Author: james

MySQL varchar not case sensative

I managed to overlook an issue with creating a varchar column in an app I have been working on. I have basically got a normalised table, with a forien key to a table of values. In this case, its a set of HTML Document Titles, keyed off an autoincrement column called Title_ID. What I want to do is look up a title, and get a Title_ID back.

Great; I can do this with a stored function, which I did, and it worked. But it was slow. So I decided that I’d normalise these en-mass with one big INSERT statement to the normalised table (protected by a unique index constraint), and then store the resulting Title_ID.

There be dragons. As one title came through as “[Q] help me” it was duly inserted and given a Title_ID.
However, when a lower case “[q] help me” came through, it matched as a duplicate of the original and therefore was not inserted again. I then pulled the strings into a Perl hash, and of course, couldn’t find a key with “[q] help me”, only “[Q] help me”.
Turns out that the issue was my column definition. varchar(x) is not case sensative. varchar(x) binary is. The Unique index I had on here was doing its job and comparing values based upon the case in-sensative column – not its fault.

ALTER TABLE Titles CHANGE COLUMN `Title` `Title` VARCHAR(600) BINARY NULL DEFAULT NULL ;

And now I see my column as “`Title` varchar(600) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL“.

PLUG Quiz Night: tonight

The time has come! The Perth Linux User Group is holding a Quiz Night tonight (doors open 7pm, Quiz starts 7:30pm), with questions very much based upon Linux & Open Source. It’s at The Moon & Six Pence in Murray Street. We’ve got a few prizes, and it’s free (through we’ll probably make you feel guilty if you’re membership isn’t 100% up to date ($10/yr)!!). Come share a drink and a laugh.

MySQL Indexes and maximum lengths

MySQL has several index types; the default and therfore probably most common is the BTree. There’s a limit in MySQL (at least as of this writing when the “current” GA is 5.5.9) is 767 bytes – across all the columns being indexed. Of course, varchar columns can now be bigger than 255 chars, so this limit is probably more easily reached these days.

In my case, I had a table with one column of a URL’s path, and a URL’s Query String, both of which can be larger than the old 255 chars. I also have an index that cover these two plus a few other columsn – normalised protocol, normalised domain, and TCP port.

In trying to move to longer columns (1K) I had to modify my index to restrict the number of characters from these varchar columns to ensure I remained under the 767 limit – I couldn’t just change one of these columns from 255 to 1024.

I throught I’d try and simple change first – instead of increasing the oclumn, just put the restriction on the current columns as they stand – so limit the index to 255 chars (while the column still IS 255). Turns out that in at least 5.5.9, since the specified size is the current column size, it ignores this.

Lets make that clearer; you need to specify a SMALLER length for the indexed columns in order for it to stick. Once that’s done, you can then alter table to increae the column lengths.

MySQL CREATE TABLE as doesn’t take triggers with it

MySQL has a nice feature that you can make a new table exectly like an old table (as far as table column structure and indexes go):

create table FOO like BAR;

However, as I just rediscovered, any triggers on the table aren’t taken across with it. D’oh. Which reminds me, mysqldump has a specific -R flag to backup/dump routines; worth having that on too (I did).

Rule for the day: check your triggers on tables before moving/renaming.

Gingerbread fixed my WiFi

Its been a few weeks now since Android 2.3, a.k.a. Gingerbread, was delivered to my Google Nexus One mobile phone. I had been experiencing terrible problems with the WiFi reconnecting to known networks, and randomly dropping off networks it was connected to. This was most frustrating; dropping back to 3G data is expensive, slow, and sometimes becuase of this, I disabled it which meant no data. All while sitting within 10 foot of my WiFi AP.

It appears that this is no longer a problem, ever since the Gingerbread update was installed. Yay! Thanks for the fix, Google Mobile team.