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How many times does my table function execute?

A left correlation join occurs when you pass as an argument to your table function a column value from a table or view referenced to the left in the table clause. This technique is used with XMLTABLE and JSON_TABLE built-in functions, but also applies to your own table functions.

Here's the thing to remember:
The table function will be called for each row in the table/view that is providing the column to the function.  Clearly, this could cause some performance issues, so be sure that is what you want and need to do.

The following code demonstrates this behavior, for both pipelined and non-pipelined functions.

CREATE TABLE things ( thing_id NUMBER, thing_name VARCHAR2 (100) ) / BEGIN INSERT INTO things VALUES (1, 'Thing 1'); INSERT INTO things VALUES (2, 'Thing 2'); COMMIT; END; / CREATE OR REPLACE TYPE numbers_t IS TABLE OF NUMBER / CREATE OR REPLACE FUNCTION more_numbers (id_in IN NUMBER) RETURN numbers_t IS l_numbers numbers_t := …

The SmartDB Resource Center

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I put together this blog post for those interested in learning more about the SmartDB (also or formerly known as "ThickDB") architecture and how to apply it in your applications. I will update it as more resources become available.

What is SmartDB?

Bryn Llewellyn, PL/SQL Product Manager, offers this description:

Large software systems must be built from modules. A module hides its implementation behind an interface that exposes its functionality. This is computer science’s most famous principle. For applications that use an Oracle Database, the database is, of course, one of the modules. The implementation details are the tables and the SQL statements that manipulate them. These are hidden behind a PL/SQL interface.

This is the Smart Database paradigm: select, insert, update, delete, merge, commit, and rollback are issued only from database PL/SQL. Developers and end-users of applications built this way are happy with their correctness, maintainability, security, and performa…

Mutating table errors and multi-row inserts

The Oracle Dev GymPL/SQL Challenge quiz played 28 Apr - 4 May explored the interactions between row-level triggers and multi-row inserts, particularly when it comes to mutating table errors. If you didn't happen to take the quiz and already learn its lesson, here goes.

[Note: you can also click on the link above and play the quiz right now, before you read this post!]

Here's the main rule to keep in mind:
A BEFORE INSERT trigger will not cause a mutating table error as long as the triggering INSERT statement is a single row insert (INSERT-VALUES). Let's take a closer look.

I create a table and a trigger on that table:

CREATE TABLE qz_flowers ( fl_num NUMBER, fl_name VARCHAR2 (30) ) / CREATE OR REPLACE TRIGGER qz_flowers_bir BEFORE INSERT ON qz_flowers FOR EACH ROW DECLARE l_count INTEGER; BEGIN SELECT COUNT (*) INTO l_count FROM qz_flowers; DBMS_OUTPUT.PUT_LINE ('Count = ' || l_count); END; /
The trigger queries from the qz_flowers table, …

Error stack function now (12.2) includes backtrace information!

The DBMS_UTILITY has long (since 10.2) offered three functions that are very handy when either tracing execution or logging errors:
FORMAT_CALL_STACK - answering the question "How did I get here?"FORMAT_ERROR_STACK - answering the question "What was the error?" (or a stack of errors, depending on the situation)FORMAT_ERROR_BACKTRACE - answering the question "On what line was my error raised?" Therefore (and prior to 12.2), if you wanted to get the error information + the line number on which the error was raised, you would need to call both of the "*ERROR*" as in:

CREATE OR REPLACE PROCEDURE p3 AUTHID DEFINER IS BEGIN p2; EXCEPTION WHEN OTHERS THEN DBMS_OUTPUT.put_line (DBMS_UTILITY.format_error_stack); DBMS_OUTPUT.put_line (DBMS_UTILITY.format_error_backtrace); RAISE; END;
Of course, in the real world, you would not display the text on the screen. You would write them to a log table via an autonomous transaction pro…

How do I get the attribute of my object type in SQL?

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This question found its way into my In Box yesterday:

I have a table with an object type column. I want to way to get the value of an attribute of that object type in my query. But Oracle keeps telling me "ORA-00904: invalid identifier". What am I doing wrong?

Almost certainly what you are doing wrong is forgetting to use a table alias. Yeah, it's that simple.
Don't forget the table alias. Let's take a look.

I create an object type, use that object type as a column in a table, and insert a couple of rows:

CREATE TYPE food_t AS OBJECT ( NAME VARCHAR2 (100) , food_group VARCHAR2 (100) , grown_in VARCHAR2 (100) ) / CREATE TABLE food_table (id number primary key, my_food food_t) / BEGIN INSERT INTO food_table VALUES (1, NEW food_t ('Mutter Paneer', 'Curry', 'India')); INSERT INTO food_table VALUES (2, NEW food_t ('Cantaloupe', 'Fruit', 'Backyard')); COMMIT; END; /
OK, let&#…

Oracle Dev Gym gets a facelift - and more!

