The Hitchhiker’s Guide to the EXPLAIN PLAN Part 35: Robust v/s optimal query execution plans

A subset of all possible execution plans can be described as robust. While such solutions are not always quite optimum, they are almost always close to optimum in real-world queries, and they have desirable characteristics, such as predictability and low likelihood of errors during execution.(read more)

The Hitchhiker’s Guide to the EXPLAIN PLAN Part 34: SQL Aaargh!

The biggest problem with SQL query optimization is that semantically equivalent SQL queries are not guaranteed to perform equally well. When I encounter problems like this, I sympathize with the folks who got frustrated with RDBMS performance and created NoSQL.(read more)

Way Too Invisible Columns

Oracle Database 12c introduced "invisible columns": they are only visible when you name them explicitly in the SELECT list. Unfortunately, they seem to be even more invisible when you access them through a database link! Here are some surprising results from SELECT and MERGE statements.

The Hitchhiker’s Guide to the EXPLAIN PLAN Part 33: The mother of all SQL antipatterns?

The mother of all SQL “anti-patterns” is the ad-hoc query. Here, the end-user of the application can specify any combination of parameters—for example, the job_id, the dept_id, or a combination of both—and the data is retrieved using a catchall query.(read more)

SQL Features in unterschiedlichen RDBMS

Eine sehr interessante Präsentation hat Markus Winand offenbar gerade auf der FOSDEM-Konferenz gehalten. Dafür spricht jedenfalls der Foliensatz dazu, den man auf seiner Webseite findet. Darin zeigt er, was seit SQL-92 alles an interessanten Features für SQL verfügbar wurde und liefert vor allem auch Informationen dazu, wann diese Möglichkeiten in den verbreitetsten RDBMS eingeführt wurden. Unter den vorgestellten Features sind:
  • Lateral Join
  • With clause (aka "subquery factoring" aka "common table expression (CTE)"; in postgres wird (more...)

Call vs Exec

Just a reference to a really simple difference between CALL and EXEC.
I thought I had mentioned this before but couldn’t find it so…

EXEC/EXECUTE is a SQL*Plus command which wraps the proc call in an anonymous BEGIN … END; block.

CALL is a SQL command hence it is limited to SQL data types and there are other restrictions which the documentation sums up pretty well.

Because CALL is SQL, there is on key behavioural (more...)

The Hitchhiker’s Guide to the EXPLAIN PLAN Part 32: Pretzels created by someone experimenting with hallucinogens?

In an interview for the NoCOUG Journal, Steven Feuerstein said: “Some people can perform seeming miracles with straight SQL, but the statements can end up looking like pretzels created by someone who is experimenting with hallucinogens.” As if to prove Steven correct, NoCOUG held three international SQL challenges. The winning entries were mind-boggling indeed.(read more)

MERGE and invisible columns = invisible documentation?

Oracle 12c introduced "invisible columns" to help us add columns to tables without breaking existing applications. The documentation explains how they work with SELECT and INSERT, but not MERGE. Here's what happened when I tried MERGE.

The Hitchhiker’s Guide to the EXPLAIN PLAN Part 31: How to Avoid a Salted Banana

In the February 2015 issue of the NoCOUG Journal, Lothar Flatz introduces the term “salted banana” for a certain kind of problem in SQL tuning.(read more)

Share your interesting SQL or PL/SQL experience on stage, on camera, or in print

If you have had interesting experiences with SQL or PL/SQL in your career, here’s your opportunity to share them on stage, on camera, or in print. Send your stories to contact_nocoug@nocoug.org and we’ll forward them to the SQL evangelists team headed up by Steven Feuerstein. You may be selected to tell your stories on stage at the YesSQL! presentation at the winter conference next Tuesday or your stories may be recorded on camera or (more...)

STEW_COMPARE_SYNC: Introducing the package

After many blog posts about comparing and synchronizing tables, I have united all the techniques I presented in one place. The STEW_COMPARE_SYNC package generates SQL for
  • Comparing tables, views and queries, both local and remote.
  • Synchronizing, or applying changes to target tables from either source tables or "Change Data Capture" input.

