A look into into Oracle redo, part 4: the log writer null write

This is the fourth blogpost on a series of blogposts about how the Oracle database handles redo. The previous blogpost talked about the work cycle of the log writer: https://fritshoogland.wordpress.com/2018/02/12/a-look-into-oracle-redo-part-3-the-log-writer-work-cycle-overview/. This posts is looking into the execution of the kcrfw_redo_write_driver function executed in the ksbcti.

Now that we are familiar with how the logwriter works in general, we need to take a closer look to the kcrfw_redo_write_driver function. First let me be clear: (more...)

A look into Oracle redo, part 3: log writer work cycle overview

This is the third part of a series of blogposts on how the Oracle database handles redo. The previous part talked about the memory area that stores redo strand information: https://fritshoogland.wordpress.com/2018/02/05/a-look-into-oracle-redo-part-2-the-discovery-of-the-kcrfa-structure/.

The single most important process in the Oracle database for handling redo is the log writer, which primary task is flushing the redo information other Oracle database processes put in the public redo strands to disk. Now that we have investigated the (more...)

Oracle Exadata Smart Flash Logging

What is Exadata Smart Flash Logging?

In an OLTP environment, it is crucial to have fast response times to redo log writes i.e. low latency.  When multiplexing redo logs for high availability i.e. to protect against hardware failure, redo log writes are only acknowledge when redo is written to all redo log members i.e when the slowest disk completes the write.  By this nature, whenever a disk slows down even (more...)

A look into Oracle redo, part 2: the discovery of the KCRFA structure

This is the second post in a series of blogposts on Oracle database redo internals. If you landed on this blogpost without having read the first blogpost, here is a link to the first blogpost: https://fritshoogland.wordpress.com/2018/01/29/a-look-into-oracle-redo-part-1-redo-allocation-latches/ The first blogpost contains all the versions used and a synopsis on what the purpose of this series of blogposts is.

In the first part, I showed how the principal access to the public redo strands is (more...)

A look into Oracle redo, part 1: redo allocation latches

This will be a series of posts about Oracle database redo handling. The database in use is Oracle version 12.2.0.1, with PSU 170814 applied. The operating system version is Oracle Linux Server release 7.4. In order to look into the internals of the Oracle database, I use multiple tools; very simple ones like the X$ views and oradebug, but also advanced ones, quite specifically the intel PIN tools (https://software.intel. (more...)

How to fix queries on DBA_FREE_SPACE that are slow

I found myself in a situation where OpsView a monitoring tool, was having difficulty monitoring the tablespaces for a particular pluggable database.

Upon investigation it was found the queries against the dictionary table DBA_FREE_SPACE were taking a very long time:

SQL> set timing on
SQL> select nvl(sum(dfs.bytes)/1024/1024,0) from dba_free_space dfs where dfs.tablespace_name = 'USERS';

NVL(SUM(DFS.BYTES)/1024/1024,0)
-------------------------------
 70.75

Elapsed: 00:00:10.98

There are 60 tablespaces in this pluggable database, which the time varied (more...)

How to Enable Exadata Write-Back Flash Cache

Please check the following blog post “How to check if Exadata Write-Back Flash Cache is Enabled” for:

  • What is Exadata Write-Back Flash Cache?
  • What are the Performance Benefit of Exadata Write-Back Flash Cache?
  • How to check if Exadata Write-Back Flash Cache is Enabled?
  • Pre-requisites and minimum versions.

You can also get more info from My Oracle Support (MOS) note:
Exadata Write-Back Flash Cache – FAQ (Doc ID 1500257.1)
OTN Article: Oracle Exadata Database (more...)

How to check if Exadata Write-Back Flash Cache is Enabled

What is Exadata Write-Back Flash Cache?

Exadata Write-Back Flash Cache provide the ability to cache not only read I/Os but write I/O to the Exadata’s PCI flash on the storage cells.  Exadata storage software 11.2.3.2.1 or higher and Grid Infrastructure and Database software 11.2.0.3.9 or higher is required to use Exadata Write-Back Flash Cache, which is persistent across storage cell restarts.

The default since April 2017 for (more...)

Adapting and adopting SQL Plan Management (SPM)

Introduction

This post is about: “Adapting and adopting SQL Plan Management (SPM) to achieve execution plan stability for sub-second queries on a high-rate OLTP mission-critical application”. In our case, such an application is implemented on top of several Oracle 12c multi tenant databases, where a consistent average execution time is more valuable than flexible execution plans. We successfully achieved plan stability implementing a simple algorithm using PL/SQL calling DBMS_SPM public APIs.

Chart below depicts a typical (more...)