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Over the weekend of April 21, we upgraded the Oracle Dev Gym site to v3 (code name: ORANGE). Here's the v2 home page:


and now v3:

Now you see the reason for the code name. It's orange!

Here are the key changes you will find on the Dev Gym:
Orange theme: all that red was hurting our eyes, but the main reason to switch to orange was to make it visually clear that this site, as with AskTOM, is part of the broader Oracle Developer initiative.Site search: type in a keyword, such as "FORALL" or "listagg" in the search bar on the home page, and we will find all quizzes, workouts and classes that match your criteria. You can further hone your search on the results page.The tournament quizzes are now offered on the home page; no need to click on the Tournaments tab to see them. These quizzes are produced fresh each week, and often focus on the latest features in SQL, PL/SQL and Oracle Database.Your recent activity on the site is available on the home page so that you…

Tips for a great presentation

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There's no shortage of people giving advice on how to improve your presentation skills and impact. I offer a short list of links at the bottom of this post. 

I though I'd take a few moments to share some tips I follow to help me make the most of my time in front of audiences.

Why listen to me? I've been doing talks on the PL/SQL language since 1992 and I am pretty sure that only 3 members of all those audiences ever fell asleep during my talk.

What are the (at most) three key takeaways?

Most attendees will forget most of what you said soon after leaving the session. Certainly almost every single technical detail will be lost. So you need to decide before you start your talk what  are the at most three things you want an attendee to remember.

Then put those in a slide and tell them right at a start.

Remind them during your talk when you are getting to one of those top 3 things.

Use the slide again at the end of your talk to drive the points home.

I also find it helpful to remi…

Nested blocks, autonomous transactions and "Where do I commit?"

This question rolled into my In Box today:
If I have a procedure that is AUTONOMOUS_TRANSACTION that does an insert and then it calls a procedure with an insert, does the second procedure need a commit, or will the procedure with the AUTONOMOUS_TRANSACTION handle the commit? If you don't know off the top of your head, don't worry, I can build a test. First of all, if you ever find yourself writing something like "If you don't know off the top of your head, don't worry, I can build a test." then please by all means go right ahead and build yourself a test script.

By doing so, you will better understand the feature in question and remember what you learned. Plus you end up with a script you can share with the community on LiveSQL.

But I don't mind answering such questions. That way I get to better understand the feature in question, remember what I learned, share a script on LiveSQL (link at bottom of post), and also add to my blog. :-)

So here goes: the ans…

A new name - and amazing new future - for PL/SQL

[You might think that this was published on April 2nd, but in fact it was published on April 1st.]

PL/SQL, the database programming language from Oracle, introduced in 1991 and used by millions over the years to implement data APIs and business logic in mission critical applications from which billions of humans benefit daily, is undergoing a radical transformation in order to stay relevant for, and meta-cool to, future generations of developers.

After a careful examination of all modern programming languages and the definitive StackOverflow developer surveys, the PL/SQL development team implemented a super-secret plan (yes, that’s correct, even the Distinguished Product Manager for PL/SQL, Bryn Llewellyn, is unaware of what you are about to read. So don’t bother him about it, OK?).

I am, therefore, inordinately pleased and honored to be the first to announce the following changes for PL/SQL in Oracle Database 20c:
PL/SQL will now be a case-insensitive language. Sort of.Only lower-case…

Rankings for 2017 PL/SQL Championship on the Oracle Dev Gym

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Thirty-six Oracle Database technologists competed on March 22nd in the 2017 PL/SQL Annual Championship at the Oracle Dev Gym. With five tough quizzes by yours truly, the competition was fierce! Congratulations first and foremost to our top-ranked players:

1st Place: li_bao of Russia
2nd Place: mentzel.iudith of Israel
3rd Place: NielsHecker of Germany

Next, congratulations to everyone who played in the championship. We hope you found it entertaining, challenging and educational.

Finally, our deepest gratitude to our reviewer, Elic, who has once again performed an invaluable service to our community.

In the table below of results for this championship, the number next to the player's name is the number of times that player has participated in a championship. Below that table, you will find another list showing the championship history of each of these players.

