A common sight in databases is a table that contains the start and end values of a range - usually dates. One or both of the dates are typically part of the primary key; sometimes they are the entire key. Some examples:
- History tables
History tables record the changes made to a row over time. Instead of updating a row and losing the prior values of each non-key attribute, a new row in inserted. Each row is differentiated by two columns that record the date the row was created (
START_DATE) and the date it was superceded (
One glance at my golf clubs would be enough to determine that I'm a terrible golfer. The pitching wedge is dirty. Nine-iron: dirty. Same with the eight, seven and six irons. Five, four and three irons are fairly clean. Woods: pristine. I play percentage golf (actually 110%, if you count penalties); I figure a 5-iron 150 meters down the fairway is a better bet than a 3-wood 200 meters into the trees.
So I've got a golf bag with 2 clubs that I paid for but never use. Madness? Well no, not really; but then I'm not paid to play golf. Can you imagine a professional golfer never using the driver? It wouldn't happen.
Can you picture an Oracle programmer never using the most powerful join method available? No? Get a mirror.
Never complain and never explain.
Disraeli was a lot of things, but Oracle Programmer was not amongst them. To be fair, perhaps he wasn't talking about Explain Plan?
SQL is a goal-oriented language. Unlike procedural languages, we tell the database what we want rather than how to get it. Oracle's Cost Based Optimizer comes up with an execution plan that is hopefully the most efficient way to resolve the query, but for many reasons it will often choose a sub-optimal plan.
A common complaint in SQL is that "it runs in 5 seconds in SQL*Plus, but takes hours in Production. Why?"
The reason is because SQL*Plus and most GUI SQL tools display rows as soon as they are fetched. In this way, you can
SELECT * FROM big_big_table and it will display the first 20 or so rows in the table in a fraction of a second, then go back for more. The SQL is not really finishing in seconds; if you timed how long it took to retrieve every row, you'd see that it takes just as long as in Production.
I'm not really an "early-adopter" of technology. Don't get me wrong; I love it, I just don't want to feed the addiction. When I do get a new piece of technology though, it's like a fever; I can't concentrate on anything until I've read the manual from cover to cover and found out everything it can do, every built-in gizmo, and every trashy piece of after-market merchandise that can be plugged into it.
And I don't think I'm alone here.
Where were you in 1990? Nelson Mandela was being freed from Victor Verster Prison after 26 years behind bars, Saddam Hussein was starting the Gulf War by invading Kuwait, and Tim Berners-Lee was inventing the World-Wide-Web at CERN in Geneva. Me? In 1990, I was writing an insurance system in Oracle SQL*Forms v2.3.