Search

OakieTags

Who's online

There are currently 0 users and 33 guests online.

Recent comments

performance

New Version Of XPLAN_ASH Utility

A new version 4.22 of the XPLAN_ASH utility is available for download.

As usual the latest version can be downloaded here.

This version primarily addresses an issue with 12c - if the HIST mode got used to pull ASH information from AWR in 12c it turned out that Oracle forgot to add the new "DELTA_READ_MEM_BYTES" columns to DBA_HIST_ACTIVE_SESS_HISTORY - although it got officially added to V$ACTIVE_SESSION_HISTORY in 12c. So now I had to implement several additional if/then/else constructs to the script to handle this inconsistency. It's the first time that the HIST view doesn't seem to reflect all columns from the V$ view - very likely an oversight rather than by design I assume.

Oracle 12 and latches, part 3

This post is about manually calling and freeing a shared latch. Credits should go to Andrey Nikolaev, who has this covered in his presentation which was presented at UKOUG Tech 15. I am very sorry to see I did miss it.

Essentially, if you follow my Oracle 12 and shared latches part 2 blogpost, which is about shared latches, I showed how to get a shared latch:

SQL> oradebug setmypid
Statement processed.
SQL> oradebug call ksl_get_shared_latch 0x94af8768 1 0 2303 16
Function returned 1

Which works okay, but leaves a bit of a mess when freed:

[Oracle] Understanding the Oracle code instrumentation (wait interface) - A deep dive into what is really measured

Introduction

This blog post is inspired by a question from an attendee of Sigrid Keydana's DOAG 2015 conference session called "Raising the fetchsize, good or bad? Exploring memory management in Oracle JDBC 12c". Basically it was a question about what the wait event "SQL*Net more data to client" represents and what it really measures. In general you may use the following steps, if you don't know what a particular wait event means:

Oracle 12c in-memory option and IO

This article is about the Oracle 12c in-memory option, and specifically looks at how the background worker processes do IO to populate the in-memory column store.

Hardware: Apple Macbook with VMWare Fusion 7.1.3.
Operating system: Oracle Linux 6.7, kernel: 3.8.13-118.el6uek.x86_64.
Database version: Oracle 12.1.0.2
Patch: opatch lspatches
19392604;OCW PATCH SET UPDATE : 12.1.0.2.1 (19392604)
19303936;Database Patch Set Update : 12.1.0.2.1 (19303936)

But first things first, let’s setup the in-memory option first with a test table. The first thing to consider is to create the in-memory area to store the objects. I only want a single table stored in the in-memory area, so I can very simply look at the size of object:

TABLE ACCESS INMEMORY FULL – but there may be more

While preparing to teach a class this week I have had the time to look into the In Memory (cost) option a bit closer. I noticed a few interesting things along the way and would like to share one of these here.

Background

One of the questions I was asking myself was:

“What happens if I scan segments that are within the IM area, and some are not?”

I was primarily thinking of joins in a DWH environment, but in order to keep the test case simple enough and reproducible I decided to go with a partitioned table where the current partition is assigned to the IMCS, and the rest is not. For this test I’m relying on the Swingbench SH schema. All of this runs on my laptop in a VM so I had to be selective when it comes to indexes. I also chose to NOT partition the tables at this stage, I wanted to chose my own partitioning scheme. For reference, here is the command that created the SH schema:

Pragma UDF – Some Current Limitations

There are currently some limitations to when pragma UDF will speed up your calls to PL/SQL functions from SQL.

In my post introducing the new pragma UDF feature of Oracle 12c I explained how it can be used to reduce the impact of context switching when you call a PL/SQL function from SQL.

In my example I showed how running a SQL-only SELECT statement that formatted a name for display over 100,000 records took 0.03 seconds went up to 0.33 seconds when the formatting SQL was put in a user defined PL/SQL function. This impact on performance is a shame as it is so beneficial to encapsulate business logic in one single place with PL/SQL. Stating that the PL/SQL function is a user defined one with the pragma UDF option reduced the run time to 0.08 seconds – which is removing most of the context switching overhead. Check out the prior post for full details.

Little things worth knowing: automatic generation of extended statistics in 12c Part II

In my last post I demonstrated an example how to use a PL/SQL package to monitor a workload in order to identify correlated columns. Such columns are good candidates for the creation of extended statistics since the optimiser does not assume that information stored in different columns may be related. Before starting my tests I invoked DBMS_STATS.SEED_COL_USAGE, which in turn increased Oracle’s attention level trying to find correlated columns. Eventually these have been identified (as shown in the output of DBMS_STATS.REPORT_COL_USAGE) and a subsequent call to DBMS_STATS.GATHER_TABLE_STATS caused extended statistics to be created, including histograms. This is one way you can get extended statistics automatically, but it requires you to enable monitoring of a workload by invoking a PL/SQL API.

Pragma UDF – Speeding Up your PL/SQL Functions Called From SQL

A new feature for PL/SQL was introduced in V12, pragma UDF. UDF stands for User Defined Functions. It can speed up any SQL you have that uses PL/SQL functions you created yourself.

{please see this second post on some limitations of pragma UDF in respect of IN & RETURN data types and parameter defaults}.

We can create our own functions in PL/SQL and they can be called from both PL/SQL and SQL. This has been possible since V7.3 and is used extensively by some sites to extend the capabilities of the database and encapsulate business logic.

Where do my trace files go? V$DIAG_INFO

Where do oracle trace files go? I don’t know why this piece of info will not stick in my head, I seem to have to look it up 3 or 4 times a year.

If only I had an easy way to find out. There is a very easy way to find out – and that piece of info won’t stay in my head either. So this really is a blog post just for stupid, forgetful me.

V$DIAG_INFO has been available since oracle V11. All the trace files go into the Automatic Diagnostic Repository (ADR) by default.