RankNameTotal Time% CorrectTotal Score1li_bao (4)27 m78%55922mentzel.iudith (4)44 m78%55203NielsHecker (4)43 m76%53744Oleks…

Qualified expressions (aka, constructor functions) for collections and records in 18c

As anyone who has followed me over the years knows, I like the Oracle PL/SQL language. Sure, it's not the newest, coolest kid on the block (it probably never was). But then, either am I. :-) PL/SQL is, on the other hand, a delightfully straightforward, easy to learn and write language that serves its purpose well: implement APIs to data (SQL) and business logic, right inside the database.

To serve that purpose, of course, PL/SQL needs to support lots of "big ticket" functionality: super-smooth and easy native dynamic SQL, canonicalization of static SQL to minimize the need for hard-parsing, invoker rights (AUTHID CURRENT_USER) and so much more.

But I must confess: the features of PL/SQL that I love the best are the relatively "little" things that make it easy for me to be productive as I churn out the packages (and, yes, I still do write lots of PL/SQL code, most lately for the Oracle Dev Gym, an "active learning" website featuring quizzes, workouts a…

Mining Application Express data dictionary views: find unconditional processes

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I ran into a problem yesterday on the Oracle Dev Gym (offering quizzes, workouts and classes on Oracle technologies). A number of rows of data were incorrectly deleted. I was able to use Flashback Query to restore them (thank you, thank you, Flashback Query!). Crisis averted. 
But how did this come about?

I recruited Chris Saxon to help me figure out how this could have happened. In relatively short order, we narrowed down the culprit to a process in the Dev Gym Application Express definition that was unconditionally "removing previews" - but was in fact removing all rows, "previews" or not. Ugh.

So we fixed that.

But it got me wondering and worrying: what other processes in my app are unconditional? And should they be?

While some processes fire unconditionally on a page (for example, to get the data from tables and display them on the screen), many are (or should be!) restricted to a button press, the result of a conditional expression, or an authorization scheme…

An appreciation of UI developers from a database developer

From what I can tell, JavaScript developers write much more complicated code, to handle much more challenging requirements, than I do, with my SQL and PL/SQL programming in the Oracle Database.

I am quite certain that JavaScript developers feel this sentiment even more strongly. People like me (veterans of multiple decades of database-centric application development) are seen as dinosaurs, using ancient, uncool technologies.

The things we do seem distant, unimportant, even simplistic, compared to the tough stuff they deal with on a daily basis: asynchronous! streams! containers! microservices! promises!....and so forth.

Heck, I haven't even had to change my "framework" for years and years! :-)

But here's something we should all of us keep in mind:
Sometimes relatively simple tasks can also be critical tasks,
in which case simplicity becomes a significant advantage (a feature, not a bug).
Now, don't get me wrong. I am not saying that I think the task of UI develope…

Don't test PL/SQL features with trivial code

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On the one hand, when you test something, you want to keep your test code as simple as possible so that you can focus on the issue you are testing.

On the other hand, if you make your code too simple you might find yourself baffled at the resulting behavior.

Why? Because the PL/SQL compiler is just too darned smart.

Today, I got a DM on Twitter asking me why the package body below was compiling without any errors, even though he specified that the PLW-06009 warning should be treated as a compile error.

The code:

ALTER SESSION SET plsql_warnings = 'Error:6009'; CREATE OR REPLACE PACKAGE pkg_test AS PROCEDURE test_job (p_test_parameter IN OUT VARCHAR2); END pkg_test; / CREATE OR REPLACE PACKAGE BODY pkg_test AS PROCEDURE test_job (p_test_parameter IN OUT VARCHAR2) IS BEGIN NULL; EXCEPTION WHEN OTHERS THEN NULL; END test_job; END pkg_test; /
Certainly seems like that exception handler allows the OTHERS handler to exit test_job without executing…

Using JSON_TABLE to move JSON data to a relational table

We are using Zoom to host the webcasts for our AskTOM Office Hours program. We schedule the meetings automatically, using their API. We can then also retrieve the meeting information as JSON documents through that same API.

Blaine Carter, the Developer Advocate who did all the heavy lifting around the Zoom API, suggested we take a daily snapshot of all our meetings, so that in case anything goes wrong, we can check back in time, grab the meeting ID, and still get that session going. Great idea!

He also suggested that I use JSON_TABLE to get the job done. Another great idea!

JSON_TABLE, introduced in 12.2, "enables the creation of an inline relational view of JSON content. The JSON_TABLE operator uses a set of JSON path expressions to map content from a JSON document into columns in the view. Once the contents of the JSON document have been exposed as columns, all of the power of SQL can be brought to bear on the content of JSON document." (quoting product manager Mark Drake fr…