Compare and Sync without Primary Keys

I have written a lot about comparing and synchronizing tables. My examples always had both primary keys and non-key columns, so I could do updates along with inserts and deletes. What about the other tables? Here's a technique that works for them.

How to avoid a salted banana! (SQL tuning)‏

Dear NoCOUG members and friends,

You’re going to enjoy the new issue of the NoCOUG Journal. Click here to download it.

  • Lothar Flatz talks about the biggest issue in SQL tuning: the “salted banana.” We predict that the phrase “salted banana” will become as widely known in the Oracle community as “compulsive tuning disorder.” Click here to go directly to Lothar’s article.
  • Janis Griffin—a.k.a. the Looney Tuner—explains why the Oracle (more...)

MySQL Locking

Have you ever wondered if MySQL does Row Level Locking? Or wondered why are you getting the error:

ERROR 1205 (HY000) : Lock wait timeout exceeded; try restarting transaction

You get the error because your allocated time to hold a DML lock in a transaction exceeds the set limit. Usually the default limit to hold a DML row lock, set by innodb_lock_wait_timeout db parameter, is 50 seconds. If your transaction doesn't commit/rollback within 50 seconds (more...)

The Hitchhiker’s Guide to the EXPLAIN PLAN Part 30: Throw Away That EXPLAIN PLAN! (Part IV)

We tend to overuse the tool—EXPLAIN PLAN in this case—that we are most familiar with. There is an expression—attributed to behavioral scientist Abraham Maslow—that “if all you have is a hammer, everything looks like a nail.” Maslow also tells the story of a drunk searching under a street lamp for his house key, which he had dropped some distance away. Asked why he didn’t look much where he had dropped it, the drunk replied, (more...)

Oracle 12c WITH inline PL/SQL

I've been having a bit of a play with the Oracle 12c database over the past few days and I thought I'd mention a gotcha I encountered.

Of course, oracle-base is a great place to start for clear & concise information on new features and I was trying out some of the WITH clause enhancements (a.k.a. subquery factoring clause). As a developer I'm pretty excited about these in particular.

Creating inline functions (more...)

The Hitchhiker’s Guide to the EXPLAIN PLAN Part 29: Throw Away That EXPLAIN PLAN! (Part III)

We tend to overuse the tool—EXPLAIN PLAN in this case—that we are most familiar with. There is an expression—attributed to behavioral scientist Abraham Maslow—that “if all you have is a hammer, everything looks like a nail.” Maslow also tells the story of a drunk searching under a street lamp for his house key, which he had dropped some distance away. Asked why he didn’t look much where he had dropped it, the drunk replied, (more...)

The Hitchhiker’s Guide to the EXPLAIN PLAN Part 28: Throw Away That EXPLAIN PLAN! (Part II)

We tend to overuse the tool—EXPLAIN PLAN in this case—that we are most familiar with. There is an expression—attributed to behavioral scientist Abraham Maslow—that “if all you have is a hammer, everything looks like a nail.” Maslow also tells the story of a drunk searching under a street lamp for his house key, which he had dropped some distance away. Asked why he didn’t look much where he had dropped it, the drunk replied, (more...)

Boosting APEX menu SQL performance

If you're using dynamic SQL to source your menus and you're utilising the apex_application_pages dictionary view to build you list of menu options - I suggest you consider using Materialized Views.

We have a reporting application that does just this; and the query also links to an authorisation table that decides what types of users see which reports.

You can create a snapshot (materialized view) using something as simple as
create materialized view apx_application_pages
as (more...)

Approximate Count Distinct (12.1.0.2 new feature)

With the release of the Oracle Database 12.1.0.2 there was a number of new features and options. Most of the publicity has been around the in-Memory option. But there was lots of other features for the DBA and a few for the developer.

One of the new SQL functions is the APPROX_COUNT_DISTINCT(). This function is different to the tradition count distinct, COUNT(DISTINCT expression), in that is performs an approximate count distinct. (more...)