Onine DDL operations are much more polite than offline DDL operations. They usually wait patiently for transactions that hold resources they need until these transactions end, and they do not block new DML statements.
As I wrote in the past, adding a constraint as Enabled and Validated (which is the default for new constrtaints) is an offline operation, but if we split it into two DDL statements – one for adding the constraint as (more...)

ODC Appreciation Day : Pattern Matching in SQL

Here’s my contribution to the ODC Appreciation Day.

Pattern Matching in SQL, using the MATCH_RECOGNIZE clause, is one of my favorite features, but only recently I’ve used it “for real”.
MATCH_RECOGNIZE allows us to perform enhanced analysis of row sequences, and to detect sequences that match complex patterns.
This feature gave a significant boost to the analytical capabilities of SQL. It enables solving various types of problems in a simpler way than before, in much (more...)

Index Hints and Distributed Queries

There are two ways to specify indexes in optimizer hints: by the index name, or by the index’s (leading) columns:

The latter is usually preferred, as the writer’s intention is clearer, and it’s immune to changes of the index name.

Note: one can present the opposite argument, that specifying the index name is immune to changes of column names

Recently I’ve used the option that I prefer – specifying the column names and not the (more...)

Dropping Virtual Columns Causes Unnecessary Invalidation

When you drop a column from a table (or set a column unused), any view that references this column becomes invalid.
As of Oracle 11gR1, where Fine Grained Dependency Tracking was introduced, views that reference other columns of the table, but do not reference the dropped column, should not become invalid. And this is usually true, but this week I discovered a case where it’s not.

When dropping a virtual column, even views that (more...)

EBR – Part 10: Data Dictionary Views for Editioning Views

This is part 10 of a post series about Oracle Edition-Based Redefinition.

Visit the index page for all the parts of the series


In a previous post I wrote about Editioning Views and their significant role in online application upgrades using EBR.
In this post we’ll see how editioning views are represented in the data dictionary views.

I’ll discuss only the USER_* views, but everything is true for the corresponding ALL_*, DBA_* and CDB_* (more...)

Wrong Results with IOT, Added Column and Secondary Index

I found a “wrong results” bug yesterday, easily reproduced in 11g, 12c and 18c.

In short, we may get wrong results under the following circumstances:

  • We have an Index-Organized Table (IOT) with multi-column primary key, populated with rows
  • The table has a secondary index on part of the primary key columns
  • We add another column to the existing IOT
  • We select from the IOT while accessing it via the secondary index

Following is a simple (more...)

EBR – Part 9: Adding a New Column

This is part 9 of a post series about Oracle Edition-Based Redefinition.

Visit the index page for all the parts of the series


In part 7 (“Editioning Views”) I introduced our third development use case – adding a new column that represents a new business logic.

I emphasize the fact it’s a new business logic, because sometimes we add new columns that come instead of existing columns, for replacing an existing business logic. (more...)

EBR – Part 8: The Last Planned Downtime

This is part 8 of a post series about Oracle Edition-Based Redefinition.

Visit the index page for all the parts of the series


I concluded the previous post by stating that the application code should never reference tables directly; instead, every table should be covered by an editioning view and the application code should reference the views.

Starting from Scratch?

If you start developing a new system from scratch, I strongly recommend to follow (more...)

EBR – Part 7: Editioning Views

This is part 7 of a post series about EBR.

Visit the index page for all the parts of the series


Our next use case is adding a column that represents a new logic to the PEOPLE table, and making the corresponding changes in the PEOPLE_DL and APP_MGR packages. Of course, as we speak about EBR, the upgrade from the previous version to the new one should be online.
An online upgrade means that (more...)

The Performance of the FIRST and LAST Functions


One of the first posts I wrote in this blog (almost five years ago) was about the FIRST and LAST aggregate functions.
These functions are, in a way, extended versions of the much more popular aggregate functions MIN and MAX.
MIN and MAX allow you (conceptually) to sort a group of rows by some column and return the value of that column from the first or last row in the sorted group.
The FIRST (more...)