All posts by ajohnson

Daily Links

Links: 1-17-2006

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Daily Links

Links: 1-11-2006

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ASP, Systems Administration

ASP, ServerVariables, Server_Name, Windows 2003, IIS6

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We’re in the process of upgrading our webservers to Windows 2003 and thus IIS6. One of the issues we ran into was that the Server_Name key of the ServerVariables collection in VBScript started returning the computer name of the machine rather than the host name of the website. I spent a couple hours googling and found nothing, my recent hire found the hot fix on microsoft.com in about 5 minutes today (which shows how important good google skills are). In short, there’s a security issue with IIS (surprised?) whereby sending a GET request to an IIS web server without sending a host header would result in the web server returning the IP address of the web server in the content location field of the response TCP header. The easy fix is to run:

cscript adsutil.vbs set w3svc/UseHostName false

which modifies all IIS web sites to return the website host name rather than the machine name / IP address when getting the Server_Name key of the ServerVariables collection.

Personal, Software Development

CIS 552: Database Design

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I took the final exam today in my CIS 552 Database Design course. I think I did ok on the exam but more importantly, I learned a lot this semester. I think the main takeway from the course is that it helps to realize that a database application (meaning an application like MySQL, Oracle or SQL Server, not a database instance like Northwind) is more than just SQL, in fact the bottom level of a database management system stack is “just” reading and writing files to the file system, albeit with transactions and atomicity and multiple users and well, a bunch of other stuff but bear with me. See, when you first start out, you do things like SELECT * FROM table and then you read somewhere how inefficient it is to bring back all the records from your table and so you use SELECT id FROM table. And then pretty soon you hit a wall where you have a query that takes like 5 seconds to run and you learn about indexes. Well, if you’re like me, you just learn that indexes make things go faster… you might read SQL Server books online and see that an index in SQL Server is a B-tree, but you don’t go any further than that and so you just come away knowing that indexes make your application run fastertm. After that, you read find someone somewhere that says that the order of your selection criteria matters and that you should try and give hints to the query optimizer when in fact, the query optimizer can do whatever the hell it wants with your declarative SQL statement and probably will.

And then you take a class like this and you learn how a database management system stores records in pages and you get to write some code that stores and retrieves bytes to and from said pages, all the while paying attention to the page header which stored (in our case) the page offset, the number of records in the page and a record size / pointer data structure, all of which is just part of a big file. After that you write code that looks up records in an index, which turns out to be a file as well. Finally, you use a block nested loop to process a join query and it turns out that again, you’re just reading bytes from a file system.

So now I understand why the guys at viaweb didn’t use a database (although I’d never go there myself) and I can have a somewhat intelligent conversation about the differences between a linear search, a binary search, an index scan, a B-tree index seek, and a hash index lookup. And oh man do I wish I was a math major in college.

Other cool stuff I saw along the way:

· On the Goodness of Binary Search by Tim Bray

· General Purpose Hash Function Algorithms

· Binary logarithm

· Google SparseHash a project that contains several hash-map implementations in use at Google, with different performance characteristics, including an implementation that optimizes for space and one that optimizes for speed.

· The Anatomy of a Large-Scale Hypertextual Web Search Engine: “Google’s data structures are optimized so that a large document collection can be crawled, indexed, and searched with little cost. Although, CPUs and bulk input output rates have improved dramatically over the years, a disk seek still requires about 10 ms to complete. Google is designed to avoid disk seeks whenever possible, and this has had a considerable influence on the design of the data structures.” (emphasis mine)

· Lucene architecture

· Quicksort

Software Development

SQL Server Group By Datetime

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If you’ve ever used SQL Server, you know that there’s no such thing as a Date or Time, only Datetime. According to SQL Server Books Online: “If only a time is specified when setting a datetime or smalldatetime value, the date defaults to January 1, 1900. If only a date is specified, the time defaults to 12:00 A.M. (Midnight).”, which is all is fine and good but it becomes an issue when you want just the date or just the time from a column that stores both. Say for instance that my boss wants to see the number of orders per day over the last couple years to see which days have the most orders (‘cyber monday!‘). Unfortunately, there’s no function called date() or time() which returns just the date portion of the datetime or just the time portion of the datetime:

-- doesn't work...
SELECT date(mydate) as thedate, count(id) as perday
FROM orders
GROUP by thedate
ORDER by perday DESC

Turns out there’s a hack that does though:

SELECT distinct convert(varchar,mydate,111) as thedate, count(id) as perday
FROM orders
group by convert(varchar,mydate,111)
order by perday DESC

Hope that makes your day.