Post-summer update

I can’t believe it’s September already.  The weather is starting to become cool and wet, days are becoming shorter, marking the end of what has been an incredible summer (and year to date).  It’s been a while since I’ve written here, so with the changing season I thought I’d share a bit of an update of 2016 so far.

Some of these warrant their own blog posts, but until I have time to write the full thing here is a summary. Continue reading “Post-summer update”

Using Multidex to solve INSTALL_FAILED_DEXOPT errors on Android

At work, we have been receiving reports of people, particularly on Android 2, not being able to install from the Google Play Store citing an error message “Package file is invalid“. At the same time, I was trying to test one of the features I was developing on a couple of Android 2 test devices we have in the office, but I was repeatedly getting an INSTALL_FAILED_DEXOPT error from adb. It seemed that both of these issues could be related.
Continue reading “Using Multidex to solve INSTALL_FAILED_DEXOPT errors on Android”

Checking for a Null value in SQL

Yesterday I fell into a very simple but dangerous SQL trap.  Can you identify it?

SELECT first_column, another_column
FROM my_table
WHERE first_column = 1
    AND (another_column >= 100 OR 
         another_column = null)

Found it?  It’s the comparison to null.  My intention was to get rows where another_column was null, however the query did not do so.  It didn’t give me any results where another_column was null, but didn’t give me any error either.  Why?

Null in SQL is not a real value.  Null means “lack of a value” or “unknown”. Anything compared to null leads to an “unknown” result. Therefore, in SQL there are actually three values conditions can evaluate to: true, false, and unknown/null. This is called three-valued logic. SQL WHERE clauses only return rows that evaluate to true, not rows that evaluate to false or “unknown”.

The proper way of checking for null is by using the IS NULL or IS NOT NULL predicates. The correct SQL query would have been as follows:

SELECT first_column, another_column
FROM my_table
WHERE first_column = 1
    AND (another_column >= 100 OR 
         another_column IS null)

So if you’re used to languages where you check for null with equal signs, remember that in SQL, you must use the special IS NULL or IS NOT NULL predicates instead of the equals sign or other comparison operators. Don’t get tripped up like I did.

If you want to read more about, Wikipedia has a detailed article on null in SQL.

iOS Screen Resolutions

iOS screen resolutions from an app developer’s point of view.

2007: In the beginning

Back in 2007 when the first iPhone and iPod Touch were launched, iOS developers only had to worry about one resolution: 480×320.  Its aspect ratio is 3:2.

Life was great.

2010: The retina displays

Fast forward three years and three generations later.  In 2010, the iPhone 4 and iPod Touch 4th generation were launched with the first Retina Displays, which doubled the resolution of the screens.  The new resolution thus was 960×640.  This was great for developers as the new resolution had the same aspect ratio of 3:2.  This meant that layouts and graphics were pretty much backwards compatible.  The only thing would be that using non-retina images on a retina display would just look a little worse.  All app developers needed to do was double the size of the images used in their app and call it a day.

The iPad

Also in the same year (2010), the iPad launched with a resolution of 1024×768 (aspect ratio 4:3).  Obviously a tablet will have a different aspect ratio than a phone.  So now, essentially developers needed to maintain two layouts for the app, one for iPhone, one for iPad.

Life was still quite good.

2012: A longer iPhone

Moving forward to 2012, Apple released the iPhone 5 and the iPod Touch 5th generation.  Apple finally decides to jump on the 16:9 train, so these devices have a 1136×640 resolution (it’s close enough to 16:9).

The iPad retina displays

The iPads also get retina displays in 2012.  They have a resolution of a whopping 2048×1536 pixels (which can’t even fit on most people’s computer monitors).  But as the aspect ratio is kept the same, this is much like the iPhone retina upgrade above.  Between 2012 and 2013, the iPad Mini and its retina version are also introduced with the same resolutions as their larger siblings.

Alright so developers have to keep three variations of layouts in mind now:  3:2 for the old iPhones (plus 1x and 2x graphics for original and retina displays respectively), 4:3 for the iPad (also with 1x and 2x) and ~16:9 for the new iPhones and iPods.

Life was manageable.

2014: A bigger iPhone

Jump ahead again two years.  We now see the iPhone 6 and iPhone 6 Plus being launched.  Their resolutions follow 16×9 and are respectively 1334×750 and 1920×1080.  The iPhone 6 Plus also handles 3x image assets.

Now, for app developers, layouts that need to be considered include 3:2 (old iPhones and iPods), 4:3 (iPads), 16:9 (new iPhones and iPods).  In addition, three versions of the image assets are required, 1x (original displays), 2x (retina displays), 3x (iPhone 6 Plus).

Tools

Thankfully, Apple has also improved Xcode throughout the years with tools in order to help with this task.  Things like Auto Layout help with varying screen sizes and orientations (if you’re willing to throw away support for older devices).

And I guess it still beats the fragmentation in the Android world.

If you need help figuring this out, check out the following links:

Database query optimization is a useful skill

Recently I read an article titled “Quick tip: Improve developer habits by showing time cost of DB queries” by Dan Birken.  He claims that one common reason why websites load slowly are because of unoptimized database queries:

Websites can be slow for many reasons, but an overwhelmingly common one is time spent performing database queries. Now database queries are likely a very important part of rendering the webpage – the information to populate a page has to come from somewhere. But sometimes rendering a page is slowed down by either unnecessary queries or improperly optimized queries.

I completely agree with his view.  For most web developers, querying information from a database is quite a common action.  Therefore, I believe learning how to write efficient and optimized database queries is a good skill to have.

I was fortunate to learn about this topic when I was developing MyBB way back years ago.  At the bottom of each MyBB page, administrators are shown debugging info with a summary of loading times and other statistics.

Debug info
Debugging information presented to the administrator on each MyBB page

Clicking on the “advanced details” link yields a much more in-depth analysis of the page generation, including an explanation of all the database queries used to create the page.

Performance debugging details
Advanced performance debugging details presented to MyBB administrators.

This is very useful to determine where the bottlenecks occur when generating the page.  Some common mistakes include repeating similar queries instead of batching them together, selecting more data than is needed, and doing full table scans rather than indexed queries.

However, not all software and web frameworks has built-in performance debugging pages like what MyBB has.  If you’re programming something from scratch, it might take some extra effort to create a useful debugging interface.  What other tools can we use then?

Using the EXPLAIN query (works at least for MySQL, PostgreSQL, SQLite) will give you the details about how a specific query will be executed by the database.  All you need to do is to tack “EXPLAIN” on at the beginning of your SELECT query, and run it through your favourite database management program.  SitePoint has a thorough article on how to understand the output of an EXPLAIN query.

Of course, it’s also beneficial to have a concrete understanding of database indexes and how they work.  One of my favourite sites on this topic is Use the Index, Luke!  They also have a fun 3-minute quiz to test your knowledge, if you think you already know everything 😉  (I got 4/5 on my first try, how about you? 🙂 )

Do you know of any other resources to help with optimizing database queries?  Let me know in the comments.