Order of Predicate Execution #2

In the previous post I talked about the order of predicate execution based on the predicate position and inline view. As promised, in this post I’ll add statistics and see what happens. Creating extended statistics In this demo I’m using functions, so Oracle doesn’t really know what the output of the function is. That’s why … Continue reading Order of Predicate Execution #2

Friday Fun SQL Lesson – union all

Our office kitchen is unavailable this Friday, so the call was put out for pub lunch.

After a couple of replies I decided to enter my reply, in full nerd mode.
select * from people_coming_to_lunch;

People
--------
Kate
Scott
Karolina

3 rows selected.
And of course one of the other SQL geeks (name redacted) replied to extend the data set.
select * from people_coming_to_lunch
union
select 'Shanequa'
from dual;
And I couldn't help myself. I (more...)

Adaptive Query Performance Fixes for #DB12c Release 1

We all know adaptive query feature introduced in Oracle Database Release 1 is a problem. Recently I had the opportunity to review few 12cR1 databases and found that to alleviate the performance problems, the DBA set parameter OPTIMIZER_FEATURES_ENABLE=11.2.0.4 in Oracle Database 12c Release 1 (12.1.0.2). This observation is the catalyst for the blog “Adaptive Query […]

Creating a SQL Plan Baseline from Cursor Cache or AWR

A DBA deals with performance issues often, and having a SQL suddenly performing poorly is common. What do we do? We proceed to “pin” an execution plan, then investigate root cause (the latter is true if time to next fire permits).

DBMS_SPM provides some APIs to create a SQL Plan Baseline (SPB) from the Cursor Cache, or from a SQL Tuning Set (STS), but not from the Automatic Workload Repository (AWR). For the latter, you (more...)

Order of Predicate Execution #1

In my previous post, I wrote about the parsing operation and what happens first. In the footnote I said that the order doesn’t affect performance, the cost based optimizer doesn’t care about the order of stuff in the query, right? Well, not quite. Preparing the environment In order to check that, I’ll create a table … Continue reading Order of Predicate Execution #1

Purging a cursor in Oracle – revisited

A few years ago I created a post about “how to flush a cursor out the shared pool“, using DBMS_SHARED_POOL.PURGE. For the most part, this method has helped me to get rid of an entire parent cursor and all child cursors for a given SQL, but more often than not I have found than on 12c this method may not work, leaving active a set of cursors I want to flush.

Script below (more...)

Baselines – session creating privs v session running privs

A colleague Richard Wilkinson was telling me about an issue he had come across with baselines and I asked him to write it up as it was an interesting experience.

The following MERGE SQL runs once a day. It always uses the same plan and roughly takes between 30 and 80 minutes:

SQL_ID/ PHV 6zs5dk6t6pkfs / 3064471754

 MERGE INTO DWB_X_PLNGRM_ATTR_DAY t
 USING (
 SELECT ilv.bsns_unit_cd,
 ilv.sku_item_nbr,
 ilv.cntnt_cd ,
 ilv.sel_units_cntnt_status,
 ilv.plngrm_item_grp_cd,
  (more...)

#GoldenGate Microservices (5 of 5) … Performance Metrics Service

This is post 5 of a 5 part post related to Oracle GoldenGate 12.3 Microservices and the final one on the HTML5 web page access. This series of posts will provide some details over the new graphical user interface (GUI) that has been built into Oracle GoldenGate 12.3.

Performance Metric Service is the new performance monitoring service that comes built into Oracle GoldenGate 12.3 Microservices. This is a huge addition to the (more...)

ADF Performance on Docker – Lighting Fast

ADF performance depends on server processing power. Sometimes ADF is blamed for poor performance, but in most of the cases real issue is related to poor server hardware, bad programming style or slow response from DB. Goal of this post is to show how fast ADF request could execute and give away couple of suggestions how to minimize ADF request time. This would apply to ADF application running on any environment, not only Docker. I'm (more...)

ADF Performance Story – This Time Developer Was Wrong

ADF is fast. If ADF application is slow, most likely this is related to development mistakes. I would like to tell you one story, based on my ADF tuning experience. Problem description: ADF application runs fast in DEV, when DB size is small. Same application runs slow in TEST/PROD, when DB size is large. Question - what is slow. Answer - slow means forms are loading slow. Ok, lets go to the story.

Developer decides (more...)

Oracle 12.1 big table caching IO code path

Recently I was triggered about the ‘automatic big table caching’ feature introduced in Oracle version 12.1.0.2 with Roger Macnicol’s blogpost about Oracle database IO and caching or not caching (https://blogs.oracle.com/smartscan-deep-dive/when-bloggers-get-it-wrong-part-1 https://blogs.oracle.com/smartscan-deep-dive/when-bloggers-get-it-wrong-part-2). If you want to read something about the feature in general, search for the feature name, you’ll find several blogposts about it.

If you are not familiar with automatic big table caching, it’s a feature (more...)