of following simple example:


create table t1
as
select
rownum as id
, mod(rownum, 10) + 1 as id2
, rpad('x', 100) as filler
from
dual
connect by
level <= 1000
;
randolf.geist's picture

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) as large_vc3
from
dual
connect by
level <= 1000
;

exec dbms_stats.gather_table_stats(null, 'a')
randolf.geist's picture

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 schema using ALTER USER ... ENABLE EDITIONS and had to use the "FORCE" option since there were (according to the official ALTER USER documentation) "objects that are not editionable and that depend on editionable type objects in the schema. ...

randolf.geist's picture

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.

randolf.geist's picture

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
level <= 1e4
;

exec dbms_stats.gather_table_stats(null, 't1')

exec dbms_stats.gather_table_stats(null, 't2')

exec dbms_stats.gather_table_stats(null, 't3')
randolf.geist's picture

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, this version now differentiates between general CPU usage and in-memory CPU usage (similar to 12.1.0.2 Real-Time SQL Monitoring). This is not done in all possible sections of the output yet, but the most important ones are already covered.

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